I. AMPIIINEURA 



315 



veliger arises from the velum, a strong circle of cilia, which surrounds a 

 velar field in front of the mouth, and which serves as a locomotor organ 

 for the larva. In some cases (fig. 314, B) it is lobed like the trochus of 

 a Rotifer. The veliger recalls the annelid trochophore and serves for the 

 distribution of the species; it is therefore of great importance for animals 



A B 



FIG. 314. Veliger stages, A, of a snail; B, of a Pteropod (from Gegenbaur). o, 

 shell; op, operculum; p, foot; t, tentacle; v, velum. 



which, like most molluscs, are sedentary or slow-moving. In cases with- 

 out metamorphosis (Cephalopoda, Pulmonata, etc.) the veliger stage 

 is frequently indicated during embryonic development by a ridge of 

 cells surrounding a preoral velar field 



Class I. Amphineura. 



These forms, some of which appear in the Silurian, are clearly the most 

 primitive of molluscs, and are distinguished by a marked bilateral sym- 

 metry. The nervous system already described (p. 313) consists of pleural 

 and pedal cords with scattered ganglion cells and no ganglia, these cords 

 being connected by numerous commissures (fig. 315, B) 



Sub Class I. Placophora (Chitonidce) . 



The chitons were formerly included among the gasteropods because 

 of the presence of a creeping foot and a radula. They are at a glance 

 distinguished from them by the rudimentary condition of the head (which 

 lacks tentacles and eyes), and the peculiar shell. This last consists of 

 eight transverse plates, overlapping like shingles which allows the animal 

 to roll into a ball. The edge of the mantle extends beyond the shell and 

 is covered with spines, while in the mantle cavity beneath are, right and 

 left, a series of ctenidia. Nerves enter the shell and end with noticeable 

 sense organs (aesthetes and, in some, eyes, fig. 316). There are no stato- 

 cysts. The symmetry of the body is also expressed in the viscera. 



