140 PRIMITIVE FORMS OF LIFE 



It remains to be seen how the molluscan type originated from 

 a transformation of a more easily explicable type. We have 

 already pointed out the transitional characters of the 

 Amphineura, of which the Chitons represent the common type, 

 and whose back is protected by eight calcified plates essentially 

 similar one to another, the shell-plates or "valves ", revealing a 

 segmentation of the body. It is the less possible to escape this 

 interpretation in that the valves are not a simple covering, 

 dead and calcareous like the shell, but are rather living 

 differentiations of the integument of which they form an 

 integral part, traversed by nerves, and the seat of sensitive 

 organs which may become eyes and which are repeated regularly 

 in the same place on all the valves. It is then quite clear that 

 the integument of the Chitons is segmented like that of the 

 Worms ; incomplete partitions even separate these segments 

 inside the body, where similar organs are encountered in all 

 segments thus divided. It must therefore be admitted that the 

 Oscabrians are closely akin to the Annelid Worms whose origin 

 we have already described. Their nervous system has been 

 studied in detail ; it is the nervous system of the Worm very 

 little modified, and this confirms our conclusion. The nervous 

 system of the Pleurotoma and the Fissurella has also 

 been carefully studied and described by Bouvier and Fischer. 1 

 With the exception of those portions correlated with the dorsal 

 cone or superposed portion of the t jdy, their nervous system 

 is identical with that of the Chitons. Cuvier has said : at 

 bottom, the nervous system is the animal ; the close relationship 

 of the Chitons and the diotocardiac Gasteropods, which 

 their external form does not suggest, is here quite patent. 

 This, then, is the link showing how the Molluscs deviated from 

 the Annelid Worms. The nervous system of Nautilus, studied 

 by Gravier, brings further support to the assertion, for it is 

 manifestly formed of two rings united in front, instead of two 

 longitudinal cords. It could, however, hardly be otherwise, 

 seeing that the ventral surface of the Cephalopods, which 

 corresponds to the feet of the Gasteropods, is reduced to the 

 space between the mouth and the anus, that is to say, the 

 periphery of the mouth itself. 



We come finally to the higher animals, to those in which 



1 XL VIII, 117-272. 



