SEA-SLUGS, HETEROPODS AND PTEROPODS. 



209 



Fig. 418. Heli- 

 cina. U. States. 



Fig. 422. Bui- 

 la. U. States. 



Fig. 420. 



Acicula. 



Fig. 419. Britain. Fig. 421. 

 Clyclostoma. Tornatella. 



United States. Britain. 



SEA-SLUGS. 



These have no shells, and many of them only slightly 

 resemble the Gasteropoda before described. See Fig- 

 ures 423-426. 



Fig. 425. Elysia. 

 Britain. 



Fig. 423. Eolis. 

 Britain. 



Fig. 427. At- 

 lanta. South 

 Atlantic. 



Fig. 426. Tritonia. 

 Britain. 



HETEROPODS AND PTEROPODS. 



These live in the open sea. Some of them move in 

 immense swarms, miles in extent. Figures 428-430. 

 They much resemble the young of ordinary sea-snails. 

 They form the principal food of the Right Whale. One 

 kind, the Clio, Figure 430, is said to have upon the 

 head three hundred and sixty thousand suckers! 



