Mollusca, 3 



The body is never divided into joints or segments, as in insects, Mollusca. 

 crustaceans, and worms. There are typically three pairs of^ALLEEIBS 



Fig. 6.- 



■Thracia phaseoUna, Lamk., with its animal, s, the incurrent and 

 excurrent siphons marked by arrows ; /, the foot. 



ganglia, or nerve-centres, symmetrically disposed, but the nerves of 

 the visceral loop are much distorted, and usually asymmetrical — at 

 least, in the univalves ; but in the bivalves there is a greater 

 approach to bilateral symmetry. There is always an alimentary 



Fig. 7. — Nervous system oi Paludina 

 or Viviparus, a pond-snail. 



Fig. 8. — Nerve-ganglia and cords 

 of Anodonta, a river-mussel. 



canal and, with one doubtful exception,^ a heart, and more or less 

 distinct respiratory organs. 



The Mollusca are divided into two branches — one in which the 

 head is well developed, and the other in which it is atrophied. 

 The former division is now generally known as the Glossophora 



The genus Dentalium. 



