MOLLUSCA : LAMELLIBRANCHIATA. 283 



It is essentially a muscular organ, developed upon the ventral 

 surface of the body, its retractor-muscles usually leaving dis- 

 tinct impressions or scars (the "pedal impressions") in the 

 interior of the shell. In many the foot subserves locomotion, 

 but in the attached bivalves it is rudimentary, and in others (as 

 in the Scallops) locomotion is effected by the alternate opening 

 and closure of the valves. In some such as the ordinary 

 Mussel the foot is subsidiary to a special gland, which secretes 

 the tuft of silky threads ("byssus") whereby the shell is at- 

 tached to foreign objects. This gland secretes a viscous 

 material, which the foot moulds into threads. 



The valves of the shell are brought together by one or two 

 muscles, which are called the " adductor muscles " those 

 bivalves with only one being called Monomyaria, whilst those 

 which possess two are termed Dimyaria. In most there are 

 two adductor muscles (fig. 99, a a) passing between the inner 

 surfaces of the valves, one being placed anteriorly in front of 

 the mouth, the other posteriorly on the neural side of the intes- 

 tine. In the monomyary bivalves the posterior adductor is 

 the one which remains, and the anterior adductor is absent. 

 The adductors leave distinct " muscular impressions " in the 

 interior of the shell, so that it is easy to determine whether 

 there has been one only in any given specimen, or whether 

 two were present. 



The habits of the Lamellibranchiata are very various. Some, 

 such as the Oyster (Ostrea), and the Scallop (Pecten\ habit- 

 ually lie on one side, the lower valve being the deepest, and 

 the foot being wanting, or rudimentary. Others, such as the 

 Mussel (Mytilus), and the Pinna, are attached to some foreign 

 object by an apparatus of threads, which is called the "byssus," 

 and is secreted by a special gland. Others are fixed to some 

 solid body by the substance of one of the valves. Many, such 

 as the Myas, spend their existence sunk in the sand of the sea- 

 shore or in the mud of estuaries. Others, as the Pholades 

 and Lithodomi) bore holes in rock or wood, in which they live. 

 Finally, many are permanently free and locomotive. 



The Lamellibranchiata are divided into two sections, accord- 

 ing as respiratory siphons are absent or present, as follows : 



SECTION A. ASIPHONIDA. Animal without respiratory 

 siphons; mantle-lobes free; the pallial line simple and not 

 indented (Integro-pallialia). 



This section comprises the families Ostreida, Aviculidce, My- 

 tilidce, Arcadcz, Trigoniadce, and Unionidce. 



SECTION B. SIPHONIDA. Animal with respiratory siphons ; 

 mantle-lobes more or less united. 



