388 MOLLUSC A. 



like ancestor, an unsegmented form with a flat creeping 

 ventral surface. 



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

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

 bears those forms which live sluggishly, and have undeveloped 

 heads and no odontophore — the bivalves or Lamellibranchs. 

 The other branch bears more active forms, in which the 

 head is well developed, and the characteristic radula is 

 present in the mouth — the primitive Amphineura, the 

 Gasteropods or snails, and the Cephalopods or cuttles. 



Phylum MoUusca. 



Branch Glossophora 

 (Syn. Odontophora). 



Branch Lipocephala 

 (Syn. Acephala). 



Class Lamellibranchiata (Syn. Pelecy- 

 poda, Conchifera or Bivalves) — cockle 

 and mussel, clam and oyster. 



Ceph"i?poda{^»"'^''-- ^'"'^ cuttle-fish. 



Class / Asymmetrical snails, 

 Gasteropoda \ whelks, &c. 



{More primitive, hilater- 

 ally symmetrical forms, 

 e.g^., Chiton. 



The position of the small class Scaphopoda is uncertain. Some place it between 

 Gasteropods and Cephalopods. 



Most Molluscs live in the sea from the shore to the great 

 depths, but there are many fresh water Gasteropods and 

 bivalves, and the terrestrial snails and slugs are legion. 



The bivalves feed on microscopic animals and organic 

 debris ; the Gasteropods are carnivorous or vegetarian ; the 

 Cephalopods are voracious flesh eaters. 



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. 



The vascular system in Molluscs always communicates 

 freely with the lacunar spaces which constitute the apparent 

 body cavity. These are never lined with epithelium, and 

 are of secondary origin. A true epithelium, however, lines 

 the pericardium and the cavity of the reproductive organs, 

 which are both coelomic in origin. 



