A Course of Elementary Instruction in Practical Biology
by T. H. Huxley, asst. H. N. Martin
Rev. ed. (1892) by G. B. Howes and D. H. Scott
by T. H. Huxley, asst. H. N. Martin
Rev. ed. (1892) by G. B. Howes and D. H. Scott
[v]
The first edition of the Course of Practical Instruction in Elementary Biology appeared twelve years ago, and the motives which led to its publication are fully explained in the original preface, which is subjoined. The present edition has been carefully revised and, where necessary, enlarged by my colleagues Mr Howes and Dr Scott, assistant Professors in Zoology and Botany in the Normal School of Science and Royal School of Mines, and such additions and improvements are entirely their work. But besides these changes, the reader who compares the two editions will observe that the order in which the subjects are presented is completely changed. In the first edition the lowest forms of life were first dealt with; the series of plants followed in ascending order; and then the series of animals, from the Bell animalcule upwards to the Frog.
No doubt there is much to be said for the principle of this arrangement, which leads the student from the study of simple to that of complex phenomena; but the experience of the Lecture-room and the Laboratory taught me that [vi] philosophical as it might be in theory, it had defects in practice.
All the simplest forms of life, which are easily accessible, are of very minute size and their study involves the use of high microscopic powers. The student who begins with them is therefore not merely introduced suddenly into a region in which everything is new and strange, but he has to familiarize himself with the use of unwonted means of exploration. By taking this road, the teacher (to whom the world of the microscope is so familiar that he is apt to forget its strangeness to students) sets himself against one of the soundest canons of instruction, which is to proceed from the known to the unknown, and from familiar methods of learning to those which are strange.
After two or three years' trial of the road from the simple to the complex, I became so thoroughly convinced that the way from the known to the unknown was easier for students, that I reversed my course, and began with such animals as a Rabbit or a Frog, about which everybody knows something, while their anatomy and physiology is illustrated by innumerable analogies with those of our own bodies. From this starting point we proceeded further and further into the unfamiliar regions of invertebrate organisation until we reached the border region between animals and plants, whence there was a natural and easy ascent to the most complicated vegetable organisms.
This order is followed in the present edition; which is greatly improved by the addition of the Earthworm and the Snail in the series of animal, and of Spirogyra in the series of vegetable, types. [vii]
I have every reason to believe that our course of instruction in Elementary Biology has been found useful by many learners and teachers. But whatever the value of our attempt to carry out a certain method of instruction, I am more than ever convinced that the method itself is one which will eventually be universally adopted, not only by teachers of the biological sciences as such, but by the teachers of so much of those sciences as constitute the foundation of medicine.
No man can be competent to deal with the greater problems of biology as they are now presented to us, unless he has made a survey, at once comprehensive and thorough, of the whole field of biological investigation. The animal and the vegetable worlds are only two aspects of the same fundamental series of phenomena, and each is capable of throwing a flood of light upon the other. I know of no way by which such a broad, but not superficial, survey can be effected except the method adopted in this work.
Again, while to my mind, nothing is more to be deprecated than the compulsory waste of the invaluable time of students of medicine, upon topics so remote from the serious business of their lives as are systematic Zoology and Botany, there is no preparatory discipline so well calculated to serve as a practical introduction to the study of Human Anatomy and Physiology, as that afforded by a proper laboratory course of Elementary Biology.
Sundry experiments have left no doubt upon my mind that, by following such a course of three or four months' duration, the medical neophyte is enabled to enter upon his proper studies, provided with a practical knowledge of [viii] Anatomy, of Histology, and of the Elements of Embryology and of Physiology, such as under the present system is either not acquired at all, or is gained at the expense of time and labour which can be ill spared from practical subjects.
November, 1887.
[ix]
Very soon after I began to teach Natural History, or what we now call Biology, at the Royal School of Mines, some twenty years ago, I arrived at the conviction that the study of living bodies is really one discipline, which is divided into Zoology and Botany simply as a matter of convenience; and that the scientific Zoologist should no more be ignorant of the fundamental phenomena of vegetable life, than the scientific Botanist of those of animal existence.
Moreover, it was obvious that the road to a sound and thorough knowledge of Zoology and Botany lay through Morphology and Physiology; and that, as in the case of all other physical sciences, so in these, sound and thorough knowledge was only to be obtained by practical work in the laboratory.
The thing to be done, therefore, was to organize a course of practical instruction in Elementary Biology, as a first step towards the special work of the Zoologist and Botanist. But this was forbidden, so far as I was concerned, by the limitations of space in the building in Jermyn Street, which possessed no room applicable to the purpose of a labora[x]tory; and I was obliged to content myself, for many years, with what seemed the next best thing, namely, as full an exposition as I could give of the characters of certain plants and animals, selected as types of vegetable and animal organization, by way of introduction to systematic Zoology and Palæontology.
