TRANSACTIONS OF THE WAGNER FREE 



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

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

 e 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 " 

 Lardmm, 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 Lucina only the direct and reflected inner laminae remain 'while in 

 the Anatmacea the most chaotic condition obtains, some forms havin- both 

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

 direct laminae ; while in still others the branchiae are more or less completely 

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

 uns in the form and structure of the gills throughout the Pelecypoda as a 

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

 nces and minor details as a fundamental basis of ordinal subdivisions The 

 I to be obtamed in the development of gills is the aeration of the circulation 

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

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

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

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

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

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

 the diagnoses have often been saved from being ridiculous only by ienorine' 

 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 HOC, and 

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

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

 tum, which thus appears to pierce the ventricle. In Nucnla 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 

 e 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 sun- 

 ported. 



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

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

 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 Nucula, where it lies above the gut, or some oysters 



