222 



CHAPTERS ON EVOLUTION. 



mussels (Fig. 136), cockles, snails, whelks, and cuttlefishes the latter 

 existing at the head of the group presents us with one or two 

 typical examples of the truths and inferences of development. 

 There are at least four well-marked classes in the Mollusca, and 

 the names of these four groups may be placed before the reader 

 by way of enabling us to retain their distinctness clearly in mind. 



Thus, firstly, we find the class 

 Lamellibranchiata, or " bivalves," 

 represented by the oysters, cockles, 

 mussels (Fig. 136), clams, &c. Then 

 succeed the Gasteropoda, of which 

 the snails (Fig. 137), slugs (Fig. 

 138), limpets, whelks, chitons (Fig. 

 139), &c., are examples. The 

 Pteropoda form a small class, often 

 popularly named "sea-butterflies" 

 (Fig. 140), and of this group the 

 Clio and Hyalcza (C) may be selected 

 FIG. 139. CHITOKS. as representatives; whilst last and 



A. Upper surface, snowing the shell. i_ i ,_ .-t/^.jjj.J 



B. Under surface, showing head(/*) and foot (/). highest COme the Cephalopoda, OF 



cuttlefishes (Fig. 141), of which the 

 familiar octopus, the argonaut, and nautilus are examples. 



Such is the constitution of the Molluscan type of animals. When 

 we study the development of the three first-mentioned classes, we 

 are struck by the similarity they present in their early history. The 



FIG. 140. PTEROPODA. 

 A. Diagram of structure ; B. Cleodora ; C. Hyalaea. 



cuttlefishes, it may be mentioned, differ from the other groups in 

 development, and present us with an ancient and early specialised 

 group of beings whose early history and evolution is really a matter 

 of geological interest, and lies without the limits of the present chapter. 

 The early stages of a bivalve, such as a cockle, to select a familiar 

 member of the first of the classes just noted, exhibit the usual process 



