ACEPHALA NUDA. 427 



error by causing them to mistake the posterior opening for the true 

 moulh(l). 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 of the mantle, in such a manner 

 that it is constantly bathed by the water as it traverses that cavi- 

 ty(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 ihe 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 cu- 

 rious 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 alter- 

 nating 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 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. 



(2) Some^uthors 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. 



(3) Chamisso, loc. cit, 1, p. 4. 



(4) See my Mem. sur les Biphores, f II. 