In 1870, my friend Professor Rolleston, of Oxford, published his "Forms of Animal Life." It appears to me that this exact and thorough book, in conjunction with the splendid appliances of the University Museum, leaves the Oxford student of the fundamental facts of Zoology little to desire. But the Linacre Professor wrote for the student of Animal life only, and, naturally, with an especial eye to the conditions which obtain in his own University; so that there was still room left for a Manual of wider scope, for the use of learners less happily situated.
In 1872 I was, for the first time, enabled to carry my own notions on this subject into practice, in the excellent rooms provided for biological instruction in the New Buildings at South Kensington. In the short course of Lectures given to Science Teachers on this occasion, I had the great advantage of being aided by my friends Dr Foster, F.R.S., Prof Rutherford, F.R.S., and Prof. Lankester, F.R.S., whose assistance in getting the laboratory work into practical shape was invaluable.
Since that time, the biological teaching of the Royal School of Mines having been transferred to South Kensington, I have been enabled to model my ordinary course of instruction upon substantially the same plan.
The object of the present book is to serve as a laboratory [xi] guide to those who are inclined to follow upon the same road. A number of common and readily obtainable plants and animals have been selected in such a manner as to exemplify the leading modifications of structure which are met with in the vegetable and animal worlds. A brief description of each is given; and the description is followed by such detailed instructions as, it is hoped, will enable the student to know, of his own knowledge, the chief facts mentioned in the account of the animal or plant. The terms used in Biology will thus be represented by clear and definite images of the things to which they apply; a comprehensive, and yet not vague, conception of the phenomena of Life will be obtained; and a firm foundation upon which to build up special knowledge will be laid.
The chief labour in drawing up these instructions has fallen upon Dr Martin. For the general plan used, and the descriptions of the several plants and animals, I am responsible; but I am indebted for many valuable suggestions and criticisms from the botanical side to my friend Prof Thiselton Dyer.
London, September, 1875.
[272]
The "Common Snail", H. aspersa, and the "Garden Snail", H. hortensis, are to be found in abundance in our gardens and hedgerows, and the descriptions here given apply equally to either with the exception of the shell. This, in the 'Garden Snail', can be at once recognized by its delicate texture and predominant whitish-yellow colour, as compared with the rough-surfaced brown-banded shell of its ally. During the summer months the 'Common Snail' is to be met with, leading an active independent existence upon or in the immediate neighbourhood of fruit-bearing shrubs. It is very susceptible to cold, and retires during the later autumn to some recess in a wall or tree, where it usually remains dormant and hibernating until the following spring. It not unfrequently buries itself for the same purpose, and even in the warm season may be induced to hibernate temporarily if starved or submitted to a reduced temperature. During such a period, or during normal hibernation, the body is completely retracted within the shell, the mouth of the latter being sealed by a film of mucus secreted by the animal, which hardens on exposure to the atmosphere; this is perforated to allow of the passage of air during respiration and is best termed for obvious reasons the hybernaculum. During the hibernating period the animals are frequently to be encountered huddled together in as[273]semblages; hence it is no uncommon thing to find, during the warm season, individuals, to the exterior of whose shells there adhere one or more (often a great number) of these hybernacula, cast off by their fellows on emerging from the dormant state.
The body of the snail is soft and unsegmented, and, unlike that of any other animal dealt with in this work, asymmetrical–inasmuch as the anal, respiratory, excretory and genital orifices all open to the right side, the latter being situated far forwards near the mouth. The ventral surface of the body is thick and fleshy, giving rise to a locomotor foot, by the wave-like contractions of which the sluggish movements of the animal are performed. So delicately adjusted are these, that the creature can crawl with ease and comfort over a knife-edged surface. The anterior end of the body is differentiated into a well-marked head segment bearing two pairs of tentacles–a shorter labial pair adjacent to the mouth and a longer ocular pair situated above and behind these. The integument covering the apex of each tentacle is especially modified in connection with a nerve supply derived from a large underlying ganglion, whence it follows that the eye-bearing tentacle performs a double function. It may be that the labial one is either tactile or olfactory, but the exact functions of these sensiferous areas have yet to be fully elucidated.
Between the head-segment and the free anterior end of the pedal-disc there is a cleft, at the base of which opens a large mucus secreting pedal-gland which extends far back to the hind end of the body.
The dorsal surface of the body is produced into a spirally coiled hump, within which the whole digestive gland and portions of the alimentary and reproductive viscera are lodged–it is hence termed the visceral sac. This sac, to[274]gether with the wall of the pulmonary chamber which over lies it in front, is invested in the spirally coiled shell, the apex of which lies altogether to the animal's right side. The mouth or peristome of the shell overlies the thickened anterior border of the pulmonary sac, from which a constant addition of shelly matter is secreted during the growth of the animal, as is also the hybernaculum during repose.
