MOLLUSCA I LAMELLIBRANCHIATA. 255 



the body, its retractor-muscles usually leaving distinct impres- 

 sions, or scars (the ' pedal impressions ') in the interior of the 

 shell. In many, the foot subserves locomotion, but in the at- 

 tached bivalves it is rudimentary. In some such as the ordi- 

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

 secretes the tuft of silky threads ('byssus'), whereby the 

 shell is attached 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 bi- 

 valves with only one being called Monomyaria, whilst those 

 which possess two are termed Dimyaria. In most there are 

 two adductor muscles 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), habi- 

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

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

 Mussel (Mytdus), 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 My as, 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 (In- 

 tegro-pallialia) . 



This section comprises the families Ostreidce, Aviculidce, My- 

 tilidce, Arcades, Trigoniadce, and Unionidce. 



SECTION B. SIPHONIDA. Animal with respiratory siphons ; 

 mantle-lobes more or less united. 



Two subdivisions are comprised in this section. In the 

 first the siphons are short, and the pallial Line is simple (Integro- 

 pallialia) ; as is seen in the families Chamidw, Hippuritidce, 

 Tridacnidce, Gardiado&, Lucinidce, Cycladidce, and Cyprinidce. 



