GENERAL CHARACTERS OF MOLLUSCS. 299 



and makes the calcareous shell which in single or double 

 form is characteristic of most Molluscs, except the modern 

 cuttlefishes and the land and sea slugs. 



A muscular protrusion of the ventral surface forms the 

 " foot," by aid of which most Molluscs move. 



The nervous system includes three chief pairs of nerve- 

 centres, the cerebral, the pleural, and the pedal ganglia, often 

 with accessory viscerals, etc. In the bivalves where the 

 head-region is undeveloped the ganglia are far apart; in 

 snails and cuttlefishes, they are concentrated in the head and 

 form a nervous mass around the gullet. 



The alimentary canal and its associated digestive gland, 

 often seem as if they were too big for the body ; in bivalves 

 the gut tends to be displaced ventrally and coils about in the 

 foot, in the others it tends to be displaced dorsally, often 

 protruding on the back as a visceral hump. 



Two larval stages are very common — a trochosphere 

 resembling that of some worms, and thereafter a more 

 characteristic " veliger." 



While there are many active swimmers among Molluscs, 

 the average habit is sluggish. 



For a diagrammatic summary of the chief anatomical 

 characters, see Ray Lankester's Schematic Archi-Mollusc, 

 a reconstruction of a possible ancestor. A simplified copy 

 of this is given in Diagram I. 



General Classification. — The frequent occurrence of a 

 trochosphere larva, the characters of the very simplest Gas- 

 teropods, and other facts support the readily-made sugges- 

 tion, that in the ages of which we have, no geological record. 

 Molluscs originated from some worm-type or other. But 

 this is very vague. 



It is certain, however, that the great mollusc branch must 

 have divided at a very early stage into two. One branch, 

 bent downwards, bears those forms which live sluggishly and 

 have undeveloped heads — the bivalves or Lamellibranchs. 

 The other branch bears more active forms, with well- 

 developed heads and raspers in their mouths, the Gastero- 

 pods or " snails," and the Cephalopods or " cuttles." The 

 simplest known Molluscs are ranked with Gasteropods, but 

 it is convenient to describe Lamellibranchs first. 



