MOLLUSCS. 



197 



darkness, and from these all degrees of development may 

 be found to the extreme in the squid, where these organs 

 are scarcely inferior to those of vertebrates in structure. 



V 



For kidneys the molluscs have one or two organs 

 (nephridia) consisting of convoluted tubes opening at their 

 inner end into the pericardium and communicating with 

 the exterior at the other. 



In some the sexes are separate; in others, like our 

 land-snails, both sexes are united in the same individual. 

 Molluscs reproduce exclusively by eggs, and in some there 

 appears in development a trochophore-like larva which is 

 regarded as indicating that these animals are related to 

 the annelids (see p. 186). Molluscs are divided into five 

 groups or classes : Amphineura, Gasteropoda, Scaphopoda, 

 Acephala, and Cephalopoda. 



CLASS I. AMPHIXEURA. 



Setting aside a few rare forms 

 this division is represented by the 

 Chitons (fig. 36). These are dis- 

 tinguished from other molluscs by 

 many points of anatomy, while ex- 

 ternally they may be recognized by 

 having oval, flattened bodies covered 

 above by a shell composed of eight 

 transverse plates which overlap, from 

 in front backwards, like the shingles FIG. 36. Chiton squamosus. 



p A n j.i After Haller. 



on a root . All the species are marine. 



CLASS II. GASTEROPODA. 



The gasteropods receive their name from the fact that 

 the foot usually forms a large sole or creeping disc extend- 

 ing along the ventral side of the body. There is a distinct 



