MOLl-USCA : GASTEROPODA. 



315 



Elystadte. 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 feyes. The ner- ^'s- m — Nudibranchiata, Doris 

 vous system is extremely well de- 5'">s«''"«,oneoftheS=a-iemo„. 



veloped, and would lead to the behef 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 HI. Nucleobranchiata or Heteropoda. — This 

 order is defitiied 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. 112. — Heteropoda. Carinaria cytHiium, p Proboscis; /Tentacles: ^Branchis* 

 * Shell; /Foot; <i 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 o£ the ordinary Gasteropods ;. the fin-like tail beipg 

 16 



