xcvi Introduction. 



In having" their organs bilaterally symmetrical in relation to a 

 vertical antero-posterior plane, the Lamellibranchiata differ from 

 the Gasteropoda and Pteropoda, but not from the Cephalopoda. 

 Their bivalve shells differ from those of the Brachiopoda in being 

 placed one on either side right and left of the antero-posterior axis 

 of the body ; in being scarcely ever equilateral ; in being very fre- 

 quently equivalve, except as regards the hinge ; and in having the 

 hinge opened by the action of an elastic ligament, and closed by 

 that of one or two transversely-running adductor muscles. Their 

 foot is ordinarily compressed from side to side, so as to be hatchet- 

 or ploughshare- shaped. It may be rudimentary, and not rarely 

 secretes a ' byssus,' whereby the animal attaches itself to one spot. 

 It never developes an epipodium, nor presents the trifid division into 

 propodium, mesopodium, and metapodium. Movement is effected 

 ordinarily by means of the foot ; but in some instances, as Pecten, 

 by the alternate opening and shutting of the valves. Two or more 

 pairs of retractor, and one pair of protractor muscles, may be present 

 to act upon the foot and visceral mass from bilateral points of 

 attachment to the valves of the shell. In the absence of any pre- 

 hensile or manducatory organs, the Lamellibranchiata are dependent 

 for the ingestion of food upon the currents set up by the cilia 

 covering not only all their external organs, except the outer surfaces 

 of their mantle, but also lining their alimentary canal. The mouth 

 is provided with labial tentacles, which are homologous with the 

 arms of the Brachiopoda. 



The digestive tract has its anterior segments closely and inse- 

 parably connected with the visceral mass made up by the hepatic, 

 and in some cases by the reproductive coeca, after disengaging itself 

 from which it, with some exceptions {Ostrea, Anomia, and Teredo), 

 passes through the ventricle of the heart, before passing over the 

 main adductor muscle of the two valves to end in a cloacal 

 atriiun. The ventricle of the heart is, with the exception of 

 Area, single, whilst the auricles are, with the exception of 

 Aiiomia, bilaterally symmetrical relatively to a median vertical 

 plane, like the gills and the organs of Bojanus. Gills are uni- 

 versally present, and are in most cases two in number on each side. 

 They take ordinarily the shape of laterally-compressed multi-fene- 

 strated pouches, attached along the upper line of the mantle cavity, 

 much as the leaves of a book are attached to the interior of its 



