GLOCHIDIA — FACTORS UNDERLYING ENCYSTMENT 491 



Light or darkness is without influence on glochidial reactivity. 

 There is no response to decreased illumination through shading. 



Low temperature tends to dull the characteristic response. 



Glochidia react to appropriate concentrations of acids, alkalies, 

 alcohols, sugars, and salts, including substances which produce 

 in man the various sensations of taste and smell. This activa- 

 tion is ionic, not osmotic. Subminimal chemical solutions do 

 not notably increase irritability and thereby lower the threshold 

 to touch. 



The view that glochidia (particularly of the hookless type) 

 regularly attach to the host through a chemical activation by 

 blood, derived from gill hemorrhages, is untenable. There is like- 

 wise no evidence of any other effective chemical influence from 

 the host. The tactile response alone is adequate to insure attach- 

 ment. 



The proliferation of gill tissue to cover an attached glochidium 

 and produce a cyst may be studied on excised filaments. Funda- 

 mentally, it represents the reparative process of wound heahng 

 which restores the continuity of the epithelium. Cyst formation 

 is not initiated or controlled by any vital influence of the glochi- 

 dium, for it can be imitated by applying tiny metallic clips to a 

 filament. Nevertheless, some normal regulatory factor fails to 

 manifest itself in experiments upon excised filaments; over- 

 growth then produces large, malformed cysts comparable to the 

 excess proliferation in these cases after simple incision. 



The larval threads which entangle certain glochidia at spawn- 

 ing, the ropy mucus or gelatinous cords which embed others, and 

 the mucus which causes the massing of still others, may each 

 be useful in keeping the larvae together until they are established 

 on the bottom and become free. In this way, the chances for a 

 heavy natural infection of a host would be improved. 



