FURTHER DEVELOPMENT OF THE LARVA AND METAMORPHOSIS. 13 



shows, in its youth, a remarkable resemblance to Chiton in the arrangement 

 of its broad, leaf-like dorsal spicula. The first appearance of the shell in the 

 Molluscs will be discussed further in Chapter xxxiv. 



The dorsal plates of Chiton consist of two layers, an upper layer which is 

 continued into the thick cuticle of the mantle, the so-called tegmentum, and 

 a subjacent calcareous layer, the articulamentum (Fig. 8, t and a). In tracing 

 the origin of the shell from spines, we should have to imagine the articula- 

 mentum as arising from the latter, which, as can easily be explained by their 

 origin, remained lying beneath the cuticle where they became expanded. The 

 cuticle above the calcareous plates which have thus arisen becomes the teg- 

 mentum of the shell. The retention of single modified spines or complexes of 

 spines, which is determined by the segmentation of the shell into separate 

 plates, would then be explicable by the manner of life of the animal, the body 

 of which, at first perhaps of considerable length, was able to roll up. 



The shell of Chiton is characterised not only by its segmenta- 

 tion, but by the presence within it of cellular strands. These are 

 the aesthetes (Fig. 8, ae) which may be simple or branched, and are 

 accordingly composed of a smaller or a larger number of long cells. 

 These sometimes occur in the tegmentum and extend from its outer 

 surface, where each is covered by a cuticular cap, to a cornice-like 

 fold of the mantle at the lateral margin of the shell (Fig. 8, gs). 

 These cell-strands have developed from the epithelial cells of the 

 mantle, which underwent great elongation during the secretion of 

 the cuticle through pressure of the surrounding cells. While that 

 part of the mantle to which they belong, and which secretes the 

 tegmentum, undergoes marked lateral displacement during the growth 

 of the shell, they, in consequence of their length, are able to retain 

 their primitive connection with the surface of the shell, and yet to 

 remain attached by their bases to the epithelium of the lateral 

 cornice of the mantle (Fig. 8). This latter epithelium is also partly 

 concerned in the secretion of calcareous shell-substance and, in this 

 way, the aesthetes appear to perforate the articulamentum which has 

 in reality been secreted round their bases. These structures have 

 been held to be sensory organs. They can hardly be tactile organs, 

 but they may perhaps serve other sensory purposes which are un- 

 known to us. The eyes discovered by MOSELEY (No. 11) on the 

 shell of a few exotic Chitones must doubtless be regarded as further 

 modification of the aesthetes. 



Figs. 5-9 show clearly that the shell of Chiton is, just like that of other 

 Molluscs, a cuticular structure. While the articulamentum, in consequence 

 of lying immediately above the epithelium, can readily increase in thickness, 

 the tegmentum, from its position, must grow chiefly at its margin. 



