212 POLYPLACOPHORA. 



The dorsal surface behind the velum constitutes the mantle, 

 and becomes divided by six or seven transverse grooves into seg- 

 ment-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. 116 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 stomodseum. Such an origin for these 

 organs is without parallel amongst Mollusca. 



The larva becomes about this time hatched, and after swimming 

 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 oil the middle and sides 

 of the post-oral mantle plates (rig. 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 pecu- 

 liar ventral plate) consists in the elongation and dorsal segmentation 

 of the posterior part of the body. The velum has the normal situ- 

 ation 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 early 

 branched off from the main group of the Odontophora along a special 

 line of its own, and not that the remaining Odontophora are de- 

 scended from Chiton-like ancestral forms. The shell of Mollusca on 

 this view is not to be derived from one of the plates of Chiton, but 

 the plates of Chiton are to be derived from the segmentation of 

 a primitive simple shell. The segmentation exhibited is of a kind 

 which all the trochosphere larval forms seem to have been capable of 

 acquiring. The bilateral symmetry of Chiton, which is quite as well 



