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[96] Mr. Huxley made some observations upon the structure of the anomalous genus Sagitta, which has already been more than once a subject of discussion at the Meetings of the British Association. Mr. Huxley's statements essentially confirmed those of M. Krohn; the existence of a ciliated canal or oviduct in the outer part of the ovary being the only new fact of any importance brought forward. The very wide geographical distribution of Sagitta was alluded to, the animal having been found in all the seas through which H.M.S. Rattlesnake passed in her circumnavigatory voyage.
In discussing the zoological relations of Sagitta, Mr. Huxley's remarks were to the following effect:–Sagitta has been placed by some naturalists among the Mollusca, a view based upon certain apparent resemblances with the Heteropoda. These however are superficial; the buccal armature of Sagitta, for instance, is a widely different structure from the tongue of Firola, to which, when exserted, it may have a distant resemblance; the distinct striation of the muscular fibre, and the nature of the nervous system, equally separate Sagitta from the Mollusca.
There appears to be much more reason for placing this creature, as Krohn, Grube, and others have already done, upon the annulose side of the animal kingdom, but it is very difficult to say in what division of that sub-kingdom it may most naturally be arranged. At first sight it seems to present equally strong affnities with four principal groups, viz.–l. the Nematoid worms; 2. the Annelida; 3. the Lernsean Crustacea; and 4 the Arachnida.
1. With the Nematoid worms it is allied by its general shape and habit, its want of distinct annulation, and remotely, by the armature [97] of the mouth. But on the other hand, it differs widely from them in the nervous system, the sexual system, and the nature of the muscular tissue.
2. Sagitta has no small resemblance to certain Naiadæ, in which when young the anterior hook-like feet are directed forwards parallel to the mouth. It differs from them in the nature of its nervous system, which exhibits a concentration quite foreign to the annelid type, in the nature of the muscular tissue, and in the total absence of any water vascular system.
3. and 4. The real affinities of Sagitta are probably with one or other of these great divisions. The structure of the nervous and muscular system speaks strongly for this view, and the nature of the sexual system is not opposed to it, inasmuch as we have hermaphrodism among both the lowest Crustacea (Cirrhipedia) and the lowest Arachnida (Tardigrada).
The study of development can alone decide to which of these divisions Sagitta belongs; but until such study shall have demonstrated the contrary, Mr. Huxley stated his belief that Sagitta bears the same relation to the Tardigrada and Acaridæ, that Linguatula (as has been shown by Van Beneden) bears to the genus Anchorella, and that the young Sagitta will therefore very possibly be found to resemble one of the Tardigrada, the rudimentary feet with their hooks being subsequently thrown up to the region of the head, as they are in Linguatula.
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