GJochidia 289 



If now the soft filament of a fish's gill, or the pro- 

 jecting ray of a fin by any chance conies in contact with 

 this sensitive surface the glochidium will close upon it 

 almost with a snap; and if the fish be the right kind 

 for the fostering of this particular mollusc, it will 

 remain attached. It is indeed interesting to see how 

 manifestly ready for this reaction are these larvae. If a 

 ripe brood chamber of Anodonta (fig. 88 on p. 180) be 

 emptied into a watch glass of water, the glochidia 

 scattered over the bottom will lie gaping widely and 

 will snap their toothed valves together betimes, whether 

 touched or not. And they will tightly clasp a hair 

 drawn across them. 



Doubtless gills become infected when water contain- 

 ing the glochidia is drawn in through the mouth and 

 passed out over them. Fins by their lashing cause 

 in the water swirling currents that bring the glochidia 

 up against their soft rays and thin edges. 



Glochidia vary considerably in form and size, in so 

 much that with careful work species of mussels can 

 usually be recognized by the glochidia alone. Thus it 

 is possible on finding them attached to fishes, to name 

 the species by which the fishes are infected. 



In size glochidia range usually between .5 and .05 

 millimeter in greatest diameter. Some are more or 

 less triangular in lateral outline and these have usually 

 a pair of opposed teeth at the ventral angle of the valves. 

 Others are ax-head shaped and have either two teeth or 

 none at all on the ventral angles. But the forms that 

 have the ventral margin broadly rounded and toothless 

 are more numerous. Whether toothed or not they are 

 able to cling securely when attached in proper place to 

 a proper host. 



The part taken by the fish in the association is truly 

 remarkable. The fish is not a mere passive agent of 

 mussel distribution. Its tissues repond to the stimulus 



