CONTENTS. 
J- 
Page. 
Introductory 41 1 
Shell 412 
Mantle 413 
Burrowing 414 
Regeneration and growth of shell 414 
Mantle gland 416 
Labial palps 417 
Gills 418 
Structure of the filaments 421 
Course of the circulation in the gills 422 
Respiratory current 424 
Food-bearing currents 423 
Circulatory system 425 
Arterial system 426 
Venous system 427 
Adductor muscles 428 
Retractor muscles of the foot 428 
Visceral mass 429 
Foot and byssus 429 
Kidney 429 
Digestive tract 431 
Nervous system 432 
Sense organs 435 
Otocysts 43 S 
Osphradium 436 
Summary 436 
Bibliography 437 
Explanation of plates 438 
410 
