246 The Study of Animal Life PART m 



a sign of concentration. There is a {cerebral} pair with nerves 

 which supply the head region, another (pleural) pair with nerves to 

 the sides and viscera, a third (pedal] pair whose nerves govern the 

 foot, and often other accessory centres of which the most important 

 are visceral. In the somewhat primitive eight-shelled Chiton and its 

 neighbours, the nervous system is the most readily harmonised with 

 that of other Invertebrates ; in bivalves the three pairs of centres 

 are far apart ; in most snails and in cuttlefish the three are con- 

 centrated in the head-regions, and it is those forms with concentrated 

 ganglia which show some signs of cleverness and emotion. 



Life -History. Most molluscs pass through two larval stages 

 before they acquire their characteristic adult appearance. The 

 first is interesting because it is virtually the same as the young 

 stage of many "worms." It is a barrel - shaped or pear -like 

 embryo with a ring of locomotor cilia in front of the mouth, and is 

 known as a Trochosphere. 



After a while this changes into a more characteristic form called 

 the Veliger. It bears on its head a ciliated cushion or velum often 

 produced into lobes ; the body has a ventral " foot" and a dorsal 

 "shell-gland." In aquatic Gasteropods the visceral hump begins 

 to appear at this stage. 



The eggs of cuttlefish differ from those of other molluscs in their 

 rich supply of yolk, which serves for a prolonged period as capital 

 for the young, and the two larval stages noticed above are skipped 

 over. There are other interesting modifications in the life-history 

 of terrestrial and freshwater forms, witness the little larvae of the 

 freshwater mussel which are kept within the gills of the mother till 

 the approach of some sticklebacks or other fish, to which the 

 liberated young then fix themselves. 



History. The shells of molluscs are well preserved in the rocks, 

 and palaeontologists have been very successful in tracing long series. 



The chief types are all represented in the Cambrian strata a 

 fact which forcibly suggests the immensity of yet earlier ages. 

 They are abundant from the Silurian onwards. The snails have 

 gone on increasing, and are now more abundant than ever ; the 

 bivalves cannot be said to have diminished, but the Cephalo- 

 pod tribe has dwindled. Of the Nautiloid type of Cephalopods, 

 of which there are crowds of fossil forms, only the pearly Nautilus 

 now survives, and though there are many kinds of naked cuttle- 

 fish in our seas there are ten times as many shelled ancestors 

 in the rocks. The geological record confirms what we should 

 otherwise expect, that the lung-breathing snails and the freshwater 

 bivalves were somewhat late in appearing. 



Prof. Ray Lankester has reconstructed an ideal ancestor which 

 combines the various molluscan characteristics in a satisfactory 



