136 A Guide to the Zoological Collections in the 



In this large and uniform Class, the body is compressed and 

 is enclosed in a bivalve shell. The two halves of the shell 

 are right and left, and are articulated across the back of 

 the animal by a spring-hinge formed by interlocking teeth 

 and elastic ligaments; they can, with few exceptions, 

 be widely separated along the free or ventral surface, but 

 can also be kept tightly closed by a special muscle or, 

 more commonly, pair of muscles — the " adductor muscles 

 of the shell " — which stretch across the upper part of the 

 Javity of the shell from one valve to the other. 



The shell as usual is secreted by the superficial layer 

 of the mantle, and the latter, therefore also consists of two 

 lobes which line and are more or less adherent to the 

 corresponding valves of the shell, the line of attachment 

 being known as the 'pallial line". 



Between the two mantle-lobes lies the body, which 

 consists of a dorsal mass containing the viscera, and of a 

 large muscular tongue-shaped or hatched-shaped "foot". 



Ihe space between the body and the mantle, on each 

 side, is the mantle-chamber : it contains the gills, which 

 have the form of delicate plates or leaves, instead of 

 plumes. There is a pair of gills on each side. Immediately 

 in front of the gills, on each side, is usually found a pair 

 of small flaps very similar in appearance to the gills, 

 from which they seem to have been derived : these are 

 the labial palps, and are used for drawing food towards 

 the mouth which lies between them in the middle line. 

 The lobes of the mantle are often, like the valves of 

 the shell, separate from one another, so as to gape when 

 the shell is opened \ but just as often they are more 

 or less united together beneath the foot — a chink, however, 

 being left for the protrusion of the latter organ — thus 

 closing the mantle-chamber below. When the mantle- 

 chamber is thus closed, the lobes of the mantle grow out 

 behind to form two retractile tubes, or "siphons", which 

 are sometimes of great length. These tubes are placed 

 one above the other, the lower one (inhalant, incurrent or 

 branchial siphon) admits water, containing air and the 



