oOO 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 branchia3. 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- 

 joined, as, for instance, in the oyster, which, on opening its shell, 

 at once admits the water to its delicately fringed branchiae ; in 

 others they are more or less united, so as to form a closed sack 

 with several openings, an anterior one (h) for the passage of the 

 foot, and two posterior ones (g,f) for the ingress and egress of the 



water which the animal requires 

 for respiration. These posterior 

 openings are often prolonged 

 into shorter or longer tubes or 

 siphons, sometimes sepasate, 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 its they are chiefly developed in 



various openings. . , . i * i 1 



those species which burrow m 

 sand, mud, wood, or stone, and which therefore require to 



