ACEPHALA NUDA. 271 



wards, 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, of the mantle, in 

 such a manner that it is constantly bathed by the water as it traverses that 

 cavity. The mantle and its envelope when exposed to the sun exhibit the 

 colours of the rainbow, and are so transparent, 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 corae out from its envelope without 

 appearing to suffer pain. 



These animals are very abundant in the Mediterranean and the warmer 

 portions of the ocean, and are frequently phosporescent. 



ASCIDI, Linnaeus. They ton of the Ancients. 



The mantle and its cartilaginous envelope, which is frequently very thick 

 resemble sacs every where closed, except at two orifices, which 

 correspond to the two tubes of several bivalves. The branchiae 

 form a large sac, at the bottom of which are the mouth and the 

 visceral mass. The envelope is much larger than the mouth, 

 which is fibrous and vascular, and on which, between the two 

 tubes, is one of the ganglions. These animals attach themselves 

 to rocks and other bodies, and are deprived of all power of loco- 

 motion ; the chief sign of vitality which they exhibit, consists in 



4MB| the absorption and evacuation of water through one of their 

 orifices; when alarmed they eject it to a considerable distance. 

 They abound in every sea, and some of them are eaten. 



Some species are remarkable for the long pedicle which supports them. 



FAMILY II. 



AGGREGATA. 



THE second family consists of animals more or less analogous to the Ascidia?, 

 but united in a common mass, so that they seem to communicate organically 

 with each other, and in this respect to connect the Mollusca with the Zoo- 

 phytes; but independently of their peculiar organization, these animals, ac- 

 cording to the observations of Messrs. Audouin and Milne Edwards, at first 

 live and swim separately, only becoming united at a certain subsequent period, 

 a fact which is in direct opposition to this opinion. 



BOTRYLLUS, Garner. 



An oval form, fixed on various bodies, and united by tens or twelves, like 

 the rays of a star. The branchial orifices are at the external extremities of 



