REPRODUCTION IN THE UNIONID^E 
101 
may be given here. Diluted sea-water and solutions, varying 
in strength from 0.5 to 1.0 per cent, of NaCl, K4CI, KCl and NH4CI 
had exactly the same effect as fish's blood, although the intensity 
of the reaction varied somewhat in certain cases. Weak solu- 
tions of MgCl2 and MgS04, however, as was to be expected, in- 
hibited contractions, and glochidia, after treatment with these 
salts, could be killed in an expanded condition, if allowed to re- 
main in the solutions for a sufficient length of time. 
Reactions of hooked glochidia 
The larvae of Symphynota complanata, which are provided with 
stout hooks and as a rule find permanent lodgment only on the 
fins and other external parts of the fish, were used in studying 
the reactions of the hooked type of glochidium. In several re- 
spects they differ from the hookless forms, when removed from 
the marsupium and placed in water, they exhibit spontaneous 
contractions which occur at irregular and rather long intervals, 
and this irritability may continue in the laboratory for a day or 
two, or until the glochidia begin to disintegrate. Under such 
conditions the valves are only partially closed at each contraction 
of the muscle, which, moreover, is never strong enough to bring 
the points of the hooks into contact. It is followed at once by 
relaxation of the muscle and the shell remains widely open until 
the next snap occurs. 
Hooked glochidia, in marked contrast with the behavior of the 
hookless forms, respond very actively to tactile stimuli, and, as has 
been stated, close completely and immediately when touched 
with any object. This reaction must be the main factor in bring- 
ing about their attachment to the fish's fins, when they are brushed 
over by the latter while lying on the bottom. With glochidia 
like those of Symphynota complanata the mere contact is suffi- 
cient to produce complete closure of the valves, and, whether 
they are exposed to the fish's blood, or not, attachment is possible 
as a result of the tactile stimulus alone. The\^ do react to blood, 
however, and exhibit a few successive contractions, from five to 
fifteen, before final closure, but the way in which the response 
