128 BULLETIN OF THE BUREAU OF FISHERIES. 



differences as described by Peck, not only in Anodonia but in a number of other genera, 

 and have also determined that the gills of the male are like the inner gill of the 

 female with respect to the frequency of the interlamellar junctions and the character 

 of their epitheUum. 



Peck's description has formed the basis of all of the textbook accounts of the 

 structure of the unionid gill, and two of his figures, showing the differences between 

 the inner and outer gills of Anodonta, are reproduced in Parker and Haswell's Text- 

 book of Zoology, volume i, page 638. 



Ortraann (191 1) was evidently unacquainted with Peck's work, as he describes 

 essentially the same differences between the marsupial and respiratory gills but 

 without reference to Peck. He is the first, however, to show that the same differentiation 

 holds good throughout a wide range of genera. In this connection he states that he 

 "made a very important discovery, namely, that in all our Unionidm the anatomical 

 structure of the gills, which serve as marsupia, is permanently diperentiated" (op. cit., 

 p. 283). He then describes in detail the points of difference, showing that in the 

 marsupial gill of the non-gravid female the interlamellar junctions, besides being more 

 numerous, are thicker and wider and are covered by an epithelium which is folded and 

 thrown into wrinkles, often of considerable proportions, whereas in the male and in the 

 respiratory gill of the female the epithelium is simple and unfolded (cf. Peck). "There 

 is no question," he says, "that this peculiar structure of the septa of the marsupial 

 gills is an adaptation to their function" — a conclusion long ago arrived at by Peck. 

 It should be stated that Ortmann has discovered another differentiating character 

 between the inner and outer gill, namely, a longitudinal furrow along the ventral border 

 of the inner gill which is entirely absent in the outer. This furrow is present in both 

 males and females. A similar furrow is figured by Peck for the gill of Mytilus, but 

 the figure in which it is shown is stated to be from the outer gill (op. cit., pi. iv, fig. 10). 



Ortmann, in his careful study of the structure of the marsupium, has described a 

 number of constant differentiations, hitherto unrecognized, which distinguish the 

 several groups established by him in his system of classification. We are relieved, 

 therefore, of the necessity of a detailed description in this place, and reference may be 

 had to his interesting paper. It should also be stated that one of our former students, 

 Mr. J. L. Carter, is now engaged in making a comparative study of the unionid marsu- 

 pium in a large number of genera, and his investigation, which was undertaken pri- 

 marily for the purpose of following the changes, both anatomical and histological, 

 occurring in the gill from the pre-gravid to the post-gravid condition, is now well under 

 way. Although Ortmann's work has, in part, rendered this investigation unnecessary, 

 nevertheless Mr. Carter's study will contribute a number of facts, especially facts of 

 a histological character, which are not included in Ortmann's observations. 



Only a brief reference here to the internal structure of the marsupium is called for 

 under the circumstances, and, since we shall need to compare our observations with 

 those of Ortmann, it will be a matter of convenience to refer to them under the three 

 subfamilies which he has distinguished. As we have not had an opportunity of exam- 



