MOLLUSC A 325 



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 which is possessed by the sea-dwelling 

 Obelia. 



Soft-bodied, shelled, unsegmented, ccelomate animals with 

 Moil a mantle, foot, and nervous system like those 



of the swan mussel are known 'as Mollusca or 

 true Shellfish. Snails and cuttlefish belong to this group. 

 The body of a snail is flattened, not from side to side as in a 

 mussel, but from above downwards, and the visceral hump is 

 twisted. The mantle cavity is represented by a deep sack, 

 containing the anus, which the twisting causes to lie over 

 the back. In the snail, but not in the related whelks, this 

 sack is converted into a lung, and the gills are lost. In a 

 cuttlefish the body is flattened from before backwards, and 

 the foot forms a funnel, the squirting of water through 

 which from the mantle cavity causes the animal to move in 

 the opposite direction. The sucker-bearing tentacles which 

 surround the mouth are said also to represent part of the 

 foot. There are two feather-like gills; and an ink-gland, 

 which opens with the rectum into the mantle cavity, enables 

 the animal to cloud the water behind it in escaping from 

 its foes. Snails and cuttlefish have heads, with eyes and 

 a rough tongue or radula, which are wanting in mussels. 



