GLOCHIDIA — FACTORS UNDERLYING ENCYSTMENT 465 



will serve to bring about a firm, locking attachment to the fish 

 host. 



This hooked type, although found in but few genera (Ano- 

 donta, Strophitus, Symphynota, Arcidens, and a few others), is 

 familiar, since it is the one usually figured and described in texts ; 

 the commoner hookless type for some reason has been largely 

 ignored. The remaining general points of structure are shared 

 equally by all glochidia and will be considered in the detailed 

 description of the hookless group which follows. 



2. Hookless glochidia. This far more abundant type of glochid- 

 ium lacks hooks and has rather delicate valves of a blunt spoon 

 shape (figs. 3 and 4) . The actual form and contour of the shells 

 vary widely among the members of currently accepted genera. 

 In size this group ranges from the minute glochidium of Margari- 

 tana, which measures 0.050 mm. or less, to that of Quadrula 

 granifera, 0.290 by 0.355 mm. 



The shell, although brittle and weaker than those of the hooked 

 glochidia, is quite firm, and can be subjected to reasonably rough 

 treatment without injury. This is due chiefly to its limy con- 

 stituents and not to the overlying chitinous cuticula. A thicker 

 ridge of lime is present as a border about the free edge ; this shows 

 in external surface view as a double-contoured margin (fig 3). 

 Along the ventral border of the valves, opposite the ligamentous 

 hinge, is an incurved cuticular flange of considerable transparency 

 (fig. 4). Viewed in profile from the side, it may appear decep- 

 tively like a hook. The sharp flange edge serves to bite into the 

 tissues of the host w^hen the glochidium becomes attached. 



Of the internal organs, the most conspicuous is the adductor 

 muscle. This, in the opened larva, stretches between the valves 

 as a broad, rounded band (fig. 4). In the closed glochidium it 

 appears in prominent circular outline, placed nearer the future 

 anterior end (fig. 3). The component fibers have the typically 

 elongated nuclei of smooth muscle, but often branch near their 

 insertions on the valves. 



Lining the valves is the larval mantle, formed of large, flat 

 cells, and reputed to be active in digesting the enclosed tissue 

 of the host during the early stages of encystment (fig. 4). Cer- 



