FURTHER DEVELOPMENT OF THE LARVA AND METAMORPHOSIS. 11 



According- to Kowalevsky, the spines of the larval Chiton originate 

 within the cells — the cells, at those points where the spines arise 

 later, being richly vacuolated ; within these cells, the spicula are said 

 to appear as rudiments and only later break through the surface of 

 the body. The description of the embryonic development of the 

 spines given by Kowalevsky is not easily reconcilable with the 

 observations made by other zoologists on their formation in the adult 

 Chiton {e.g., those of Keincke, No. 11, and recently those of 

 Blumrich, No. 1), and other Amphineura (Proneomenia, Thiele, 

 No. 1). According to these observers, the spines arise as cuticular 

 secretions in depressions of the mantle-epithelium, the whole being- 

 covered by a thick cuticle (Figs. 6-8, r). 



In the mantle-epithelium there are papilla-like swellings (Fig. 6 A) in which 

 at a later period the formation of spines takes place. These papillae become 

 differentiated in the following way : broad cells appear at their bases, and to 

 these smaller cells become applied laterally (Fig. 6 B), the whole structure thus 

 assuming the character of an ectodermal depression. A special basal cell (bz\ 

 is said to be pre-eminently the formative cell of the spine, which is first to be 

 recognised as a small rounded structure within the papilla. It then increases 

 in size (Fig. 6 B), presses apart the cells of the papilla and passes out of the 

 latter (C and D). As the epithelial cells round the papilla continually secrete 

 cuticular substance, the spine is pressed upwards. The basal (formative) cell 

 from which it formerly rose is during this process drawn out like a thread 

 (Fig. 7 E), the spine for a long time remaining in connection with it. The 

 base of the spine, in contradistinction to the main calcareous shaft, consists of a 

 chitinous substance and surrounds the latter like a cup, the whole being now 

 far removed from the smaller papilla-cells. The latter, probably, give the 

 spine its characteristic markings. The formation of the spine is completed 

 by the secretion of a peg-like terminal knob by the formative cell, from which 

 the spine then detaches itself. The neighbouring cells may also secrete 

 another chitinous ring composed of several pieces round the base of the 

 actual spine. 



It is a remarkable fact that these spines somewhat resemble in their origin 

 the setae found in the Annelida in ectodermal depressions ; this has already 

 repeatedly been pointed out (Reincke, Semper, v. Jhering) and Hatschek 

 and Thiele have again recently laid great stress on this point. We shall refer 

 to it again later (Chapter xxxiv.). 



While the spines show a formation sui generis, the dorsal plates 

 are comparable with the shell-structures of other Molluscs. Their 

 position on the back of the larva corresponds to that of the shell- 

 gland in the larvae of the Lamellibranchia and (iastropoda (ef. Figs. 

 4 and 5 ; Figs. 56, p. 135, and 57, p. 137). As in these latter forms, 

 a cuticle first appears above the epithelium (Fig. 9, c) and, beneath 

 this cuticle, the calcareous substance is then secreted (Fig. 9, /•). 



