MOLLUSCA : GASTEROPODA. 



Elysiadce. Specimens of the Sea-slugs and Sea-lemons may at 

 any time be found creeping about on sea-weeds, or attached 

 to the under surface of stones at 

 low water. The head is furnished 

 with tentacles, which appear to be 

 rather connected with the sense of 

 smell than to be used as tactile 

 organs ; and behind the tentacles 

 are generally two eyes. The ner- Fig. i 39 . Nudibranchiata. Doris 

 vous system is extremely well de- ***"*' on 

 veloped, and would lead to the belief that the Nudibranchs 

 are amongst the highest of the Gasteropoda. Locomotion is 

 effected, as in the true Slugs, by creeping about on the flat- 

 tened foot. 



ORDER III. NUCLEOBRANCHIATA or HETEROPODA. This 

 order is denned by the following characteristics: Animal 

 provided with a shell, or not, free-swimming and pelagic ; 

 locomotion effected by a fin-like tail, or by a fan-shaped, ver- 

 tically flattened, ventral fin. 



Fig. 140. Heteropoda. Carinariacymbiuin. ^Proboscis; /Tentacles; $ Branchiae ; 

 j Shell ; f Foot ; d Disc. (After Woodward). 



The Heteropoda are pelagic in their habits, and are found 

 swimming at the surface of the sea. They are to be regarded 

 as the most highly organised of all the Gasteropoda, at the 

 same time that they are not the most typical members of the 

 class. Some of them can retire completely within their shells, 

 closing them with an operculum ; but most have large bodies, 

 and the shell is either small or entirely wanting. They swim 

 by means of a flattened ventral fin, or by an elongated tail, 

 and adhere at pleasure to sea-weed by a small sucker situated 

 on the side of the fin. These organs are merely modifications 

 of the foot of the ordinary Gasteropods ; the fin-like tail being 



Y 



