284 MANUAL OF ZOOLOGY sect. 



differing only in colour when the products are mature. Each is an un- 

 paired sac marked by a series of slight lateral constrictions. The larva 

 is a Trochosphere. 



All the Amphineura are marine. The Placophora (Chitons) occur 

 at all depths, thougn most abundant on the shores between tidal limits. 

 The shell-less forms (Aplacophora), on the other hand, are rare in very 

 shallow water, and absent altogether from the littoral zone; some have 

 been found at considerable depths (down to 1250 fathoms). The Placo- 

 phora are all vegetable feeders, their food consisting of minute algae 

 and diatoms. The Aplacophora subsist on small animals. The Placo- 

 phora, when at rest, adhere firmly to the surface of a rock or a block 

 of coral by means of the sucker-like foot. When forcibly detached the 

 animal curls itself up into a ball, and will only after a considerable time 

 slowly extend itself again. All their movements are extremely slug- 

 gish. 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. 

 They are distinguished by the possession of a shell of a 

 univalve character, consisting of a single piece, and by the 

 mantle not being developed into two lateral folds, as in the 

 Pelecypoda. There is a distinct head, bearing eyes and 

 tentacles. The body is inequilateral, and the foot is ven- 

 trally situated, forming a large creeping disc. 



If we look at a living Gastropod, such as a snail (Fig. 

 171) when fully extended, the want of symmetry appears 

 at first sight to be limited to the spiral shell, which is in 

 itself unsymmetrical, and is held obliquely, the head part 

 and the " tail " part appearing when superficially examined, 

 quite bilaterally symmetrical. But a closer examination, 

 especially after removal of the shell, shows that the depar- 



