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quainted with the arrangement of the valves in Lamellibranchiata 

 before they examine the Brachiopoda, so that they come to consider 

 the latter order with a vague impression that all bivalves must have 

 the shells in a similar relation to the animal ; and since opportunities 

 for the examination of the structure of the few living species are rare, 

 and the study of their anatomy attended with peculiar difficulties, 

 there is not much chance of an ordinary observer's correcting this 

 presupposition. But since the researches of R. Owen and the 

 anatomical investigations of Albany Hancock have given us full de- 

 tails of the structure of the living Brachiopoda, there can no longer 

 be any doubt as to the special points involved in our inquiry. 



The method of determining the longitudinal axis of the body is by 

 passing a plane through the two extremities of the alimentary canal ; 

 this axis being found, the two others, the lateral and the perpendicu- 

 lar, are derived very easily. The anterior opening of the alimentary 

 canal in Brachiopoda, in the common Terehratula caput-serpentis of 

 our shores, for instance, is situated at the base, and between the 

 brachial coils, and is thus within the socket or dorsal valve, as it is 

 called. Following it toward its termination, we see that it first bends 

 slightly toward the same valve, then turns toward the umbo, passes 

 through the fork of the occlusor muscles, this again doubling on its first 

 course, and extends across the space inclosed in the arch of the valves, 

 terminating near the opposite valve in the same vertical plane in which 

 it originated. 



It is now evident that the longitudinal axis lies in a plane described 

 by drawing a line from the back to the middle of the border of each 

 valve, and that we have here one valve before and one behind, and 

 further, that half of each valve will be right and half left. 



If bilaterality is a character in the group, we may expect to find 

 throughout the main systems of organs lateral equivalents, one on 

 either side of the longitudinal axis, each complementary to the other, 

 and presenting that peculiar reverse symmetry we find where there is 

 bilaterality. 



Considering first the exterior of the animal, we find that each side 

 of each valve is perfectly complementary to the other ; every line and 

 curve on the one side has a similar line and curve on the other, which 

 is its equal in every respect except the reversion of curve. The 

 structural features of the hinge are perfect specimens of bilateral sym- 

 metery, and if the shell is ornamented with strias, or radial plications, 

 they are precisely equivalent on the right and on the left. Passing to 

 the interior, we find the several processes which give a general support 

 to the viscera, and a special foundation to the oral arms, to be most 

 markedly bilateral. The muscular system, so complicated and beau- 

 tiful in the Brachiopoda, has each of its main divisions in pairs, one 



