340 



CHARACTERS OF INVERTEBRATE ANIMALS 



side. From creatures of this kind we can imagine Cephalopods, 

 Gastropods, Lamellibranchs, and Scaphopods derived by special- 

 ization along different lines, the nature of which has been already 

 indicated in the description of these classes. The small number 

 of forms which make up the present class have probably retained 

 to a higher degree than any other living forms the characters 



B. 



Fig. 200. Proto-molluscs 



A, Mail-Shell (Chiton], seen from above and below. 

 B, Proneomenia, right side and under surface, c, Chaetoderma. 



of these hypothetical ancestors, though they also have undergone 

 modifications of their own, and it is often a difficult problem to 

 determine which of their characteristics are primitive and which 

 not. 



The most abundant and familiar of these animals are the 

 Mail-Shells or Chitons (fig. 200), most of which live under stones 

 near low-water mark. A common British form is Chiton mar- 

 ginatus. In the bilateral symmetry, the presence of a fairly well- 

 marked head, a broad, creeping foot, and a continuous mantle-flap, 

 the Chitons are probably primitive. The same thing may be 

 said regarding the dorsal heart, which is placed posteriorly and 

 possesses the three typical chambers, and the kidneys, which are 

 paired and open far back. The rasping-organ, however, is highly 

 complex, and the gills are arranged in a row on each side, instead 

 of being two in number. Quite possibly, however, the many- 



