298 MOLLUSCA [CH. 



foot or crawling surface which they all possess (Gr. yacrryp, the belly; 

 TTOIXS, TroSos, foot). The shell is typically composed of a single piece, 

 never of paired pieces ; and from this circumstance is derived the 

 general term "univalve" often applied to the Gastropoda hy 

 collectors ; in one small division of the class (the Isopleura) how- 

 ever the shell is represented by eight pieces placed one behind the 

 other in the middle line. 



Class II. LAMELLIBEANCHIATA = PELECYPODA. 



The characters mentioned at the end of the last section sharply 



separate the Gastropoda from another class of 



Mus!!h~ water Mollusca, the Larnellibranchiata or Pelecypoda, to 



which the common mussel and innumerable marine 



FIG. 136. Shell containing Anodonta mutabilis, and behind it the inner face 

 of an empty left shell. 



1 Points of insertion of the anterior retractor (above) and of the protractor 

 muscles (below) of the foot. 2. Point of insertion of the anterior adductor 

 muscle. 3. Point of insertion of the posterior retractor of the foot. 

 4. Point of insertion of the posterior adductor muscle. 5. Lines formed 

 by successive attachments of the mantle. 6. Urabo. 7. Dorsal siphon. 

 8. Ventral siphon. 9. Foot protruded. 10. Lines of growth. 



forms, such as the Oyster, Clam, Cockle, etc., belong. The Molluscs 

 belonging to this class have a shell composed of two similar pieces, 

 the right and left valves, united by a horny flexible piece, the hinge 

 (Fig. 136). The foot is typically formed like a wedge or axe-head 



