﻿BRACHIOPODA. 



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as applied to those animals, because they are in the first place inapplicable to the 

 character of the organism, and in the next they have never been universally employed in 

 the same sense. 



The vent of the animal of the Brachiopoda is said to be situated nearer to the umbo 

 than is the mouth, and on that account the distinctions called " anterior " and 

 " posterior " have been applied. The outer portion of the shell covers the arms and the 

 greater part of the mantle, while the viscera and vital organs are in the umbonal part of 

 the shell; and I think, therefore, that such terms as visceral region for the inflated 

 portion and brachial region for that which is occupied by the spiral " arm feet " would 

 better define the different positions of the animal, and could never be reversed or 

 misapplied. There is also great confusion, or, at least, want of unanimity, in the use of 

 the distinctive terms " ventral " and " dorsal " valves, the perforated one having 

 sometimes been called ventral and at others dorsal, and vice versa. Professor M'Coy 

 has proposed that the name " entering valve " and " receiving valve " should be given 

 to the two pieces, and Mr. Davidson further suggests that the perforated valve should 

 be called " dental valve," and the imperforated one the "socket valve." However, as 

 this author still distinguishes the two valves as "ventral" and " dorsal " I have followed 

 Jiis example. 



In the Bivalvia the distinctions are founded upon the hinge furniture, either on the 

 dental character, or upon the position of the connector, but in the Brachiopoda the 

 distinctions are made dependent upon the apophysary system ; viz. on the calcareous 

 internal appendage for the support of their spiral " arm feet." The muscular system of 

 the Brachiopoda also differs from that of the Bivalvia, inasmuch as the latter open their 

 shells, as well as close them, by means of muscles, of which they have as many as eight, 

 exhibiting a somewhat complex mechanical operation for the opening and closing of the 

 valves ; and this arrangement appears to me to be far less simple than that of the elastic 

 connector of the Bivalvia, and to effect the object in a far less easy way. Whether this 

 be a proof of inferior or superior organization I am not prepared to suggest, but it does 

 appear from it that the rule that nature adopts the simplest method for attaining a result 

 is not one of universal application. 



The valves of the Brachiopoda are, with some exceptions, more or less of an ovate 

 form, the longer axis being generally from the umbo to the opposite margin, and the 

 length of the shell is measured in that direction ; the breadth, therefore, being at right 

 angles to this line, and the depth having relation only to the tumidity of the valves. 

 This is not in accordance with the measurement of the Bivalves as adopted by myself, 

 but it is the mode that appears to be generally accepted, and I have, therefore, measured 

 my shells by that standard. 



