4 MANUAL OF THE MOLLUSCA. 



divided them into three classes, founded on the modifications of 

 their feet, or principal locomotive organs. 



1. The cuttle-fishes constitute the first class, and are termed 



Fig. 1.* Oral aspect of a Cephalopod. 



Cephalopoda,^ because their feet, or more properly arms, are 

 BO attached to the head as to form a circle round the mouth. 

 2. In the Qasteropoda,\ or snails, the under side of the body 



Fig. 2. A Gasteropod.^ Fig. 3. A Pteropod.|| 



forms a single muscular foot, on which the animals creep or 

 glide. 



homologous in the vertebrata and in the mollusca. When applied to the latter, the 

 terms are vague and indefinite in meaning. 



* Fig. 1. Loligo vulgaris. Lam. \. From a specimen taken off Tenby, by J. S. 

 Bowerbank, Esq. Tlie mandibles are seen in the centre, surrounded by the circular 

 lip, the buccal membrane (with two rows of small cups on its lobes), the eight sessile 

 arms, and the long pedunculated tentacles (<), with their enlarged extremities or cluba 

 (e). The dorsal arms are lettered d, the funnel/. 



t From cephale, the head, and poda, feet. See tlie frontispiece and pi. I. 



X Gaster, the under side of the body. 



§ Fig. 2. Helix desertorum, Forskal. From a living specimen in the British Museum, 

 March, 1850. 



I Fig. 3. Hyalaa tridentata, Lam., from Quoy and Gaimard. 



