98 GEORGE LEFEVRE AND WINTERTON C. CURTIS 
because their small size makes attachment elsewhere impossible. 
The type of glochidium is constant for the genus, so far as our 
observations go, and in some cases the shape is also character- 
istic, as shown by Symphynota and Anodonta {A, B and C) in 
which the shell outline is a distinguishing feature. 
BEHAVIOR AND REACTIONS OF GLOCHIDIA 
At the time of spawning the glochidia, already freed from the 
egg-membranes and more or less loosely held together in slimy 
strings, are discharged at irregular intervals through the exhalent 
siphon. Being heavier than water, they sink rapidly to the bot- 
tom, coming to rest with the outer surface of the shell directed 
downward and the valves gaping widely apart. The belief was 
formerly general that they ^^swim" about by rapidly opening and 
closing the valves, after the manner of Pecten, and in spite of 
frequent denials by Schierholz ('88), Latter ('91), and others, 
the same statement is still occasionally encountered. In the 
recent volume on Mollusca in the Treatise on Zoology, edited by 
Lankester, this inexcusable error is repeated. ^'The glochidia," 
we are again informed, ''swim actively by clapping together the 
valves of the shell," (p. 250 j . They are on the contrary, as is now 
well known, entirely incapable of locomotion and remain in the 
spot where they happen to fall, although it is true that they may 
exhibit from time to time spasmodic contractions of the adduc- 
tor muscle which cause the valves to snap or wink, each contrac- 
tion being immediately followed by relaxation and opening of the 
shell. These movements of the valves, however, are never so 
vigorous as to cause the glochidium to move from place to place 
in the water. 
The glochidia remain in this helpless situation until they die, 
unless they happen to come in contact with the host on which 
they pass through the post-embryonic development as parasites. 
The stimulus which causes the contraction of the muscle and re- 
sults in attachment to the host is in the case of hooked glochidia, 
a mechanical one. It may be readily imitated and glochidia of 
