LIFE-HISTORY 



283 



have seen that the young of the crayfish escape the same danger 

 by holding on to the body of their mother. It is interesting to 

 note that marine relations of both these animals have free- 

 swimming larvae, and to recall that Hydra lacks the free larva 

 of the sea-dwelling Obelia. 



MOLLUSCA 



The snail and the swan mussel illustrate pretty well the general 

 features of the phylum Mollusca. Though coelomate, the animals 



v.h. J9 



IvJlm/. c. 



Fig. 211. — Diagrams of a mussel (A), a whelk (B), and a cuttlefish (C). 



after Lankester. 



-Partly 



a., Anus ; /., foot ; fun., funnel through which water is squirted by the cuttlefish in swunming ; g., gut ; 

 h., head ; ntl.c, mantle cavity ; sh., internal shell found in some cuttlefish (the " cuttle bone ") ; 

 ten., tentacles of the cuttlefish ; v.h., apex of visceral hump. 



of this phylum have usually a functional hsemocoele, the coelom 

 being reduced to the cavity of the gonads. They are the most 

 complex of unsegmented animals. There are three unique 

 features found in various forms in the different classes (Fig. 211), 

 but with little resemblance to anything found elsewhere : the 

 ventral and largely muscular foot, a dorsal visceral mass, and 

 covering this and hanging down from it to enclose a cavity 

 a special skin called the mantle. A shell is often but not invariably 

 secreted by this. The typical larva of the molluscs (not seen in 

 either of our examples) is a trochophore very similar to that of 

 the anneUds. Intracellular digestion is common. There are five 

 classes. 



CLASS I—AMPHINEURA 



These are primitive forms with a poorly developed head and 

 spicules in the mantle. Chiton (Fig. 212) is somewhat limpet-like, 

 but has eight transverse shell-plates, and is marine. 



