8 GLOCHIDIA OF FRESHWATER MUSSELS. 



grunniens), from its food habits, is oftener found infected with 

 glochidia than probably any other fish; specimens of this fish taken 

 August 9, 1910, from whicli the figure shown was made (fig. 41, pi. iii), 

 and again July 20, 1911, have been found to be the hosts for many 

 young mussels (P. donaciformis) , all deeply encysted on the gill fila- 

 ments and showing the same remarkable growth found in laevissima. 

 By reference to the figure (fig. 41, pi. iii) it will be noted that the 

 growth is extraordinar}', the length havmg increased during para- 

 sitism more than five times over the length of the glochidial shell, and 

 with increase in depth in proportion. 



As pointed out by Lefevre and Curtis "■ in various species studied 

 by them the normal growth during the parasitic period is very slight, 

 so far as the shell is concerned; "the mussel leaves the fish with only 

 a very narrow margin of adult shell protruding beyond the glochid- 

 ial outline. The shape is still that of the glochidium, * * *." 

 Experiments conducted by the writer at the Fairport laboratory 

 confirm this in the case of L. recta, L. anodontoides, and Olovaria 

 ellipsis, in which scarcely any marginal growth at all is discernible. 



In the key for identification of the species of unionid glochidia, 

 which follows, the average measurements of the glochidium, in frac- 

 tions of a millimeter, are given immediately following the name of each 

 species. The length, a line across the widest part of the shell (anterior 

 to posterior edge) parallel to the hinge line, is given first, followed by 

 the depth, which is a vertical line from the highest point of the hinge 

 to the extreme ventral margin. These measurements are followed by 

 reference to figure numbers of specimens shown on the plates, an 

 arrangement which it is hoped will facilitate the use of the figures in 

 identification. 



Key for Identification of Unionid Glochidia. 



Anodonta type: 



Glochidium large, subtriangular in shape, visually longer than deep, M-ith a spine at 

 tip of each valve. 



1. Hinge line straight, or nearly so. 



a. Length greater than depth. 



Alasmidonta calceola, 0.300 by 0.255 mm. (fig. 1). 

 Anodonta imhecillis, 0.310 by 0.290 mm. (fig. 2). 

 Strophitus edentulus, 0.350 by 0.285 mm. (fig. 3). 

 Symphynota compressa, 0.353 by 0.313 mm. (fig. 44). 

 b. Length and depth about equal. 



Anodonta grandis, 0.410 by 0.420 mm. (fig. 45). 

 Anodontoides/. subq/lindraceus, 0.330 by 0.330 mm. (fig. 43). 



2. Hinge line irregular, undulate. 



aa. Length and depth almost equal. 



Anodonta corpulenta, 0.350 by 0.350 mm. (fig. 4). 

 Arcidens confragosus, 0.355 by 0.350 mm. (fig. 5). 



Op. cit., Bulletin of Iho Bureau of l-'isliories, vol. xxx, p. 176. 



