MOLLUSCA 545 



the cavity of the gonads and at the sides the two coelomoducts 

 ("kidneys"). In the nervous system there were as in annehds and 

 arthropods, a circumoesophageal commissure or brain which may or 

 may not have been ganglionated, ventral pedal cords, a visceral 

 commissure coming from the pleural part of the brain, and a pallial 

 commissure in the mantle edge. From this beginning diverged the 

 different groups which we know to-day. The chitons (Amphineura), 

 which have departed least from the ancestral structure, became elon- 

 gated but limpet-like forms (Fig. 373 B), their visceral hump being 

 protected by 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. 373 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. 373 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 the head, rendered unnecessary and withdrawn into the mantle cavity, 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 flat sole 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. 402 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. 374 B), but all extension takes place by secretion at 



