310 Animal Biology 



a series of calcareous plates (usually eight) on the dorsal side. Several pairs of 

 gill filaments may be present for respiration. 

 Example: Chiton (Fig. 115). 



Class 2 — Gastropoda (gas -trop' o da) (Gr. gaster, belly or stomach; pous, 

 foot). — The body is more or less spirally coiled with part of the digestive tract in 

 the muscular foot. Gastropods possess a distinct head, foot, and mantle cavity. 



Respiratory 

 aperture 



Velum 

 I 



A 



T ' 1 ' 



Gcnita/apertur. ^^^^^^ 



Mouth 



nespiratory aperture 



stalked eye 



Zdqz of vnantie 

 foot — :^^- — 



-Tenfcacfc 



' Mouth 



Genital aperture 



Fig. 116. — A, Fresh water snail {Lymnea sp.) ; B, land snail (Humboldtiana 

 sp.). Bodies are expanded from the shell. (From Potter: Textbook of Zoology, 

 TheC. V. Mosby Co.) 



The latter contains a mantle for respiration. The shell is in one piece and coiled. 

 In such types as the slugs the shell is absent. The shell is asymmetrical, but 

 the head and foot show bilateral symmetry. 



Examples: Snails (Figs. 116, 117, and 119), slugs, limpets, and whelks. 



