240 



LIFE, IX ITS INTERMEDIATE FORMS. 



cept that there is an opening in the side for the protru- 

 sion of a moveable fleshy organ called the foot ; but ex- 

 ternally there is this peculiarity, that, in the substance of 

 the coat, there are formed two valves or convex plates of 

 stony substance (shell), which are united along one side 

 by a hinge, and enclose the soft parts, as the leaves of a 

 book are contained within the covers. 



This, then, is a bivalve shell-Jlsh, as it is commonly 

 (though incorrectly) named, and it is the representative 

 of an extensive Class of animals, found all over the world, 

 in fresh and salt waters, and designated Coxchifep.a, or 

 u Shell-bearers." There is, it is true, very great diversity 

 in the details of form and structure that we find in this 

 immense assemblage of animals ; but all these variations 

 can be easily traced by insensible gradation to this primal 

 form, and thence to those lower types which we have 

 already described. Sometimes the orifices for receiving 

 and discharging water are prolonged into two distinct 

 tubes with fringed extremities, as in that lovely and deli- 

 cate shell that inhabits our sandy beaches, called, from 

 its diverging rays of pink and yellow, the Setting Sun 

 (JPsammobia vespertina), where the two tubes, when fully 

 extended, are twice the length of the shell. At other 

 times we find the tubes again reduced to simple openings, 

 and one of these forming a mere slit, scarcely to be dis- 

 tinguished from the common opening of the coat or mantle, 

 as in the Mussels (Mytilidce). This separation of the 

 mantle, again, occurs in various degrees, from its condi- 

 tion as a mere orifice for the protrusion of the foot, to 

 that of the Oysters (O&treada), where it is open all round, 

 a fleshy counterpart of the shelly valves, bordered by a 



