FERA.
The mollusks included in the class of Conchifera present peculiar characters which prevent their being confounded in any point of the series with the other classes of the same sub-kingdom. They are all contained within a bivalve shell, generally articulated after the manner of a hinge; to this shell the animal is attached by one or several muscles, and the shell itself is secreted by a fleshy envelope, generally thin, but having the edge thickened, to which naturalists agree in giv ing the name of mantle. The animal, of a structure more simple than other mollusks, has no head; the mouth is pierced at the anterior extremity and is the entrance to organs of di gestion, consisting of a stomach, an intestine of different lengths, an anus, and an organ for secreting bile. Circulation is performed by means of a heart generally symmetrical, the ventricle of which surrounds the rectum. Respiration is effected by means of four bran chial leaflets, equal in size and symmetrical, arranged on either side of the body. Gene ration is simple; the Conchifera are endowed with hermaphrodism adequate to the continu ation of the species ; every individual has an ovary included among the general mass of the viscera. The nervous system does not form a complete ring around the cesophagus; ganglia are found towards the anterior and posterior parts of the animal, and lateral and very long filaments form a ring within which the visceral mass is included.
Before entering upon the more particular description of the organs which have just been mentioned, it is essential as a preliminary to institute some order among the members of the class Conchifem, to throw them into a few grand divisions by which the labour of de scription, in many particulars, will be very much abridged.
Lamarck divided the Conchifera into two grand orders, Dimyaria and Monomyaria. We are of opinion that this division may be preserved with some slight modifications ; and, farther, that it is necessary to establish a third order equal in importance to the two others, and including the Brachiopoda. The ana tomical inquiries of Cuvier, and those, still more recent in their date, of Mr. Owen into the structure of the Brachiopoda will not allow us any longer to regard these animals as per taining to the family of monomyary Conchi fers. These inquiries also prove that Cuvier,
in forming the Brachiopoda into a particular class of Mollusea, disjoined them in too great a degree from their congeners. It is from re garding both of these views as carried too far, that we have been led to propose a new divi sion which to us appears to be called for, and to be preferable to either of the others; this is to restore the Brachiopoda to the type of pro per Conchifera, and to establish a third order of this family for their especial reception, to which the title of Polymyaria might be given. Instead of placing this order at the end of the Conchifera, however, it appears better to set it at the head, especially if the analytic method of Lamarck be adopted as the basis of the classification. The Conchifera we should, then, propose to arrange in the following order : The organization of the Brachiopoda being more simple than that of the other Conchifera, renders it proper to place this order at the be ginning of the class. The Diinyaria having an oroanization somewhat less complex than the Mbonomyaria constitute an intermediate order, which is the most numerous of the three; the Monomyaria terminate the series.
To facilitate tbe comprehension of the brief descriptions which we shall give of the dif ferent parts of the Conchifera, it seems neces sary to state precisely the position in which the animal must be placed in order to be suit ably observed. The animal, then, is supposed to be walking before the observer, included within six planes to which its different parts are referred. The head or the oral aperture indicates the anterior extremity of the creature. This extremity is directed forwards, its pos terior extremity backwards. The back cor responds to the superior plane ; the belly and foot correspond to the inferior plane, and the flanks of the animal to the lateral planes, one of which is to the right, the other to the left. The two accompanying figures (fig. 345) will suffice to give an idea of the relations of one of these animals to the different planes within which it is supposed to be included.