276 CLASSIFICATION OF ANIMALS 



sists of three ^ranglia called the cerebral, pedal and visceral 

 ganglia. 'J^hese are connected by paired nerves. The coelomic 

 cavity is almost obliterated by a mesenchymatous parenchyma. 

 A pair of nephridia connect the remnant of the body-cavity 

 with the exterior. There is also a pair of gonads. The body 

 is fundamentally bilaterally symmetrical but in one large group 

 a twisting of the visceral mass results in more or less asymmetry. 

 The mollusca are primarily aquatic animals but a large number 

 have become adapted to a terrestrial life. 



593. Class I. Amphineura. — This is a small class and only 

 one example need be described. Chiton is bilaterally sym- 

 metrical and flattened dorso-ventrally. There is a partially 

 differentiated head but the mantle fold includes the head 

 as well as the body. On its dorsal surface the mantle forms a 

 single series of eight plates. The foot is very broad and mus- 

 cular and is used for locomotion and as a sucking disc for a 

 hold-fast. In the groove between the mantle and the side of 

 the body is a series of ctenidia, or comb-like gills. The digestive 

 tract consists of a mouth cavity with radula and a pair of 

 ''salivary" glands, an oesophagus, stomach with a pair of 

 digestive glands and a coiled intestine. The vent is at the 

 posterior end, opening into the mantle cavity. The nervous 

 system consists of an oesophageal ring or nerve collar and two 

 pairs of longitudinal nerves, one ventral and one lateral. Be- 

 sides, there are several smaller ganglia and numerous connectives 

 between the longitudinal nerves. There is a pair of nephridia 

 and a double gonad with paired ducts. All the Amphineura 

 are marine. 



594. Class n. Conchifera. — This group is distinguished 

 from the preceding by the fact that the mantle fold does not 

 include the head and by the way in which the shell is formed. 

 The latter consists of numerous spine-like pieces in the Amphi- 

 neura while in the Conchifera it is formed in layers as a single 

 structure. 



