THE EVIDENCE FROM DEVELOPMENT. 233 



In so far as the life-histories at which we have just glanced are 

 concerned, the general conclusions to be drawn from their study lie 

 on the surface of the subject. Beginning with the worms and their 

 transformations, we find a type of larva, presenting a rounded body 

 with variously disposed cilia (Fig. 157, B), which simply becomes 

 segmented, and with little further change becomes the worm. From 

 the worms to the " Lampshells " is an easy transition, for in the 

 development of the latter (Fig. 154) we find the clearest reproduction 

 of the features of the young worm larva (Fig. 157) in the body divided 

 into its three segments and exhibiting its cilia and eye-spots ; whilst, 

 as Huxley remarks, the resemblance to the worm-larva is increased 

 when we find the young lampshell developing bundles of bristles 

 (Fig. 1 54, F, G), such as the worm possesses, on the middle joint of its 

 body. From such resemblances, Huxley is more than justified in 

 remarking that, whilst the lampshells bear a likeness in development 

 to the plant-like "Sea-mats" (Fig. 190) and their neighbours, their 

 development " no less strongly testifies to their close relations with the 

 worms." Thus the evolution of a race of lower shellfish from a 

 worm-stock is plainly enough taught by development ; and such a 

 fact testifies directly enough to the possibilities of other molluscan 

 developments having had a similar origin. 



Coming next in order to the Molluscs themselves, we find two 

 classes the bivalves and the gasteropods in each of which certain 

 primitive forms of development may be traced. The "Veliger- 

 stage" (Fig. 142, A) may be regarded as common to both groups ; and 

 the common origin for both classes may reasonably enough be 

 argued for and maintained on this ground alone, and apart from any 

 plain agreement in structure. It is, however, in the lowest members 

 of each group that we may expect to find the most marked likeness to 

 the primitive type and root-stock from which these classes have been 

 derived. Hence, it is to Chiton (Fig. 139) and Dentalium (Fig. 144) 

 that we turn for aid in solving the problem before us. The young 

 "Toothshell" (Fig. 145, C) is unmistakably a worm. Its barrel-shaped 

 body, its circlets of cilia, its end-tuft of these appendages all are 

 characters which reproduce before us the embryo worm (Fig. 157, B). 

 Nor is the early history of chiton materially different from that of 

 the "Toothshell." The young chiton (Fig. 146, A) leaves the egg, 

 as we have seen, with a ciliated girdle in the middle of its body, 

 and a long tuft of cilia on its head ; whilst this embryo seems to 

 proceed even further on the worm-track, when we find its body to 

 become segmented or divided as in the worms (B, C); these divisions 

 becoming the shell-plates of the mature Chiton. Thus Chiton may 

 be regarded, without exaggeration, as a worm-form existing under a 

 molluscan guise. And when we arrive at the higher gasteropods, with 

 their " veliger-stage " and " trochosphere," we see produced before us 