The aperture of the mouth is bounded by soft fleshy lips, and it leads into a spacious buccal cavity the walls of which are excessively thick and muscular. A denticulate horny upper jaw or beak is present, and the floor of the mouth is raised up into a cushion-shaped odontophore or tongue which is in turn surmounted by a dentigerous lingual-ribbon or radula. This is thrown into a licking rasp-like motion during feeding, by the activity of an underlying musculoskeletal apparatus, the odontophoral cartilages connected with which are worthy of note as composing an endoskeleton.
The mouth itself leads into a long tubular œsophagus, which passes straight back and, on entering the visceral sac, opens into a small stomach which receives the secretion o the digestive gland. The stomach in turn gives origin to a coiled intestine which, on nearing the exterior, skirts the lower right-hand border of the pulmonary sac, terminating in an anus which lies to the right of the respiratory orifice. The middle segment of the œsophagus is enlarged to form a distensible crop, applied to which there are a pair of salivary glands, confluent above and pouring their secretion into the mouth by means of two elongated ducts.
The digestive gland is a paired structure; its lobes are asymmetrical–the smaller right one lying altogether within the top whorls of the shell. Microchemical examination shows that it performs a complex function, serving both as a [275] storehouse of combustible fatty carbo-hydrate material and as a centre for secretion of a digestive ferment.
The pulmonary sac or mantle arises as a fold of the body wall, in which pulmonary vessels appear during development. At the hinder end of the enclosed pulmonary chamber there are situated, side by side, the heart and kidney. The heart is enclosed in a definite pericardium, the floor of which is in open communication with the excretory organ by means of a short ciliated reno-pericardial duct. The excretory organ itself lies altogether to the right side of the body and debouches on to the exterior by a long duct, running parallel with the rectum.
The heart consists of a single auricle and ventricle, the valves between them being so disposed as only to admit of a current passing from the lung sac to the body. It therefore transmits only aerated blood, and as it is in no way concerned with the propulsion of the blood to the respiratory organs it is termed–like that of the Crayfish already considered–a systemic heart. The ventricle gives origin to a single aorta which, on entering the body-cavity, subdivides into two branches. The anterior of these supplies all parts of the body which lie in front of the heart, and the posterior is restricted to the visceral sac and its contents. These arterial trunks break up into minute ramifications, which pass either into capillary systems or lacunar spaces, all of which converge, directly or indirectly, towards a great sinus which lies at the base of the pulmonary sac. From this, afferent pulmonary vessels arise on all sides; the branches of these, reuniting in the substance of the lung-sac, form a system of efferent pulmonary vessels, which unite to form a large pulmonary vein which enters the heart. The efferent pulmonary vessels of the right side pass, on their way to the heart, through the excretory organ, in the sub[276]stance of which they break up into a second (renal) capillary network.
The blood contains amœboid corpuscles, which float in an opalescent serum; it assumes a bluish tinge on exposure to the atmosphere.
The central nervous system is enclosed in a membranous circum-œsophageal sheath. It consists of three yellowish ganglionic masses; the supra-œsophagual or cephalic lying above the gullet and giving off nerves to the head segment and related parts; the podal which supplies the foot and body-wall; and the parieto-splanchnic which distributes fibres to the body-wall and viscera, and all parts lying behind its point of origin, irrespective of the foot. These ganglionic centres are connected together by lateral commissures; and from the cerebral mass there arise a system of buccal nerves in relation with the buccal mass and its odontophore, and others distributed to the sense organs. The latter are, a pair of small auditory vesicles to be hereafter described (see Sect . J. 3) and visual and tegumental sense organs borne by the tentacles, to which reference has already been made.
The snail is hermaphrodite and the sex-organs are highly complicated. With the exception of the hermaphrodite gland or ovotestis, a portion of the duct of the same and its appended albumen-secreting gland–all of which are lodged in the visceral sac, they fill the greater part of the spacious body cavity and can be at once recognized by their deadwhite colour. As the hermaphrodite duct approaches the exterior it suddenly divides into distinct oviduct and vas deferens; the base of the latter is enlarged to form a swollen eversible intromittent organ or penis, which opens, side by side with the oviduct, into an integumental pit or genital cloaca. Appended to the whole apparatus there are several accessory glands and diverticula. Chief among these is a [277] cœcal diverticulum of the base of the penis which secretes a mucilaginous investment for the spermatozoa; the spermatophores or packets of spermatoza thus formed are transferred, during copulation, to a corresponding cœcal diverticulum of the oviduct known as the receptaculum seminis. Fleshy valves are developed within the lips of the genital cloaca and at the orifices of the genital ducts which open into it, and the whole condition of the organs is such as to obviate the possibility of self-fertilization.
