284 MANUAL OF ZOOLOGY SECT. 



will only after a considerable time slowly extend itself again. All 

 their movements are extremely sluggish. The Aplacophora are unable 

 to fix themselves in this way ; many of them occur twined round the 

 stems of zoophytes, sometimes attached by a thread of viscid mucus. 



3. THE GASTROPODA 



The class Gastropoda comprises the Snails and Slugs, 

 Limpets, Whelks, Periwinkles, Sea-hares and the like. These 

 are distinguished by the possession of a univalve shell, i.e. a 

 shell consisting of a single piece, and by the mantle not 



oc.tenl 



lent 



FIG. 159. Helix nemoralis. an. anus; gen.ap. genital aperture; oc.tent. pos- 

 terior eye-bearing tentacles ; pulin. opening of pulmonary sac ; tent, anterior 

 tentacles. (After Pelseneer.) 



being developed into two lateral folds, as in the Pelecy- 

 poda. There is a distinct head, bearing eyes and tentacles. 

 The body is inequilateral, and the foot is ventrally situated. 

 If we look at a living Gastropod, such as a Snail, when 

 fully extended (Fig. 159), the want of symmetry appears at 

 first sight to be limited to the shell, which is in itself un- 

 symmetrical and is held obliquely, the head part and the 

 " tail " part of the animal appearing, when only super- 

 ficially examined, to be quite bilaterally symmetrical. But 

 a closer examination, especially after removal of the shell, 



