226 



CHAPTERS ON EVOLUTION. 



regarded, by the strict rules of comparative anatotny, as lower 

 organisms than the whelks and their relations. But if 'antiquity of 

 origin be esteemed in the Gasteropods, as it is in higher circles, a cri- 

 terion of respectability, then the chiton race may claim a superior rank 

 to many of their neighbours, and may maintain that when the univalve 

 race was but in the infancy of its development, they possessed a stable 

 and well-founded family connection. The chitons begin their fossil 

 history in the lower Silurian rocks, and appear at the present time 

 with but little variation from their past structure. They are, therefore, 

 unquestionably an ancient series of beings, which have most probably 

 sprung from a far back root-stock, whence the Gasteropods themselves, 

 and other molluscs likewise, may have branched off to become the 

 superior shelled races and tribes of to-day. What, then, is the course 

 of chiton development? As we should expect, it is much more 



primitive, much nearer 

 the type of the worms 

 and of Dentalium de- 

 velopment, than that of 

 other univalves. The 

 researches of Loven 

 have made us ac- 

 quainted with the early 

 history of the chiton 

 group. From the egg, 

 the infant chiton (Fig. 

 146, A) issues forth as an oval speck possessing a circle of cilia 

 surrounding its body near the front extremity, and likewise bearing 

 a tuft of cilia on its head, The likeness between the young chiton 

 and the young cockle (Fig. 142, A) is clearly traceable. An eye- 

 spot soon appears on each side of the ciliated 

 circlet, and the body next becomes annulated or 

 ringed in appearance (B), such an aspect remind- 

 ing one most forcibly of the young stages of the 

 worms (Fig. 157). Even when the young chiton 

 exists in this free swimming state, the segments 

 of the shell begin to appear (C), and correspond 

 with the rings into which the larval body is 

 divided ; whilst subsequently the broad " foot " 

 is developed, and the animal settles down into 

 a sedentary and placid existence on the rocks and stones of the 

 coast. Chiton development thus tells a tale of early origin, and of 

 alliance with the worm stock. In this respect it forms a worthy 

 companion to Dentalium itself. 



The development of the familiar pond snail (Lymneus), as studied 

 by Professor Ray Lankester and others, may render us acquainted 



FIG. 146. DEVELOPMENT OF CHITON (LOVEN). 



FIG. 147. POND SNAIL, 

 GASTRULA-STAGE. 