In the spiculum amoris, an accessory to the female portion of the apparatus (see Sect. G. 3 f) we have a structure, almost without parallel in the whole animal kingdom. It reaches maturity during the breeding season, and is forcibly ejected from individual to individual during the amorous overtures, which last for a period of some hours.
The spermatozoa are long filiform bodies, each with an enlarged nucleus-bearing "head." The ova are chiefly noteworthy on account of the absence of a distinct vitelline membrane; they are comparatively large and are provided with a nutritive food-yolk.
The reproductive elements of opposite sexes ripen alternately, the maturation of a given batch of spermatozoa preceding that of the ova they are destined to fertilize, and in view of the facts above related it follows that the spermatozoa must be transferred, during copulation, to the body of the second individual–there to await the descent of the ova.
Fertilization takes place as the ova leave the body, and there are to be found in the haunts of these animals during the summer months, usually beneath some stone or decaying wood or more rarely buried in the earth itself, aggregates of 100 or more eggs, each invested in an albuminous envelope, within which the early developmental phenomena are undergone.
[278] The conditions of development of the "Pond Snail" (Lymnæus stagnalis) are much more favourable for observation than those of the "Common Snail," and they are accordingly here dealt with.
The "Pond Snail" is a sluggish carnivorous animal which may be found in abundance during the summer months in ponds and stagnant waters; it is moreover an indispensable acquisition to the aquarium. If well fed, the animals will deposit their eggs upon the vessel, in aggregates, imbedded in the albuminous secretion aforementioned. The more important developmental changes are undergone within this investment.
The transverse diameter of the fertilized ovum is about the 1/200th of an inch. The segmentation is holoblastic and unequal, and the changes undergone during the early developmental period are substantially such as have been already described for the Earthworm (cf p. 247), resulting in the formation of a simple two-layered gastrula.
Prior to segmentation there appear on the surface of the egg some two or more minute protuberances, which finally become constricted off and lost.1 A portion of the nucleus of the egg cell is carried away with each body thus formed. Various interpretations have been put upon these polar bodies;they are cellular in nature and of general occurrence in the animal kingdom, but further discussion concerning them is beyond the scope of this work. They are alluded to here, as the conditions for observation are exceedingly favourable.
After the gastrula phase is passed, the embryo assumes a somewhat spherical shape, during which period the mouth is formed as a median involution of the epidermis. There now appears on the surface, immediately in front of the [279] mouth, a thickened zone incomplete ventrally. The surface of the body becomes ciliated and especially so this zone, whereupon there results a rapid rotation of the embryo within its albuminous investment. This being so, the zone in question is termed the trochal ridge, the larva possessed of it being said to be in the trochosphere stage. This, in turn, gives place to a more advanced veliger stage, so called on account of the changes undergone by the trochal ridge, which now becomes more marked, being produced out into a hood-shaped pre-oral lobe or velum. During this stage the mantle arises as a fold of the body-wall, which, as age advances, takes on the characters of a lung sac.
During the final stages of larval metamorphosis the left side of the body grows much more rapidly than the right one. Thus it is that the originally bilaterally symmetrical larva becomes converted into an asymmetrical adult, a fact which renders clear the displacement of the orifices (other than those of the mouth and pedal gland) and of the organs connected therewith, the suppression of the excretory organ of one side, and the enormous increase in size of the left lobe of the digestive gland, as compared with the smaller right one.
Very early in the history of the larva the locomotor foot arises, as a median ventral outgrowth between the mouth and anus; and as the importance of this structure becomes more marked, the cilia of the velum undergo a reduction. Consequent upon these changes the rotatory movements of the animal, so characteristic of the trochosphere, give place to a sluggish creeping motion. The velum itself does not entirely vanish in Lymnanus, but persists throughout life as a couple of so-called subtentacular lobes which lie immediately above the mouth. These are wanting in the "Common Snail."
LABORATORY WORK.
If the snail be killed, by immersion in water heated to a temperature such as the hand can comfortably bear, the shell will readily part company with the muscles to which it gives attachment. This being the case, no difficulty will be experienced in removing the animal, if holding it in the left hand, the shell be twisted off by the finger and thumb of the right.
In dissecting the internal organs it is advisable to remove the visceral sac in starting. This may best be done by cutting away its thickened edge with a pair of scissors, and tearing it off with a couple of pairs of strong forceps.
