254 MOLLUSCA. 



aperture is not tumid, and the internal lip is finely striated; the general form 

 of the shell is that of a cone, of which the spu-e forms the base. They in- 

 habit the rivers of the Antilles. 



ORDER II. 



NUDIBRANCHIATA. 



The Nudibranchiata have no shell whatever; neither are they fur- 

 nished with a pulmonary cavity, their branchiae being exposed on 

 some part of the back. They are all marine animals, frequently 

 swimming in a reversed position with the foot on the surface, con- 

 cave like a batteau, and employing the margin of their mantle and 

 their tentacula as oars. In the 



DoEis, Cuv. 

 The branchiae are arranged in a circle round an opening in the posterior 

 part of the back, under the form of little arbusculae, the whole resembling 

 a sort of flower. The mouth Is a small proboscis, situated under the ante- 

 rior margin of the mantle, and furnished with two little conical tentacula. 

 The species are numerous, and some of them large. They are found in 

 every sea, where their ova, resembling gelatinous bands, are diffused over 

 stones, sea-weeds, &c. 



PiocAMocEEos, Leuck. 

 The anterior margin of their mantie is ornamented with numerous branched 

 tentacula. 



POLYCEBA, Cuv. 

 The branchijc, as in Doris, on the hind part of the body, but more simple, 

 and followed by two membranous laminae, which cover them in moments of 

 danger; anterior to the claviform tentacula, similai* to those in Doris, are 

 four, and sometimes six others, simply pointed. 



Tkitonia, Cuv. 

 The body and superior tentacula, as in Doris; but the orifices through which 

 the peculiar liquid is excreted, are on the right: the branchiae, which re- 

 semble little trees, are arranged along the sides of the back, and the mouth, 

 provided with broad membranous lips, is armed inside with two homy and 

 trenchant lateral jaws, which may be compared to a pair of sheep-shears. 



