256 POLYPLACOPHORA. 



The dorsal surface behind the velum constitutes the mantle, 

 and becomes divided by six or seven transverse grooves into 

 segment-like areas, which may be called mantle plates (fig. 116 

 iv.). These areas would seem (?) to correspond to so many 

 flattened-out shell-glands. Immediately behind the velum the 

 eyes appear as two black spots (fig. 1 16 IV.). 



While the above external changes take place the archenteron 

 undergoes considerable modifications. Its anterior section gives 

 rise, according to Kowalevsky, to a dorsal (?) sack in which the 

 radula is formed ; while the liver arises from it as two lateral 

 diverticula. 



From the above statements it would appear that Kowalevsky holds that 

 the oesophagus and radula sack are both derived from the walls of the 

 archenteron and not from the stomodasum. Such an origin for these organs 

 is without parallel amongst Mollusca. 



The larva becomes about this time hatched, and after swim- 

 ming about for some time attaches itself by the foot, throws off 

 its larval organs, cilia, etc., and develops the shell. 



The shell appears first of all during larval life in the form of spicula on 

 the middle and sides of the head, and later on the middle and sides of the 

 post-oral mantle plates (fig. 116 v.). The permanent shell arises somewhat 

 later as a series of median and lateral calcareous plates, first of all on the 

 posterior part of the velar area, and subsequently on the mantle plates behind. 

 The three calcareous patches of each plate fuse together and give rise to the 

 permanent shell plates. The original spicula are displaced to the sides, 

 where they partly remain, and are partly replaced by new spicula. 



The nervous system is formed during larval life as four longitudinal 

 cords : two lateral the branchial cords, and two ventral the pedal. 

 Paired anterior thickenings of the pedal cords meet in front of the mouth to 

 form the cesophageal ring. The pedal cords and their derivatives are 

 believed by Kowalevsky to be developed from the lateral parts of the plate 

 formed by the metamorphosis of the blastopore. The median part of the 

 plate is still visible after the formation of these parts. 



The chief peculiarity of the larva of Chiton (apart from the 

 peculiar ventral plate) consists in the elongation and dorsal 

 segmentation of the posterior part of the body. The velum has 

 the normal situation and relation to its mouth. The position of 

 the eyes behind it is however abnormal. 



The elongation and segmentation of the posterior part of the 

 trunk is probably to be regarded as indicating that Chiton has 