A. General external characters.
1. In the living animal observe:
a. The body; produced ventrally into an expanded locomotor disc or foot; and dorsally into a spirally coiled visceral -sac, invested in the single coiled shell or exoskeleton.
b. The head segment, a freely projecting anterior lobe of the body, which overhangs the foot. It bears two pairs of retractile tentacles, a smaller lower-most labial pair, situated at the sides of the mouth, and a longer dorsally placed ocular pair, at the summit of each of which there is borne, when fully extended, a minute black eye.
c. The apertures. Examine the animal en face, and note:
[281] • The mouth, surrounded by a thick circular lip, externally to which there is, on either side, a well-marked lateral lip. Observe that the mouth is bounded above by a denticulate horny beak, lying within the circular lip.
• The podal gland, opening by a wide aperture at the base of a depression between the head segment and the foot. Insert a seeker into it–it can be readily introduced for a distance of more than an inch.
•With the animal still in this position, note that the shell (and visceral sac which underlies it) is carried altogether to the creature's right side.
Place the snail right side uppermost, and examine:
• The genital orifice, situated a short distance behind the bases of the tentacles of the right side.
• Gently raise the shell, and note, underlying its free edge, the thickened glandular border of the pulmonary sac. Enclosed by valve-like folds of this there lies the large respiratory orifice. Situated side by side with, and a little to the right of, this, is the anus. (The excretory orifice also opens at this point, but within the lip of the respiratory one. It is not visible without dissection. Cf. Sect. C. i d.)
• The genital furrow, a feebly defined integumental groove, extending from the base of the pulmonary sac to the genital orifice.
B. The shell or exoskeleton.
a. It forms a continuous investment. Its free edge is produced into a whitish porcellanous reflected [281] border or peristome. It is thrown into a spiral of four whorls.
b. Note its texture and colour (cf. p. 272). As the apex (first formed part) is approached, a smooth friable texture and a pearly lustre are assumed.
c. The columella. If, with the peritreme directed towards you, the shell be carefully opened up with scissors, the columella will be seen as a central pillar or axis. Cut into this with care; it will be found to be hollow, and closed in below by an overgrowth of the peritreme–the shell is thus a spirally coiled tube.
In young shells the columellar cavity opens freely below, by an aperture or umbilicus.
d. The columellar muscles. Remnants of the glistening white tendons of these are often to be found, attached to the upper end of the shell axis. (Cf. Sect. E. 1 a.)
e.The hybernaculum or epiphragm. Examine a dormant specimen, and note that the mouth of the shell is completely closed by this. Remove it and examine it under a low power; note the perforation of its central area.
C. The pulmonary sac and its associated structures.
1. Examine the pulmonary sac from above. It consists of a membranous expansion of the body-wall, which overlies the entire antero-dorsal region of the visceral hump. Large blood vessels are developed within its walls; and there is visible through it, on the right side, the yellow excretory organ. Insert a scissors [283] into the respiratory orifice and made an incision along the right-hand border of the sac, cutting clear of the excretory organ. Carry a second incision along the thickened base of the sac towards its left side, and eflect the whole. Note:
a. The floor of the pulmonary chamber; thin and transparent, there being seen through it the dead-white reproductive organs.
b. The rectum, a thin walled tube coursing along the floor of the pulmonary chamber at its extreme right hand border.
Follow it to the anus. It opens, at the base of a groove-like depression in the lip of the respiratory sac, below and to the right of the respiratory orifice.
c. The excretory gland, a considerable yellowish mass, lying to the posterior right hand end of the pulmonary sac. Make an incision into it and wash out its contents; note the thickened spongy texture of its glandular lining, which is thrown into a series of folds by the underlying blood-vessels.
d. The excretory duct, a yellow thin-walled tube, running to the right of and parallel with the rectum. Its orifice lies a little above and to the right of the anus. Insert a blowpipe into this and inflate the whole; note that the duct is continued along the right hand border of the gland to its summit.
If examined minutely, it will be seen that the excretory duct is continued on beyond the above-named orifice as a well marked excretory groove, whose walls are contractile. This courses over the base of the rectum dorsally to the anus, and, passing [284] backwards and downwards, terminate to the left of the respiratory orifice.
e. The pericardium, a small sac lying in a recess of the left hand border of the excretory gland. Remove its front wall and examine the heart ; it is subdivided into a single auricle and ventricle.
2. Cut away the upper part of both the excretory organ and pericardium, and remove so much of them as remains, together with a portion of the adjacent visceral sac. Wash carefully until clear of sediment, and examine in water under a low power.
A short ciliated duct will be found, passing from the base of the pericardium to the excretory gland into which it opens by a reno-pericardial aperture.
D. The alimentary organs.
1. Place the animal on its left side, and pin it down through the muscular foot. Remove the lung-sac and liberate the rectum from its surroundings. Next dissect away the right half of the body wall and visceral sac.
