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Show 426 MOLLUSCA. ORDER II. ACEPHALA NUDA( 1 ). The naked Acephala(2) are not numerous, and are sufficiently removed from the ordinary Acephala, to form a distinct class, were such a division considered requisite. Their branchire assume various forms, but are never divided into four leaflets ; the shell is replaced by a cartilaginous substance which is sometimes so thin that it is as flexible as a membrane. We divide them into two families. FAMILY I. SEGREGATA(3). This family comprises those genera· in which the individuals that compose them are insulated and without any mutual or· ganic connection, although frequently living in society. In the BIPHORA, Brug.-TnALIA, Brown)-SALPA. and DAGYSA, Gmelin, The mantle and its cartilaginous envelope are oval or cylinddcal, and open at the two extremities. Near the anus, the opening is trans· verse, wide, and furnished with a valve which permits the entrance of water, but not its exit; near the mouth, it is simply tubular. Mus· culat· bands embrace the mantle and contract the body. The animal moves by taking in water at the posterior aperture, and forcing it out through that near the mouth, so that it is always propelled backwards, a circumstance which has led some naturalists into ( 1) Since c:11led by De Blain ville AcEra.uoPnoRA HETl:ItOllRANcniAT.A. As to Lamarck, he makes a separate class of them, which he calls the TuNIC.ATA, and which he places between his Radiata and his Vermes; but these animals having a brain, nerves, a heart, vessels, liver, &c. this arrangement is inadmissible. (2) Or the .!lcepltales sans coquilles of our author. .11m. Ed. (3) As this family has received no name from our author, I have been com· pelled, in conformity with. the plan adopted from the commencement of the work,, to reme~y the omission, fot• such 1 consider it, by the above word; 'in the selectwn ofwluchi have been governed by that which the Baron himself affixes to the second family, or his .!lggreges. .Jlm. Ed. ACEPHALA 'NUDA. error by causing them to mistake the posterior opening for· the true mouth(l). It usually swims on its back. The branchire form a single tube or riband, furnished with regular ves~els, placed obliquely in the middle of the tubulat· cavity of the mantle, in such a manntr that it is constantly bathed by the water as it traverses that cavity( 2). The heart, viscera, and liver are wound up near the mouth and towards the back; but the position of the ovary varies. The mantle and its envelope when exposed to the sun exhibit the colours of the rainbow, and are so diaphanous, that the whole structure of the animal can be seen through them: in many they are furnished with perforated tubercles. The animal has been seen to come out from its envelope without appearing to suffer pain. The most curious circumstance respecting them, is their remaining united for a long time, just as they were in the ovary, and thus swimming in long chains where the individuals are disposed in different ways, but each species always according to the same order. M. de Chamisso assut·es us that he has verified a still more sin• gular fact relative to these animals; it is, that the individuals which have thus issued from a multiplex ovary, are not furnished with a similar one, but produce insulated young ones of various forms, which have an ovary like that which produced their parent, so that there is, alternately, a generation of a few insulated individuals, and another of numerous and aggregate ones, and that these two alternating generations do not resemble each other( 3). It is very certain that in some species little individuals have been observed adhering to the interior of large ones, by a peculiar kind of sucker, which were different in form from those that contained them( 4). These animals are very abundant in the Mediterranean and the warmer portions of the ocean, and are frequently phosphorescent. (1) This has also ha.ppe,1ed to M. de Chamisso, in his Dissert. de Salpi.~, Berl., 1819, and to others after him, but it is evident that there is no good reason for changing the denomination of parts in an animal merely because it swims on its back, with the head behind. It is thus that naturalists have been led into error with respect to the organization of the Pterot?·aclwata, which always swim on their back, a mode of natation common to numberless Gasteropoda both testaceous and naked. (2} Some authors assert that this tube is perforated at both ends, and that the watet• traverses it; I have endeavoured to convince myself of the truth of this assertion, but in vain. (3) Chamisso, loc. cit., I, p. 4. (4) See my Mem. sur les Biphores, f. n. |