I20 BULLETIN OF THE BUREAU OF FISHERIES. 



GENERAL STRUCTURE OF THE MARSUFIUM. 



In connection with our investigations on fresh-water mussels we have had occasion 

 to give quite a little attention to the anatomical and histological structure of the mar- 

 supium in a number of genera, and, furthermore, we have been particularly interested 

 in the changes that occur in the gills during the period of gravidity. We have already 

 published a brief account (1910b) of some of our observations on the marsupium, with 

 illustrations of the more important types, but, as Ortmann has since added a number 

 of new facts to the subject, it is advisable to present our results in greater detail and 

 with additional illustrations. For this purpose it will be more convenient to follow 

 Simpson's arrangement, and we shall refer to the species examined by us under the 

 several groups established by him. It will also be convenient in connection with the 

 description of the marsupium to refer somewhat incidentally to certain observations on 

 breeding habits, characteristics of the embryos, and related matters. The finer struc- 

 ture of the marsupium is reserved for a subsequent section of this report. 



TetrageruE. — The marsupium in these forms comprises all four gills, a condition 

 which is undoubtedly the most primitive one among the Exobranchiae. It is the con- 

 dition occurring in the genus Quadrula, in which, following Ortmann, we include Trito- 

 gonia. We have encountered it in the following species: ebena Lea, heros Say, lachry- 

 mosa Lea, metanevra Rafinesque, obliqua Lamarck," plicata Say, pustulosa Lea, trigona 

 Lea, tuberculata Barnes (Tritogonia tubcrcidata), and iindulata Barnes. 



No special structural modifications are present beyond the usual glandular folded 

 epithelium covering the surface of the interlamellar junctions which, as has been known 

 since the work of Peck (1877), are closer together in the marsupial than in the purely 

 respiratory gill. The gills when gravid, although somewhat distended and padlike in 

 appearance, never become swollen to the extent that is seen in many other genera. 

 Figure 5, plate vii, which is drawn from a gravid female of Ouadrida ebena, illustrates 

 the typical appearance of the marsupium in this group, although the gills shown in the 

 figure are not as fully distended as is frequently the case. 



In ebenaandtrigonai\\& ovarian eggs and the embryos are frequently brilliantly colored 

 red or pink and when the marsupium is charged the color shows through the colorless 

 transparent walls of the gills, which present a striking appearance on removing the shell. 

 In all of the other species of Quadrida observ-ed by us the pigmentation is absent, but in 

 ebena and trigona the color is found in a majority of the gravid females, the number of 

 such cases being somewhat greater in trigona (over two-thirds of all gravid females 

 examined in this species) than in ebena. The red pigment, however, whenever it occurs, 

 does not persist, but on the contrary totally disappears in the later stages of embryonic 

 development, and by the time the glochidia are fully formed no trace of it is left. We 

 have never seen a single case of a red or pink glochidium either in these two species of 

 Quadrula or in any other genus in which pigmented eggs and embryos occur. It is true 



a Ortmann (op. cit.. 1911, p. 330) states that only the outer gills serve as the marsupium in obliqua, and on this groimd he 

 has removed the species to Pleurobcma. If we have made no mistake in the identification of our specimens, our observations 

 on this species are not in accord with his. 