There will be seen filling up the greater part of the body cavity the dead-white generative organs; remove these en masse, whereupon there will be clearly visible–
a. The crop; a large sac-like organ, filling a considerable portion of the "body cavity". It is generally rendered the more conspicuous on account of the yellow colour of its contents. Its lining membrane is thrown into a series of longitudinal folds, these are visible through its thin walls, giving it an apparent longitudinal striation.
[285] b. Follow the crop forwards, it will be found to arise from the roof of the thick-walled muscular buccal mass, as a simple œsophageal tube.
c. The sac of the radula, a small backwardly directed diverticulum of the floor of the buccal mass.
d. The salivary glands, two irregular whitish masses closely applied to the sides of the crop. They are confluent postero-dorsally, and each is connected with the roof of the mouth by an elongated salivary duct. Follow one of these forwards, its base becomes enlarged as it enters the mouth, at the re-entering angle between the œsophagus and buccal mass.
e. Follow the crop backwards, it will be found to pass into the visceral sac under cover of the right lobe of the digestive gland (a yellowish brown mass, filling the apical whorls of the visceral hump). Turn this gland downwards and forwards, and note, lying beneath it, the small sac-like stomach, into the hinder end of which the œsophagus is seen to pass.
f. The digestive gland. Buried up in its small right lobe will be found the white ovotestis (remove this with care). Its left lobe is much the larger of the two; it is subdivided into three lobules which adapt themselves to the coils of the intestine. Arising from these are seen well-defined ducts; follow them and note that they unite to form a single short digestive duct.
g. The stomach. Open this from the side, being careful not to injure the right lobe of the digestive gland, and wash out its contents. It is a simple [286] sac, slightly subdivided into two by a longitudinal fold. Four apertures will be found in its walls; they are–
• The œsophageal orifice, opening into its hinder end.
• The origin of the intestine, lying below and to the right of [above].
• The apertures of the digestive ducts; opening–that of the left side just above the intestinal orifice–that of the right into its apex (this duct skirts the free edge of its gland).
• The intestine. After leaving the stomach it passes upwards and forwards, altogether to the left side. It then makes two turns upon itself, approaching the right side as it does so, and finally courses along the right border of the pulmonary sac. (Cf. Sect.C. § 1 b.)
E. The buccal mass and odontophore.
1. Examine the buccal mass from the side and make out its muscles. The constrictor fibres have already been referred to, as constituting the muscular wall of the whole structure. Of those which remain the more important are–
a. The retractor; a large sheet, arising from its floor and side walls below the sac of the radula, and passing back, side by side with the immense retractor pedis fibres arising from the foot, to be inserted into the columella or shell axis. (Cf Sect. B. d.)
b. The protractors; delicate muscles arising from its side walls, and passing downwards and forwards, to be inserted into the cephalic integument.
[287] c. The levators; delicate muscles arising just above the protractors, and passing tentacles.
d. The depressors, small muscles underlying the protractors, and passing obliquely backwards.
Carefully remove one half of the buccal mass, cutting to the near side of the middle line. Examine under water and note–
a. The odontophore, a cushion-shaped elevation of the floor of the mouth, completely covered in mucous membrane. There overlies it a yellowish ribbon-shaped band–the lingual ribbon or radula; follow this back into its sac (see Sect. D. I. c) and note that as that is approached it assumes a whitish colour and membranous texture.
b. The radula. Remove this bodily, with a pair of forceps; transfer it to a glass slide, cover in water and examine under a low power. Note the presence of an immense number (between twelve and thirteen thousand) of chitinous teeth. Those which are functional can be readily distinguished by their yellow colour and sharp pointed cusps. Examine that portion of the radula which lay within the sac. Note the transparent immature teeth, becoming simpler and more papilla-like as the hindermost border is reached. Examine in like manner the front end. It is beset by teeth whose cusps are worn down, and reduced, in many cases, to the condition of mere functionless rudiments.
[288] c. Examine the functional region under a 1/4 or 1/6 objective. Note that the bases of the two sets of lateral teeth (uncini), by which the radula is for the most part beset, are concave externally. Follow those of one side inwardly–a single median longitudinal row (rachis) will be reached, the bases of whose teeth are concave on either side.
The rasping surface of each median tooth is produced into a pointed cusp. In the lateral teeth there appears, on the inner side of this, a second smaller one which increases in size relatively, as the free edge of the radula is approached. There appears, at the same time, a smaller external accessory cusp, which is not represented in the median teeth.
d. The edontophoral cartilages; two gristly masses to whose presence the elevation of the floor of the mouth is due. There are attached to their bases a series of small intrinsic muscles, arising from the side walls of the buccal mass.
e. The horny beak ; seen to be formed in relation with a special fold of the lining membrane of the roof of the mouth. It lies wholly within the circular lip, seen in section to be thick and fleshy.
f. The salivary duct; opening into the roof of the mouth by a minute orifice situated just above the odontophore.
