222 MOLLUSKS: HETEROl'ODS AND PTEROPODS. 



in an aquarium, where they are also very useful, devour, 

 ing the green confervje that grow on the glass. 



Fig. 416. — Heli- ^^^1 Fig. 418. — 

 cina. U. States. ^ Acicula. 



Fig. 417. — Great Fig. 419 — 



Clyclostoma. Britain. Tornatella. 



United States. Great Britain 



Fig. 420. — Bul- 

 la. U. States. 



Sea Slugs. 



These have no shells, and many of them only slightly 

 resemble the Gastropods before described. See Fig- 

 ures 421-425. 



Fig. 422. — Doris. Great Britain. 



Fig. 423. — Elysia. Fig. 424. — At- 

 Great Britain. lanta. South 



Atlantic. 



Fig. 421. — Eolis. Fig. 425. — Tritonia. 



Great Britain. Great Britain. 



HETEROPODS AND PTEROPODS. 



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

 immense swarms, miles in extent. Figures 426-428. 



