56 ZOOLOGY. 



including some with accessory pieces. From the laminated form of the 

 gills, two of which nsnally hang like a curtain on each side, between the 

 mantle and the l)ody, thev are named Lamellibranchia by Blainville. 



In the Conchifera the back of the animal is under the hinge of the shell ; 

 and when the shell is removed, the heart may be observed in some families 

 beating at the anterior part of the back. Beneath this, at the anterior 

 * extremity, is a simple opening constituting the appendages named labial 

 'palpi., the vibrill[e of which cause currents, which bring nourishment 

 within reach. 



Upon comparing the shell of an oyster with that of a freshwater mussel, 

 a discolored impression -will be found near the centre of the former, and two 

 impressions in the latter, situated towards each end. These are the 

 inusciilar impressions^ serving for the attachment of the adductor muscles, 

 which draw the valves together ; and when they are relaxed the shell is 

 opened by the elasticity of the dorsal ligament^ which may be either 

 external or internal. 



Some conchifera remain permanently fixed, like the oyster, which is 

 attached by its left valve. Others, as Pinna {pi. T6, jig. 18), are attached 

 by a bimch of fibres, named the hgssus, which is secreted by the foot. 

 Some can swim by alternately opening and closing the valves, of which 

 Pecten {figs. 27-9) affords an example ; whilst others move by means of a 

 foot, which enables them to burrow in the mud, or move along in the sand. 

 The foot is situated below the mouth ; and when present, the mantle must 

 be open to allow it to pass, ^qq p>l. l^r^fig. 50, left-hand end. 



Posteriorly the mantle has two openings, one above the other, forming 

 siphons for respiration and excretion. These are sometimes made merely 

 by the partial contact of the ends of the mantle, which may project but 

 little, althouo-h at other times it extends and firms lono- perfect tubes 

 {fig. 50). The inferior tube is named the hranchial siphon, and is used in 

 breathing; the upper one is the anal sijyho?}, and serves for the excretions. 

 The curve in the pallial impression is caused by the displacement of the 

 mantle, to afford room for the retracted siphons when their size is 

 considerable. 



The hinge is usually provided with projections of calcareous matter, 

 named teeth from their shape ; and those of one vah-e are fitted into 

 corresponding vacancies or depressions in the other. These present so 

 many modifications that they have been made a principal character in the 

 construction of genera. Those situated under the beak of the shell are 

 termed cardinal teeth ; and those anterior and^^o5if<?nor to these the lateral 

 teeth, a badly selected term, all the teeth being equally lateral. Indeed, the 

 hump of a dromedary, or the dorsal fin of a fish, might as well be termed 

 " lateral." This misapplication arose when the length of a bivalve mollusc 

 from the mouth towards the vent was named its breadth, and Lamarck, Say, 

 and others, named that end " posterior" at which the mouth is situated, and 

 wMph precedes in locomotion. A few minutes devoted to observation and 

 dissection of the animal would have prevented this error, which must be 

 borne in mind in reading the descriptions of these authors, or they cannot 

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