2l8 MOLLUSCA. 



In accordance with these considerations, the Lamdlibranchi- 

 ata are divided into two sections, according as respiratory 

 siphons are present or absent, and according to their nature 

 when they exist. 



SECTION A. ASIPHONIDA. Animal without respiratory si- 

 phons ; mantle-lobes free; the pallial line simple and not in- 

 dented (Integro-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 Chamidcz, Hippuritidce, 

 Tridacnidce, Cardiadce, Ludnidcs, Cycladida, and Cyprinidce. 



The second subdivision (Sinu^allialiq) is distinguished 



by the possession of long respiratory siphons, and a sinnated 



pallial line, and it comprises the families Veneridcs, Mactridce, 



Tellinida, Solenidce, Myaculce, Anatinidce, Gastrochcenidce, and 



Pholadidce. 



Besides the impressions left by the muscular border of the 

 mantle, and by the retractor muscles of the siphons, when 

 these are present, there are other impressions caused by the 

 insertion into the shell of the muscles by which the valves are 

 brought together the " adductor muscles." The number of 

 adductor muscles never exceeds two, but there may be only 

 one ; and in accordance with this distinction the Bivalves have 

 been divided into the two groups of the Dimyaria and Mono- 

 myaria. These divisions, however, are of small actual value. 

 In most Bivalves there are two adductor muscles passing be- 

 tween the inner surfaces of the valves, one being placed 

 anteriorly, in front of the mouth, whilst the other is situated 

 posteriorly, in the neighbourhood of the vent. In the Mono- 

 myary Bivalves it is the posterior adductor which remains, 

 and the anterior adductor is absent. The adductors leave 

 distinct "muscular impressions," or scars, in the interior of the 

 shell, so that it is easy in any given specimen to determine 

 where there was only one adductor, or whether two were pre- 

 sent (see fig. 1 68). 



The habits of the Lamellibranchiata are very various. Some, 

 such as the Oyster (Ostrea), and the Scallop (Pecteii), 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," 



