REPRODUCTION AND ARTIFICIAL PROPAGATION OF FRESH-WATER MUSSELS. 1 25 



marsupium in this region. The depth of this fold varies with the fullness of the mar- 

 supium, as the greater is the distension of the latter the farther forward it is tucked 

 under the anterior respiratory region. Posteriorly the two portions of the gill are sharply 

 defined by a deep cleft, as shown in the figure. The surface of the marsupium is thrown 

 into an irregular series of low undulating folds which are more prominent in the more 

 heavily charged females. In two of the females the marsupium is a salmon pink, while 

 the third is colorless, but here, as in the other cases described in which glochidia are 

 present, the color is due to unfertilized eggs. 



The record in our notes of the three females is as follows: 



No. I, small specimen, 44 by 39 mm. Marsupium colorless, only slightly distended 

 and not thrown into folds or undulations; no anterior fold, merely a notch; glochidia 

 colorless. 



No. 2, larger specimen, 57 by 52 mm. Marsupium salmon pink, much fuller than 

 no. I, and thrown into distinct folds; deep anterior fold; glochidia colorless, but many 

 pigmented unfertilized eggs and abnormal embryos mixed with them. (This is the 

 specimen from which the figure was drawn.) 



No. 3, largest specimen, 58 by 55 mm. Marsupium with just a tinge of pink, more 

 heavily charged than either of the others and showing prominent folds or undulations; 

 deep anterior fold; glochidia colorless, and a few pigmented unfertilized eggs and 

 abnormal embryos present. 



It is evident from this comparison that the smaller, and therefore presumably the 

 younger, females are less heavily charged than the larger and older ones; and, further- 

 more, that those changes in the gill which are the mechanical effects of gravidity, like 

 the folds, vary directly with the degree of distension of the marsupium. This conclu- 

 sion holds good for all the Unionidae which we have had an opportunity of examining, 

 and also applies to the experience of other observers. 



The glochidia of Dromus dromus, which are excessively minute and of unusual 

 form, being kidney shaped, are referred to later. 



INTERNAL STRUCTURE OF THE MARSUPIUM. 



The marsupium of the Unionidae furnishes a beautiful illustration of a remarkable 

 diversity of form in the adaptation of an organ for a specialized function. One can not 

 study this structure in the North American Unionidae without being forcibly impressed 

 with the great variety of detail which one and the same general adaptation is capable 

 of exhibiting. But whatever be the special direction which the modification has taken, 

 even in the most bizarre forms of the marsupium, like that of Cyprogenia, there is never 

 any doubt as to the relation between the structural specialization and the function 

 which it is adapted to perform. The structural basis of the marsupium — one might 

 almost say the unit of structure — is the water tube, and it is from an investigation of 

 its finer structure and its relation to other tubes, similarly modified, that an under- 

 standing of the unionid marsupium is gained. The fundamental adaptation is a series 

 of compartments in the interior of the gills provided with a specialized glandular 

 85079° — Bull. 30 — 12 9 



