484 LESLIE B. AEEY 



anoglossus, N/200; Holothuria, N/500. For man the limiting 

 concentration in taste lies below N/250, probably nearer N/100. 

 4. Chemical excitation in nature. In a previous section the 

 conclusion was reached that all the glochidia posses a rather acute 

 and nearly equal tactile sensibility. This is not in agreement 

 with the statements of Lefevre and Curtis ('12). The data on 

 chemical excitation, just presented, also show that, to KCl at 

 least, the hooked forms are the more delicately reactive. At the 

 outset, accordingly, the bare evidence would not appear favora- 

 ble to the view which Lefevre and Curtis advanced, that the usual 

 effective stimulus causing hookless glochidia to attach was 

 chemical in nature, of hooked glochidia mechanical. Finding, 

 moreover, that the blood of vertebrates causes the larvae to 

 snap shut, they concluded that blood, through the activity of its 

 salts, was the agent responsible for the attachment of hookless 

 glochidia to gills. They write (p. 154) : 



Since the hookless glochidia, which are essentially gill parasites and, 

 when taken into the mouth of the fish lodge among the gill filaments, 

 produce abrasions of the delicate epithelium covering the latter, a more 

 or less extensive hemorrhage from the blood capillaries occurs, as may 

 be readily seen from a microscopic examination. It is therefore evident 

 that blood exuding from the gill filaments in the immediate neighborhood 

 of the glochidia must have the same effect as in our experiments, and, 

 by exciting vigorous contractions of the adductor muscle, furnish an 

 efficient stimulus in bringing about a firm and permanent attachment to 

 the filaments. It is true that hookless glochidia will occasionally secure 

 an attachment to the edge of fins and other external parts of the fish 

 but it is quite evident that they are not adapted to such locations, as 

 they rarely succeed in remaining there. It is possible that when they do 

 become attached to the fins the closure of the valves is due to the pres- 

 ence of blood on the latter; but, since hookless glochidia occasionally 

 close when touched repeatedly, the attachment in these situations is 

 probably'brought about by a sluggish response to contact with the edges 

 of the fins. Their characteristic place of attachment, however, is the 

 gill filaments, and this definite reaction to the fish's blood constitutes a 

 most striking functional adaptation to the special habit of hookless 

 glochidia as gill parasites. 



Blood of vertebrates causes glochidia to close promptly. 

 Adding the blood of fish, frog, or man to a watch-glass containing 

 Lampsilis luteola glochidia, however, has never resulted in throw- 

 ing them into ''rapid and violent contractions, alternating with 



