REPRODUCTIOX IX THE UNIONID^ 
111 
mine whether the simple mechanical causes oi over-infection, deli- 
cacy of fin, or configuration of the mouth parts can giv^e a satis- 
factory explanation; or whether the histological changes which 
the fish is capable of, under stimulation, by the glochidium, must 
be regarded as the causes of its immunity. We have not yet 
carried out sufficiently rigorous experiments to feel sure tliat the 
simpler explanations can be excluded. In any case, it is interest- 
ing that fish hke the minnows and darters, which live close to the 
bottom, are not likely to become heavily infected b}^ some of our 
most common glochidia. 
Duration of the parasitism 
The duration of the parasitism appears, to be influenced very 
considerably by the temperature as stated by Schierholz ('88) 
and Harms ('07 -'09). Our records show a difference in the time, 
spent by a given glochidium upon the same fish, whicti varies 
with the season. For example, we have records of Symphy- 
nota complanata completing its metamorphosis and leaving the 
fish in from ten to fourteen days during December, and others 
in which the parasitism, begun March 25th, was continued for 
about forty days. S. costata, in an infection begun during Jan- 
uary, remained upon the fish for upwards of seventy days. Infec- 
tions with Lampsilis glochidia have shown a variation in the dura- 
tion at the temperature of the laboratory' (16°-20° C.) of from 
36 to 14 days. The glochidia of Unio complanatus and of Quad- 
rula plicata in July infections gave a period of from twelve to 
fifteen days. In all these cases, the temperature is probably the 
cause of the d:fferences, as Harms believes. 
HISTOLOGICAL CHANGES IN CYST-FORMATION 
As has been described, the glochidium attaches itself to the fish 
by closing its shell firmly over some projecting region which can 
be grasped between the valves, like the free border of a fin or 
the gill filam.ent. In so doing, a portion of the epithelium and- 
underlying tissue, including blood vessels and lymphatics, and 
