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how fully the idea of bilateral symmetry is developed in the order. 

 We have seen that it is exhibited throughout the organization, from 

 the general form of the shell to the minute ramifications of the vascu- 

 lar system. Nor is this bilaterality confined to simply having organs 

 similar in their functional characters alone ; but, more than this, every 

 line and curve of one side has its mate on the other side, so that view- 

 ing the organization as a whole, we find even as much, if not more, of 

 that lateral symmetry than we have in the human body. 



One thing remains to be noticed in this connection. In Terehratula, 

 in Lingula, and in most of the genera intervening between these ex- 

 tremes of the order, there is a more or less evident tendency to form a 

 groove and ridge along the plane of the perpendicular of the shell, and 

 consequently dividing the surface of the valves equally into right and 

 left. The only purpose which this ridge and groove can have, since it 

 seems to have no structural relations, is as an index of the lateral 

 equivalence of two sides, — it is an effort to carry this differentiation 

 of right and left to the surface of the animal ; and since right and left 

 cannot have a surface expression under normal conditions, as it has in 

 the oysters and clams, it results in this sinus ; and to show that this is 

 the true meaning of the feature, in one group, the Diphyes of the 

 Jurassic period, the sinus is so far developed that it nearly sunders 

 the right half from the left half of the shell. 



Since our best, if not only, guide to the homologies of the Mollusca 

 is to be found in the relative position of organs, it will be readily seen 

 how much importance is to be attached to this mode of considering the 

 Brachiopoda as a key to the relations existing between them and the 

 other Mollusca. It is not possible for me to consider here the homolo- 

 gies which could be made between the Brachiopoda and the other 

 Acephala, but a glance at the various portions of the organization will 

 show how probable it is that the oral arms correspond to the labial 

 palpi in Lamellibranchiata. The heart is, by all the tests we can 

 apply to it, homologous with the same organ in Lamellibranchiata. 

 The position of the liver-like organs is comparable with what we 

 have in the Ostrea. The nervous centre or oesophageal ganglionic 

 ring seems to correspond in every important feature with the same 

 structure in Lamellibranchiata. But notwithstanding the assistance 

 derived from a proper understanding of these axial relations, it will be 

 necessary to seek the aid of embryology before we can hope to come 

 to a full understanding of the character and extent of the homologies 

 which unite this order with the other groups in the class. The homol- 

 ogies which I have referred to as probable, are given with the hope of 

 disabusing the minds of some naturalists who have questioned the 

 existence of any homological relations between the Brachiopoda and 

 the other Mollusca. 



