12 



AMPHINEUKA. 



Each plate begins to form independently ; the calcareous secretion 

 takes place first at the anterior border of each segment, and proceeds 

 backwards from this point. 



The segmentation of the shell is a peculiarity of the Chitones and does not 

 occur in other Molluscs. Although the impression of segmentation is given 

 (Figs. 3 C, and 9), this term is not, strictly speaking, applicable here, since there 

 is no corresponding segmentation of the body. The shape of the shell is perhaps 

 rather due to its mode of origin. We have already seen that the spines are 

 cuticular structures, and this is of special interest since, in Molluscs as 

 primitive as the Solenogastres, they are the only hard structures of the integu- 





S«SS**\ c 



rt^^K, 



Fig. 8.— Portion of a transverse section through a Chiton (diagrammatic, chiefly after 

 Blumrich) : a, articulamentum ; ae, aesthetes ; c, mantle-cuticle ; &p, ectoderm 

 covering body and mantle, forming papillae in the mantle-region : /, foot ; gs, cornice- 

 like projection of the mantle ; k, gill, with the branchial artery cut through on its left 

 and the branchial vein on its right, between the two lies the lateral (visceral) nerve- 

 strand (n); Hi, body-cavity; m, mantle; st, spines; t, tegmentum, with another 

 thinner and specially differentiated cuticular layer above it. 



ment besides the cuticle. It has already been mentioned that the spines may- 

 assume a plate-like shape, and if we bear in mind the development of the 

 spines in such forms as very young Dondersia (cf. Fig. 10 C, p. 17), it appears 

 probable that the dorsal plates of the Chitones may have developed out of such 

 modified spines,* either through the broadening of individual spines or through 

 the fusion of several to form a single plate. In this respect Dondersia is of 

 special interest, for this form, in which actual dorsal plates do not occur, 



* The derivation of the dorsal plates of Chiton from its spines was attempted 

 by Gegenbaub (in his Grundriss der vergl. Anatomie) as early as 1878; his 

 views have recently been adopted by Blumrich, but Thiele assumes different 

 origins for the dorsal plates and the spines, and attempts to explain a part of 

 the former (the articulamentum) as an inner integumental skeleton. 