F. The Pedal Gland.
This is best examined at this stage, by removing the foot to one side of the middle line. It has the appearance of a white fluffy-looking mass, lying immediately above the pedal disc, and extending back for two-thirds the length of the same. Open it with care–it consists of a cœcal [289] diverticulum of the integument, opening below the mouth by an expanded orifice. (Cf. Sect. A. I. c. •.)
Compare the same, as seen in a transverse section across the foot. Note its thick glandular walls and central lumen. It is accompanied on either side by a well-defined lateral pedal blood sinus.
G. The Reproductive Organs.
1. Pin the animal down as directed in Sect. D., and remove the right half of the body-wall posterior to the genital orifice, together with the rectum. Dissect off the visceral sac and examine in order–
a. The ovotestis or hermaphrodite gland ; a small white mass, buried up in a fossa of the right lobe of the digestive gland. Remove sufficient of the latter to fully expose it.
b. The duct of the ovotestis or hermaphrodite duct; a short highly convoluted glistening white duct, passing upwards from the ovotestis towards the main mass of the reproductive apparatus.
2. Remove sufficient of the right lobe of the digestive gland to liberate the ovotestis and hermaphrodite duct; note–
a. The albumen gland; a large greyish white structure lying, apex downwards, to the immediate left of the duct of the ovotestis.
This gland swells up very rapidly under water, in the manner of the glandular segment of the Frog's oviduct already described (p. 53). Dissection may conveniently be carried on under weak alcohol.
[290] In the vicinity of the base of this gland, the genital organs come into close apposition with the intestine and aortic vessels. Carefully liberate them en masse from their surroundings and pin down under water–the albumen gland with its flattened face uppermost. Now follow in order–
b. The further course of the hermaphrodite duct.. On reaching the albumen gland it becomes suddenly constricted, to form an exceedingly delicate threadlike tube. As this is usually of a dead-white colour it can be seen through the more transparent albumen gland, in the superficial portion of which it lies buried. It passes for a short distance towards the apex of this gland, and then, turning sharply upon itself, is continued straight back towards the base of the same; it here becomes suddenly enlarged to form a convoluted duct with sacculated walls, which is continued on towards the genital orifice.
The transition between the two segments of the hermaphrodite duct here described, is so sudden as to make it appear that they are distinct structures; hence it is that the sacculated segment is sometimes spoken of as an oviduct.
c. Its walls are beset externally by a continuously straight series of glandular follicles, making up an accessory gland (so-ca]led prostate).
d. Open up the albumen gland and note its spacious central duct. It will be found to enter the enlarged "head" of the sacculated portion of the hermaphrodite duct. Follow the hermaphrodite duct onwards–its sacculations gradually disappear, and it suddenly divides into two–oviduct and vas deferens.
[291] e. The oviduct; a very short thick-walled tube, entering into immediate relationship with a large white sac–the dart sac (§ 3 e).
f. The vas deferens; a thin-walled tube, arising side by side with g, but of lesser calibre than it. Its inner end is tied down to the female duct; leaving this it passes under the retractor muscle of the right tentacles and becomes enlarged to form the penis, apparent now as the elongated swollen end of the vas deferens. It passes to the exterior side by side with the female duct.
It is attached to the body-wall of the left side by a ribbon-shaped retractor muscle. Cut this across and displace the whole organ.
g. The spermatophoral gland of flagellum ; an elongated thread-like diverticulum of the base of the penis. It is much coiled.
3. Remove the entire reproductive system from the body, together with a portion of the body-wall immediately adjacent to the sexual orifice. Pin the whole down carefully under water, and open up the larger ducts and their appendages, as they stand related to the exterior. Make out in order:
a.. The genital cloaca or vestibule ; a shallow integumental pit, opening on to the exterior by the single genital orifice.
b. The penis. Remove a portion of its enlarged eversible base. It opens into the front wall of the vestibule; its lining membrane is produced into a fold or prepuce, above which there is usually an eminence.
[292] a. The oviduct. Follow this down; its first segment already described (§ 2 c) is sometimes termed the vagina. This opens by a tumid lip into a thin-walled terminal segment, which enters the vestibule side by side with the penis.
d. The spermatheca or receptaculum seminis (receptacle of the spermatophores); an elongated diverticulum of the above-named terminal segment of the oviduct, bound down during life to the hermaphrodite duct (§ 2 b). Liberate it from this, and note that it arises by a short neck which subdivides into two cœcal diverticula–a longer and stouter coiled one, and a shorter one which terminates in a globular enlargement (this lies, during life, tucked away in the bend of the first coil of the intestine. Cf. § 2 a).
e. The dart-sac; an immense melon-shaped diverticulum of the base of the oviduct, in life backwardly directed. Open it up and note its powerful muscular walls; lying within it will be seen
f. the dart or spiculum amoris, a four-bladed calcareous projectile. Its head overlies a papillate ingrowth of the lining membrane of the sac.
g. The accessory mucus or digitate glands ; two branching masses opening side by side into the basal segment of the oviduct, immediately above e.
