MOLLUSCA : GASTEROPODA. 



413 



tion, and the branchiae (rarely absent, as in Limapontia and 

 Rhodope] are always placed externally on the back or sides of 

 the body. This section comprises the families Doridce (Sea- 

 lemons), Tritoniadcf) sEolidce, Phylltr/widce, and Elysiadce. 



Fig. 217. Scalaria Gran- 

 landica, a Holostoma- 

 tous Univalve. 



Fig. 218. Fusus tornatns, a Si- 

 phpnostomatous shell. Post- 

 Pliocene. 



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 ten- 

 tacles, which appear to be rather 

 connected with the sense of smell 

 than to be used as tactile organs ; 

 and behind the tentacles are gen- 

 erally two eyes. The nervous sys- 

 tem is extremely well developed, 

 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 flattened foot. 



ORDER III. NUCLEOBRANCHIATA or HETEROPODA. This 

 order is denned by the following characteristics : Animal pro- 

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

 effected by a fin-like tail or by a fan-shaped, -vertically-flattened, 

 ventral fin. 



F'fg. 219. Nudibranchiata. Doris 

 Johnstoni, one of the Sea-lemons. 



