REPRODUCTION AND ARTIFICIAL PROPAGATION OF FRESH-WATER MUSSELS. 121 



that the marsupium may still be more or less deeply tinged with red, even when it con- 

 tains fully developed glochidia, but this is due to its containing a variable number of 

 unfertiUzed eggs, which do not lose the color, and not to the glochidia which are always, 

 as stated, entirely colorless. 



The occurrence of unfertilized eggs is very common in all of the species of Quadrula 

 which have come under our observation, and their presence is more characteristic of 

 certain species than of others. They are quite rare in plicata and pusttdosa, for example, 

 less so in metanevra, common in ebena, while in trigona, in which they occur more fre- 

 quently than in any other species of Quadrula, they were found in a large majority of 

 cases. The number of unfertilized eggs in different females of a given species varied 

 from cases in which only a few such eggs were scattered among normal embryos all the 

 way to cases in which the marsupium contained no normal eggs or embryos at all. Eggs 

 which have not been fertilized, after remaining in the marsupium, become swollen and 

 stratified (see below), frequently forming exovates and undergoing fragmentation before 

 final disintegration. 



There seems to be a definite correlation between the presence of unfertilized eggs in 

 the marsupium and the occurrence of trematode parasites in the testis of the male; in 

 species like plicata, in which unfertilized eggs were rare, only occasionally were the testes 

 infested with worms, but in trigona, for example, the trematodes were found in a large 

 number of males. It is not at all improbable that the amount of sperm available in a 

 given locality is greatly reduced as a result of the castration of males by this testis infest- 

 ing parasite. 



The abortion of embryos and glochidia, which is so characteristic of the genus 

 Quadrula, and the significance of this peculiarity will be referred to later on. 



HomogauE. — The condition in which the entire outer gills only are utilized as a 

 marsupium is present in 1 6 genera, according to Simpson." We have verified its occur- 

 rence in Alasmidonta truncata Wright; Anodonta cataracta Say, grandis Say, implicata 

 Say; Arcidens confragosus Say; Pleurobema (Esopus Green; Symphynota complaiiata 

 Barnes, costata Rafinesque; and in Unio complanatws Dillwyn and gibbosus Barnes. 



As has already been stated, Ortmann has disrupted the group, placing Pleurobema 

 and Unio in his subfamily Unioninae, while segregating Alasmidonta, Anodonta, and 

 Symphynota in his Anodontinae. This he has done chiefly because of a differentiation of 

 the ventral border of the marsupium and of a secondary division of the water tubes of 

 the marsupium in those genera included in the Anodontinae. These differences will 

 be referred to below. 



The marsupium when filled with embrj'os or glochidia may be greatly distended 

 beyond its normal dimensions, and in this condition is an enormously swollen padlike 

 structure, with a smooth surface, filling a large portion of the mantle chamber. Figure 3, 

 plate VI, represents the gravid marsupium of Symphynota complanata, which may be 

 taken as typical of the Homogense, although in Pleurobema and Vnio the distension 

 is not so great. 



■> Maroariiana is placed in this group by Simpson, but as it utilizes all four gills as the marsupium it should be included 

 with the TetratensE. 



