PHYLUM MOLLUSC A 185 



In detail of structure, the members of the various classes 

 differ so greatly that morphology will be discussed separately 

 under the headings for the five classes. 



Class Amphineura 



The Amphineura seem to represent the most primitive group 

 of the Mollusca. The chitons are the typical representatives. 

 They are strictly marine, living at all depths but occurring 

 in special abundance in shallow water where they move freely 

 over the rocks by the action of the powerful foot. The head 

 region is not sharply set off. There are no tentacles and usually 

 no eyes. The eight transverse plates with which the dorsal 

 surface is covered overlap like the tiles on a roof. The arrange- 

 ment of the plates rather strongly suggests origin from segmented 

 ancestors, though present-day molluscs lack metamerism. The 

 plates are so articulated that the animal may roll into a ball. 

 Sometimes, they are completely covered by the mantle, but more 

 commonly the mantle only partially covers the plates and extends 

 beyond them along the sides of the body where it is covered 

 with spines. Mantle folds on the ventral surface produce a 

 series of small gills or ctenidia. Nerves penetrate the skeletal 

 plates and in the outer, less dense layer of each plate there are 

 frequently small sense organs called aesthetes and in some 

 instances eyes. The mouth and anus are located near the 

 anterior and posterior extremities of the body on the ventral 

 surface. The internal organs are disposed bilaterally along the 

 median plane marked off by the oral-anal axis. 



All representatives of this class are marine. The oval or 

 flattened body, which is bilaterally symmetrical, may or may 

 not have a differentiated head. The nervous system consists 

 of a pair of cerebral ganglia connected by a circumesophageal 

 ring and four longitudinal nerve cords which pass, two laterally 

 and two ventrally, through the body. There is no centralization 

 of the nerve cells in these cords to form ganglia. In many 

 instances, the longitudinal cords are connected by numerous 

 cross-commissures. 



Some amphineurans are simpler in structure than the chitons. 

 These wormlike forms without shells occur at fairly great 

 depths of the ocean where they burrow in the mud or sand or 

 are associated with various colonial coelenterates. The mantle 

 covers the body completely and bears calcareous spicules instead 



