472 THE INVERTEBRATA 



eight shell plates, their mantle cavity extended all round the foot 

 while instead of a single pair of ctenidia many such pairs arose. The 

 Gasteropoda remained as short creeping forms (Fig. 349 C); they are 

 characterized by the growth of the visceral hump dorsally, but 

 unequally so that it has coiled in a spiral (which is covered by a single 

 shell). This caused a readjustment of the visceral hump which has 

 revolved (usually to the right) on the rest of the body through 180° 

 (torsion) and the mantle cavity is now anterior. The Lamellibranchiata 

 (Fig. 349 D) are flattened from side to side, the whole body being 

 covered by two mantle lobes secreting two shell valves united by a 

 median hinge. The ctenidia inside the greatly enlarged mantle cavity 

 have developed into huge organs of automatic food collection and so the 

 head, rendered unnecessary and withdrawn into the mantle cavity, has 

 become vestigial. Similarly the foot has lost its creeping character 

 and has to be extended out between the valves to move the animal. 



In the Cephalopoda, though there is an unequal growth of the 

 visceral hump relative to the rest of the body, as in gasteropods, it is 

 coiled in a plane spiral, but there is no torsion, the mantle cavity re- 

 maining posterior. The primitive forms in the group (Fig. 377 A) have 

 an external shell which is divided into chambers, and those behind the 

 body chamber contain gas. This has had a great effect on the develop- 

 ment of the group , for by diminishing the specific gravity of the animals 

 it has enabled them to become more or less free-swimming. They 

 have tended, with the loss of the shell, to become more and more 

 efficient swimmers, and this is associated with the development of 

 their predatory habits. The anterior region shows a kind of trans- 

 formation new to the molluscs in its partial modification into circum- 

 oral prehensile tentacles for seizing food. Lastly, and in connection 

 with all these changes, the brain and sense organs have become 

 enormously developed and the cephalopods are seen to be one of 

 the most progressive groups of invertebrates. 



Characteristically the ectodermal epithelium of the mantle secretes 

 a shell in the Mollusca and in most of them the method of secretion 

 is the same. The original shell is laid down by the mantle of the 

 veliger larva (Fig. 350 B), but all extension takes place by secretion at 

 its edge (Fig. 353). The outer shell layer, periostracum, formed of 

 horny conchiolin, is first produced in a groove and then the prismatic 

 layer ^ largely consisting of calcite or arragonite, is secreted underneath 

 it by the cells of the thickened edge. The innermost nacreous layer 

 (also mostly CaCog) is, however, formed by the cells of the whole of 

 the mantle, and under such conditions as occur in the formation of 

 pearls this general epithelium is capable of secreting any of the three 

 shell layers. 



In the Mollusca the development of the trochosphere takes place in 



