275 



finds expression in Mollusca ; I think it gives in a few words the most 

 important features of the fundamental thought of the type. As there 

 defined, this thought is, a massive concentration of the structure, with 

 a differentiation of the weight of organization on to the sides of the 

 body. 



In attempting to apply this characterization to the different groups 

 which are unquestionably included within the type, we have always 

 found an unexpected difficulty. Applying with equal force to all the 

 members of the Cephalopoda and Gasteropoda, there have always been 

 certain contradictions apparent when it came to be apphed to some 

 members of the third and lowest class, viz., Acephala. Nowhere in 

 the t}^e is the feature of laterality more conspicuous than in those 

 bivalves with which our eyes are everywhere made familiar ; in the 

 oysters, the clams, and the fresh-water mussels, this laterality is so 

 presented as to be evident to the most cursory examination. The 

 two valves are one right and one left, with the end of the alimentary 

 canal in the same plane as the junction line of the valves. The 

 breathing organs, or gills, are one upon the right and one upon the 

 left ; the labial palpi, and many other parts of the organization, share 

 in this lateral arrangement. Indeed, it would not be too much to 

 state that the whole aspect of the animal is determined by this 

 character. 



But in examining the structure of the Brachiopoda, the order next 

 below the Lamellibranchiata, naturaHsts have very generally failed 

 to find any evidence of bilaterality in their organization. If we con- 

 sider one of their valves as right and the other as left ; or if, as is 

 most usual, we take one to be dorsal and the other ventral in their 

 relation to the organization, then the feature of bilaterality will seem 

 to be altogether wanting. Supposing that one valve is right and the 

 other left, we shall then have all the breathing organs or oral arms 

 upon one side, both oviducts upon one side, the cavities of the body 

 divided unequally, and a full negation of all those relations of the 

 sides of the animal, which we find throughout the type, and especially 

 marked in the next order in the scale of development. If this view 

 of the relations of Brachiopoda were correct, we should have good 

 reasons to suspect the value of bilateraHty as a type character, and a 

 doubt would, to most minds, be thrown upon those generalizations on 

 the subject of type characters, which have made the study of rela- 

 tive structure a study of personified thought. 



But I am fully convinced that these views of the organization of 

 Brachiopoda are erroneous, and that in this order we have as beau- 

 tiful an instance of bilateral symmetry as can be found in the whole 

 range of MoUusks. 



A fruitful source of trouble has been that Malacologists are ac 



