388 PLEUROBRANCHHm NUDIBRANCHIATA DORID^. 



ally furnished with a shell, which is limpet-like, and covers the 

 back of the animal, but is generally more or less concealed by 

 the mantle. The gills are confined to one side of the body, and 

 placed between the margin of the mantle and the foot. These 

 animals feed on vegetable matters, and have an exceedingly com- 

 plicated stomach. The shell of one species is known as the 

 Chinese umbrella. The nearly allied PHYLLIDIID^E have no 

 shell, and the branchiae are placed on both sides of the body. 



SECTION B. NUDIBRANCHIATA. 



1005. The animals of this Section are found on all rocky 

 coasts, where they usually inhabit rather deep water, creeping 

 about upon sea-weeds, and occasionally swimming. The species 

 are numerous, and some of them attain considerable size ; the 

 Tethys fimbriata (Fig. 680), a native of the Mediterranean, at- 

 taining a length of one foot. The number of those existing on 

 our own shores is much greater than has been usually supposed ; 

 a large number of species having been discovered by recent in- 

 vestigations. These, many of which are exceedingly elegant, 

 are described and beautifully figured in the admirable monograph 

 of the British Nudibranchiata, by Messrs. Alder and Hancock, 

 lately published by the Ray Society. The section has been 

 divided into several families, characterised principally by peculi- 

 arities in the branchial organs. 



1006. The DORID^, or Sea-Lemons, are distinguished by 

 having the branchiae placed in a circular form at the hinder part 

 of the body, around the anal aperture (Fig. 678). The upper 



surface is formed by a broad mantle, 

 strengthened by the deposition of 

 calcareous particles ; the foot is com- 

 paratively narrow ; and near the 

 anterior margin of the mantle is a 

 FTG. GTS. DOR* ARGO. pair of tentacles, usually laminated 



and retractile, and often accompanied by filamentous prolonga- 

 tions of the mantle. These animals are found on most shores 



