96 GEORGE LEFEVRE AND WINTERTON C. CURTIS 
to a lesser extent in species of the other genera mentioned, has 
never shown any trace of the thread which is so conspicuous a 
feature of the glochidium of Unio complanatus. We have also 
examined the glochidia of Symphynota complanata many times 
with the same negative results and a smaller number of observa- 
tions confirm this for S. costata. Since many species thus have 
no thread in any way functional for attachment to the fish, the 
question arises whether the thread when present has as impor- 
tant a function in this respect as has been supposed. Our observ- 
ations upon the glochidia of Anodonta cataracta confirm the 
descriptions of Schierholz (^88) and others who have studied the 
European species of Anodonta, as to the tangling of the glochidia 
into masses by means of their extruded threads, and in this genus 
the threads do seem effective in drawing other glochidia into con- 
tact with the fish when a single one has become attached. This 
is not, however, effective for the greater part of the period dur- 
ing which the glochidium may remain alive upon the bottom, for 
the threads are dissolved within a day or two and then the glo- 
chidia become entirely free from one another. When taken 
from the parent gill, the glochidia of Symphynota are entangled 
in a ropy mucus and this acts in a manner similar to the threads 
of Anodonta, but it is usually dissolved after a few hours in the 
water. In the ripe glochidium of U. complanatus, the threads 
are extruded when the glochidia are removed from the parent 
and placed in water. When this extrusion has taken place, the 
glochidia and broken egg-membranes become united into globular 
masses from which it is difficult to separate individual specimens, 
and from observing such glochidia in contact with fish, we are 
forced to conclude that they are not so likely to become attached 
to the gills or fins as they are when separated by the disintegration 
of the threads of mucus. The glochidia of Lampsilis, which when 
fully ripe, at once spread out into masses of entirely unconnected 
individuals, appear much better able to attach to the gills of 
fishes. Accordingly, we would consider the thread as something 
to be gotten rid of rather than an organ of great importance in the 
attachment, and this is in agreement with Lillie's interpretation 
of the thread as an excretory product. 