H. The circulatory system.
Examine an animal freshly removed from its shell, before it is in any way dissected. The pulmonary vein will be seen running in the wall of the lung-sac, in front of and in a line with the excretory organ. Make an incision in this and [293] introduce a small syringe or medicine-dropper filled with injecting material. Seize the vessel and the inserted nozzle between the finger and thumb of the left hand, and inject. If this simple operation be carefully performed, all the leading vessels which carry aerated blood will be injected.
1. Lay open the pulmonary-sac, and note:
a. The efferent pulmonary vessels; a uniformly arranged series of trunks bringing in the blood from the lung-sac; they unite to form a large pulmonary vein which enters the auricle.
b. The efferent renal vessels; two or three small trunks conveying the blood from the excretory organ to the pulmonary vein.
During the process of injecting these become very rapidly filled, and in the exercise of the pressure necessary to inject the whole arterial system their walls are not unfrequently ruptured.
2. Open the heart (cf Sect. C. I e). Note the thick muscular wall of the ventricle, and the auriculo-ventricular valves–so disposed as only to admit of a passage of the blood from the auricle to the ventricle.
3. The arterial system. Remove the right half of the body wall and visceral sac, and together with them the rectum and greater portion of the genital apparatus– cutting the sacculated hermaphrodite-duct across at about its middle.
There will thus be exposed the great arteries. Work them over in the following order:
a. The aortic trunk, a very short vessel, arising from the base of the ventricle. It lies to the left of the head of the first coil of the intestine, immediately [294] below which it suddenly subdivides into an anterior and a posterior branch.
b. The anterior aorta; a well-defined vessel running parallel with the sacculated portion of the hermaphrodite-duct. On entering the body-cavity it passes beneath the first coil of the intestine and the adjacent globular head of the spermatheca (G. § 3 d); it then travels to the right side of the body and runs on, beneath the crop, to the anterior extremity. Branches are given off along its course to the genital apparatus, foot, crop, salivary glands and tentacles, with the adjacent integument; it finally perforates the sheath of the nervous system, ending in relation with the floor of the buccal mass. Special branches accompany the rectum and pedal gland.
c. The posterior aorta; a considerable vessel which accompanies, at its outset, the first limb of the intestine. It descends into the visceral sac, and is mainly distributed to those portions of the alimentary canal and its appended glands contained within the same.
4. The venous system. With the exception of certain well-defined sinuses and vessels detailed below, the venous channels consist mainly of a series of lacunar spaces, in free communication with the "body-cavity." Reference has already been made (Sect F.) to the lateral pedal sinus, and if, in an uninjured Snail, the point of the injecting apparatus be carefully inserted into this, no difficulty will be found in demonstrating the existence of the vessels here described.
Having injected the animal, reflect the lung-sac, [295] cutting close alongside the rectum, in order that that portion of it bearing the smaller pulmonary vessels may be reflected intact.
Note in order:
a. The circulus venosus pulmonis; a great sinus surrounding the base of the pulmonary sac.
b. The marginal sinus of the visceral sac; a spacious channel continuous with a. posteriorly and running within the thickened edge of the visceral sac.
On exposing the interior of the pedal sinus in an uninjected specimen, there will be seen opening into it the orifices of adjacent lacunæ. During the process of injection from the above sinus the fluid introduced passes through these–partly into the body-wall, partly into the "body-cavity" in either case finally reaching the pulmonary circulus, towards which all those channels in which venous blood circulates eventually converge.
c. The afferent pulmonary vessels; arising from the pulmonary circulus on all sides. They alternate with striking regularity with the efferent pulmonary vessels, the two sets of trunks being connected by a well-differentiated respiratory plexus.
The afferent pulmonary vessels of the left side and front wall of the lung-sac are elongated. Those of its right side are very short, but in so far as their relations to the lung-sac are concerned they differ from their fellows in no other respect; after leaving that to reach the heart however, they enter the excretory organ and break up within it into a renal plexus, from which the efferent renal vessels previously described (Sec. H. 1 b) arise.
The walls of of the vessels concerned in respiration are frequently pigmented. The entire pulmonary system can therefore be generally followed in an uninured specimen, without the aid of injection, by examining with care the wall of the lung- sac from the outside.
1 In this animal they usually rernain adherent to the fertilized egg until segmention is far advanced.
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