The Seaside Book. 2409 



fringed round the edge with innumerable slender filaments, hangs from each valve of 

 the shell. This membranous envelope, which is called the mantle, exists, though un- 

 der many modifications, in all the Mollusca, and indeed is one of their most essential 

 parts. It is by means of this organ that all the shell-coated tribes cover themselves 

 with the beautiful shells which are objects of so general admiration. The thickened 

 margin of the mantle is furnished with glands which secrete both colouring matter 

 and carbonate of lime. From the latter material, deposited in cellular substance de- 

 rived from the animal, the shell is gradually formed by constant additions to its mar- 

 gin ; while the colouring matter, poured in at the same time, gives to the outer surface 

 all the peculiar markings which characterize each kind. The outer coat of the shell 

 is therefore entirely the work of the margin of the mantle. Its increase in thickness 

 is an after-process, effected by the general surface of this organ, which throws off 

 layers of pearly substance, and adds them continually, one after another, to the inner 

 surface of the shell. Thus as the shell increases in size, its walls grow in thickness. 

 In the scallop, among the fringing processes of the margin, are found a number of 

 glittering studs of metallic brilliancy, which are supposed to be eyes — and at least are 

 the only representative of those organs observed in the class, whose habits little require 

 such a provision. Within the mantle are found the branchiae or lungs, which consist 

 of four delicate leaves formed of radiating fibres of extreme fineness. The mouth is 

 a simple orifice, bordered by membranous lips, and placed at one end of the body, 

 between the two inner leaves of the branchiae. A. great portion of the body consists 

 of an extremely firm muscle, round which the stomach, liver, and other parts, are 

 disposed, and which connects the two valves of the shell together ; by its expansion 

 allowing them to open, and causing them to close by its contraction. This most pow- 

 erful muscle alone keeps the shell closed ; and its strength must be familiar to every 

 one who has opened an oyster, whose resistance to the knife ceases only when this 

 muscle is cut asunder. 



" Such are the general features of the more simple conchiferous animals, as the 

 scallop and oyster. If we examine the cockle, we shall find some modifications, and 

 the full development of a highly organized muscular foot. This organ exists but in a 

 rudimentary form in the scallop, whose habits suggest other modes of locomotion than 

 those of running and leaping. The scallop, which inhabits deep places, where it lies 

 on a rocky or shelly bottom, swims or flies through the water with great rapidity, 

 moving itself by suddenly opening and shutting the valves. In the cockle the first 

 difference which strikes us is, that the edges of the mantle are not open all round, as 

 in the scallop, but united together, at one side, into a short tube. On cutting a little 

 deeper we perceive that the shell is held together by two muscles, one placed on each 

 side of the central hinge. The hinge itself is differently formed, the ligament which 

 connects the valves being external, and the joint furnished with a nicely fitted appa- 

 ratus of tooth-like plates. On the whole, we have a higher type of structure, while 

 the development of a large muscular foot, capable of being either wholly retracted 

 within the shell or protruded to a considerable length, marks a new feature in the 

 animal, which at once suggests a difference in habits and destiny. That the differences 

 observed in the organization of the cockle, and of the allied genera, Mactra, Venus, 

 &c, and which are found in a still more advanced state in the Myae or gapers, and 

 the Solen or razor-shell, admirably fit them for the sphere of life for which they are 

 designed, is at once obvious when we consider these modifications of structure in re- 

 ference to the habitat of the animal. 



VII T 



