50 ZOOLOGY 



foot, by the aid of which creeping movements are carried out. 

 So far as the internal organs are concerned, Mollusca are 

 remarkable for the absence of metamerism. Both primary 

 and secondary body-cavities are developed; but the former 

 includes most of the spaces which intervene between the skin 

 and the gut, whilst the latter forms the genital sacs and a 

 space called the pericardium which surrounds the heart. In 

 this last feature lies another contrast between Mollusca and 

 Arthropoda, for in the latter group there is also a space 

 called the pericardium surrounding the heart ; but this space 

 in Arthropoda is part of the primary body-cavity, and the 

 heart opens into it by open slits called ostia. Mollusca are 



FlG. 21. Side view of a living snail as an example 

 of the division of Mollusca called Gastropoda ; 

 /, foot the dotted line, as in Fig. 20, shows the 

 upper limit of the muscular portion ; m, mantle ; 

 o, opening between mantle and body ; s, shell. 



divided into groups according to the size and shape of what 

 we may term the dominating organs, i.e. the mantle, shell, 

 and foot. Thus in one large division the mantle is repre- 

 sented by two flaps situated to the right and left of the 

 animal's body, and the shell consists of stiff right and left 

 valves united by a median horny flexible piece termed the 

 hinge. This is so arranged that when it is uncompressed it 

 holds the two valves of the shell apart from one another \ but 

 when the animal is alive and is alarmed, it can withdraw the 

 foot and lower part of the body within the embrace of the 

 valves, and can then hold the two valves tightly pressed 

 together by the contraction of a powerful muscle. The foot 

 in this group is shaped like an axe or wedge, and the animal 

 when alive slowly ploughs its way through mud or gravel. 



