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. 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 œsophagus and radula sack are both derived from the walls of the archenteron and not from the stomodæum. 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 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 œsophageal 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 early branched off from the main group of the Odontophora along a special line of its own, and not that the remaining Odontophora are descended 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 marked as that of the Lamellibranchiata, indicates that it is a primitive phylum of the Odontophora.
Scaphopoda. The external characters of the peculiar larva of this interesting group have been fully worked out by Lacaze Duthiers (No. [286]).
The segmentation is unequal and conforms to the usual molluscan type. At its close the embryo becomes somewhat elongated, and there appears on its surface a series of transverse ciliated rings. As soon as these become formed the larva is hatched, and swims about by means of its cilia. Six ciliated bands are formed in all, and in addition a tuft of cilia is formed in a depression at the anterior extremity.