408 THE ANATOMY OF INVERTEBRATED ANIMALS. 



The muscles which are attached to the valves, viz., the ad- 

 ductors, retractors of the foot, and pallial muscles, give rise 

 to impressions on the inner faces of the valves, which are 

 very obvious when the latter are removed and cleaned. With 

 the growth of the animal, the distance of these impressions 

 from the hinge-line and from one another is necessarily in- 

 creased, and it is not difficult in some cases (e. g., Anodonta) 

 to trace a faint triangular mark, which has its base in each 

 adductor impression and its apex in the umbo, and which in- 

 dicates the successive shiftings of position of the muscle. 



In some cases (e. g., Lima) a Lamellibranch may perform 

 a sort of aquatic flight by the flapping of the valves of its 

 shell. 



The hard and sharp-edged valves of the shell in* Teredo 

 are probably the agents by which the mollusk carves its pas- 

 sages through the wood which it inhabits. Whether the 

 valves of the shell of the Pholades and Saxicavce are the in- 

 struments by which they excavate their burrows in hard rock, 

 or whether, as has been suggested, the foot, armed with 

 sand, is the boring instrument, is a question which has been 

 much discussed, but hardly brought to a satisfactory decision. 



The haemal face of the body is either flat or slightly 

 arched, whence, in side view, the haemal contour is either 

 straight or convex. In most Lamellibranchs the body is 

 symmetrical in relation to the median plane, but, in those 

 whicli have inequivalve shells, as the Scallop (Pecten) and 

 the Oyster ( Ostrcea), the one half is more convex than the 

 other. No Lamellibranch has a distinct head ; but, in those 

 which possess two adductor muscles (e. g., Anodonta)^ the 

 region in which the anterior adductor lies and which is situ- 

 ated in front of the mouth may be distinguished as the pro- 

 soma, from the middle region (mesosoma) which gives rise to 

 the foot ; while the part which lies behind the foot and con- 

 tains the posterior adductor may be termed the metasoma. 



The foot may be rudimentary, but it is usually large, flex- 

 ible, and employed as an organ of locomotion. The posterior 

 face of the foot not uncommonly presents a gland which se- 

 cretes a chitinous, or shelly, substance the byssus. 



From the sides of the mesosoma, close to the attachment 

 of each mantle-lobe, the branchiae project into the pallia) 

 cavity. 



In its simplest form, the branchia of a Lamellibranch con- 

 sists of a stem fringed by a double series of filaments (e. g., 

 Nucula). The next degree of complication arises from these 



