MOLLUSCA. 485 



so near a resemblance as to forbid a detailed description. 

 Some species are said to eject a coloured fluid, like the Aply- 

 sia, from the lid of the branchiae. A gland is observed in 

 the Sulla lignaria, similar to the Aplysia, in which it is 

 probable the fluid is prepared. 



The species of this genus have not been sufficiently in- 

 vestigated in a living state. When preserved in spirits, it 

 is impossible to form a correct idea of their true appear- 

 ance, as exhibited when alive in sea-water, since they usual- 

 ly exist as a shapeless mass. CUVIER has given delineations 

 of such preserved species, but they bear no resemblance to 

 the figures of MONTAGU, of the same species, taken from 

 living objects. 



M. LAMARCK is inclined to divide this genus into two, 

 distinguishing those which have the shell concealed, by the 

 term Bulltea, from such as have the shell in part exposed, 

 which he retains in the genus Sulla. The shells of the 

 genus BullfECi are thin and white, as B. aperta ; those of 

 BuUa stronger, more opake, and covered with an epidermis, 

 which, after the death of the animal, is easily detached, as 

 B. lignaria. 



51. DORIDIUM (of Mekel). Destitute of a dorsal plate 

 or shell. 



There is a cavity in the cloak, with a spiral turn. The 

 branchiae, and accompanying organs, are placed far behind. 

 There is here no appearance of a spinous tongue ; the gul- 

 let is simple, and the stomach is membranaceous. D. car- 

 nosum, a native of the Mediterranean, is the type of the 

 genus. 



Order II. -BRANCHIAE INTERNAL. 



The aerating organs are contained in a cavity, and ap- 

 pear in the form of sessile, pectinated ridges. 



