6 Guide to the Invertelrata. 



MoUusca. All the Mollusca possess a ventral muscular tliickeniDg of 

 OALLEBT [)^Q body - wall known as the foot (variously developed and 

 modified), which serves as an organ of locomotion. The snail 

 crawls upon its foot ; the cockle, and other bivalves, can burrow 

 with their foot, and also transport themselves from place to place 

 under water. The Pteropods ("sea-moths") use the expanded 

 lobes, or wing-like fins of their foot, as swimming-organs. 



Fig. 14. — Cleodora pyramidata, lAzm. Atlantic. /,/,. expanded wing-like 

 lobes of foot, used by the Pteropods as swimming -ocgans. (Nat. size.) 



I. — Cephalopoda. 



The greatest modification is to be met with in the class 

 Cepualopoda, which is also the highest division of the MoUuscan 

 Bubkingdom — exemplified by the "Cuttle-fish" and the "Pearly 



Fio. 15. — Sepiola Hondeletii, Gesner. Torbay. (Nat. size.) 



Nautilus" — in which the forepart of the foot encircles the 

 mouth and is divided into a series of "tentacles," or feelers, 



