226 Annals of the Carnegie Museum. 



and secondly by the fact that the structure of the marsupium in the 

 Unionidce becomes highly specialized, or, to express it concisely, 

 this family makes a special effort to bring the marsupial apparatus 

 to the highest degree of efficiency. All differentiation is connected 

 with two purposes: the lengthening of the breeding season, and the 

 change of the discharge of the glochidia from a "natural" to an "un- 

 natural" manner, if such an expression may be allowed. As regards 

 the first, the subfamily of the Unionince is as yet in an undifferenti- 

 ated condition, possessing a short breeding season (being tachytictic) , 

 without specialization. 4 But in the Anodontince and Lampsilince the 

 breeding season is extended over the winter and the glochidia, after 

 they are fully developed, are not discharged immediately, but retained 

 for a long period in the marsupium (bradytictic) . This renders it 

 necessary to develop special devices in the marsupium, and the most 

 urgent need apparently is to provide the necessary oxygen for the 

 glochidia enclosed in the marsupia. It is now interesting to observe 

 how this purpose is accomplished in two different ways by the two 

 subfamilies. In the Anodontince, the lateral, secondary water-tubes 

 cut off from the central ovisac, undoubtedly have the purpose of 

 keeping up a lively current of water around the swollen marsupial 

 mass. Nothing similar to this is known in the Lampsilincc, but in 

 the case ol these the whole marsupium bulges out beyond the original 

 edge of the gill, and this bulging mass is enclosed in a rather thin 

 membrane, favoring osmotic processes. Further, there is a tendency 

 to locate the marsupium in the posterior part of the gill, and to push 

 it toward the lower posterior end of the shell, so that it is close to the 

 branchial opening, where fresh and pure water enters the animal. 

 In addition, a number of the Lampsilincc develop special papillae and 

 flaps on the edge of the mantle, just at the place toward which the 

 marsupium is pushed, and these structures surely have the purpose 

 of producing a lively current of water over the marsupium. Further- 

 more, the thin membrane enveloping the protruding part of the 

 marsupium, and its position near the branchial opening, are apparently 

 connected with the peculiar discharge ol the glochidia in the Lamp- 

 silincc, which is through the edge of the marsupium, through holes 



4 Haas (1910c, p. 19) comparing the marsupium of Anodonta and Unto expresses 

 the opinion that that of Anodonta is more primitive than that of Unto, which is 

 entirely erroneous: just the opposite is the case, that of Anodonta being much more 

 complex and specialized. As long as views like this prevail, we cannot expect to 

 arrive at a proper understanding of the system of the Najades. 



