112 



MOLLUSCA. 



cular 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 error by causing 

 them to mistake the posterior opening for the true mouth*. It 

 usually swims on its back. The branchiae form a single tube or 

 riband, furnished with regular vessels, placed obliquely in the middle 

 of the tubular cavity pf the mantle, in such a manner that it is con- 

 stantly bathed by the water as it traverses that cavity f. 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 en- 

 velope 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 assures 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 J. 



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. 



These animals are very abundant in the Mediterranean and the 

 warmer portions of the ocean, and are frequently phosphorescent. 



The THALLE, Brown, have a small crest or vertical fin near the 

 posterior extremity of the back ||. 



* This has also happened to M. de Chamisso, in his Dissert, de Salpis, 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 Pterotracheata, which always swim on their 

 back, a mode of natation common to numberless Gasteropoda both testaceous and 

 naked. 



f Some authors assert that this tube is perforated at both ends, and that the 

 water traverses it ; I have endeavoured to convince myself of the truth of this 

 assertion, but in vain. 



J Chamisso, loc. cit., I. p. 4. 



See my Mem. sur les Biphores, f. II. 



|| Holothuria Thalia, Gm., Brown's Jam., xliii, 3; H. caudata, lb., 4; H. 

 denudata, Encyc. Method., Vers., Ixxxviii ; Salpa critata, Cuv., Ann. du Mus., IV, 

 Ixviii, 1, figured under the name of Dagysa by Home, Lect. on Compar. Anat. II, 

 Ixiii ; Salpa pinnata, Forsk., xxv, B. 



