356 



MOLL USCA. 



mantle, shell, or head, which occur in large groups of molluscs, 

 is not a primitive condition, but can only be explained by degen- 

 eration. The name veliger arises from the velum, a strong circle 

 of cilia, which surrounds a frontal or velar field in front of the 

 mouth, and which serves as a locomotor organ for the larva. In 

 some cases (fig. 343 B} it is lobed like the trochus of a Rotifer. 



Fia. 343. Veliger stages. A, of a snail; B, of a Pteropod. (From Gegenbaur.) 

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



The veliger recalls the annelid trochophore and serves for the 

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

 animals which, like most molluscs, are sedentary or slow-moving. 

 In cases without metamorphosis (Cephalopoda, Pulmonata, etc.) 

 the veliger stage is frequently indicated during embryonic devel- 

 opment 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 symmetry. The nervous system already described (p. 

 354) consists of pleural and pedal cords with scattered ganglion 

 cells and no ganglia, these cords being connected by numerous 

 commissures (fig. 344, B}. 



Sub Class I. PlacopJiora (CMtonidce). 



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 con- 

 dition of the head and the shell. This last is unique among mol- 

 luscs; it consists of eight transverse plates overlapping like shingles, 

 which allows the animal to roll itself into a ball. The edge of the 



