50S TRANSACTIONS OF THE WAGNER FREE 



has resulted in a most complex and divergent multitude of subordinate forms 

 of gill, which are connected by all shades of differentiation with the filibranchi- 

 ate gill and with each other. In some forms the ascending external lamina is 

 again slightly reflected at its upper edge, forming the so-called " appendix " 

 [Carduun, Mactra, sp.), in others portions of the laminae are aborted. In Lasea 

 the direct external lamina is shortened and its reflected part is absent, in some 

 species of Zz/««a only the direct and reflected inner laminae remain, while in 

 the Anatinacea the most chaotic condition obtains, some forms having both 

 direct and the inner reflected laminae; others having the equivalent of the two 

 direct laminse ; while in still others the branchije are more or less completely 

 aborted or merged in the peripedal septum. The very great variety which 

 obtains in the form and structure of the gills throughout the Pelecypoda, as a 

 Class, should be a sufificient warning against the use of their structural differ- 

 ences and minor details as a fundamental basis of ordinal subdivisions. The 

 end to be obtained in the development of gills is the aeration of the circulation 

 and incidentally the avoidance of foul water and the collection by ciliary action 

 of food particles. The development of the greatest superficial aerating sur- 

 face with the least expenditure of tissue is capable of being approximately 

 solved in so many different ways, that there is no occasion for surprise at the 

 multitude of forms which have been evolved. In cases where the minor struct- 

 ure of the gill has been taken as a systematic character of primary importance, 

 the diagnoses have often been saved from being ridiculous only by ignoring 

 the exceptions to the rule. 



The Heart, — It is probable that in the Proto-pelecypod the heart was 

 double, with a ventricle and auricle on each side connected by an aorta above 

 and one below the rectum. This state of affairs continues in the Area noce and 

 several of its allies. In the course of development the ventricles have become 

 soldered in the medium line in many groups, their extensions clasping the rec- 

 tum, which thus appears to pierce the ventricle. In Nuada and Trigonia, while 

 an appearance of duplicature persists, the ventricle is practically single and in 

 the vast majority of Pelecypods it is single, both in fact and appearance. 



It has been argued that the radical form was an unpaired ventricle dorsal 

 to the rectum, with a single anterior aorta, and this view has certain points in 

 its favor, but on the whole the view here adopted seems at present better sup- 

 ported. 



If in the process of consolidation the aortal arch below the rectum should 

 be lost by degeneration, we should have a ventricle with an anterior aorta rest- 

 ing upon the rectum, as is actually the case in some species of oyster. If, on 

 the contrary, the superior arch was aborted, we should have the ventricle ven- 

 tral to the rectum as in Pinna. 



If one anterior and one posterior limb of the double arch should abort, we 

 might expect a ventricle retaining an anterior and posterior aorta but still free 

 from the rectum, as in Nuciila, where it lies above the gut, or some oysters, 



