&00 THE INHABITANTS OF THE SEA. 
ligament connects the two valves, and opens them wide as 
soon as the muscular contraction which closed them ceases 
to act. 
While the sea-snail creeps along upon a mighty foot, the 
bivalve is frequently doomed to a sedentary life, and the former 
protrudes from its shell a well-formed head, while the latter, 
like many a biped, has no head at all. The lamellibranchiate 
Acephala have, however, been treated by nature not quite so 
step-motherly as might be supposed from this deficiency, for 
many of them have eyes, or at least ocular spots, which enable 
them to distinguish light from darkness; and even auditory 
organs have been discovered in many of them. Their circu- 
lation is performed by a heart generally symmetrical, and their 
respiration by means of four branchial leaflets equal in size, 
and symmetrically arranged on either side of the body. The 
mouth is a simple orifice without any teeth, bordered by mem- 
branous lips, and placed at one end of the body between the 
two inner leaves of the branchie. The digestive apparatus 
consists of a stomach or intestine of different lengths, a liver, 
and several other accessory organs. A simple nervous system 
brings all the parts of the body into harmonious action. 
In many lamellibranchiates the folds of the mantle are dis- 
juined, as, for instance, in the oyster, which, on opening its shell, 
at once admits the water to its delicately fringed branchiz; in 
others they are more or less united, so as to form a closed sack 
with several openings, an anterior one (i) for the passage of the 
foot, and two posterior ones (g,/) for the ingress and egress of the 
water which the animal requires 
° for respiration. These posterior 
Che openings are often prolonged 
into shorter or longer tubes or 
siphons, sometimes separate, and 
sometimes grown together so 
as to form a single elongated 
fleshy mass. The use of these 
prolongations becomes at once 
apparent when we consider that 
Bivalve deprived of shell, to show ate they are chiefly developed in 
Sl ee those species which burrow in 
sand, mud, wood, or stone, and which therefore require to 
