5G ZOOLOGY. 
including some with accessory pieces. From the laminated form of the 
gills, two of which usually hang like a curtain on each side, between the 
mantle and the body, they 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 Jabial 
palpr, the vibrille of which cause currents, which bring nourishment 
within reach. ae 
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 
muscular 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 dzgament, which may be either 
external or internal. 
Some conchifera remain permanently fixed, like the oyster, which is 
attached by its left valve. Others, as Penna (pl. 76, fig. 18), are attached 
by a bunch of fibres, named the byssus, which is secreted by the foot. 
Some can swim by alternately opening and closing the valves, of whieh 
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. See pl. 76, fig. 50, left-hand end. 
Posteriorly the mantle has two openings, one above the other, forming 
syphons for respiration and excretion. ‘These are sometimes made merely 
by the partial contact of the ends of the mantle, which may project but 
little, although at other times it extends and forms long perfect tubes 
(jig. 50). The inferior tube is named the branchial siphon, and is used in 
breathing; the upper one is the anal sephon, 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 valve 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 posterior 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 
“Jateral.” 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 
which 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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