PROPAGATION OF FRESH-WATER MUSSELS. 621 
As there are important commercial species in each class, the futility of any 
law which would prohibit the taking of mussels during their breeding season 
becomes at once apparent, for there is no month in the year when some species 
are not carrying either embryos or glochidia. 
THE LARVA, OR GLOCHIDIUM. 
Two well-marked types of glochidia occur in the Unionide, one provided 
with stout hooks on the ventral margin of the valves and the other entirely 
hookless. The former are characteristically parasitic on the fins and other 
external parts of the fish, the latter on the gills. We have given special atten- 
tion to a study of the physiological peculiarities correlated with these two types 
of structure, including their differences in behavior in attachment. 
The occurrence of the two types of glochidia in the genera which we have 
examined is shown below. 
Hookless glochidia: Hooked glochidia: 
Lampsilis (except L. leevissimus). Anodonta. 
Obliquaria. Lampsilis levissimus. 
Obovaria. Strophitus. 
Plagiola. Symphynota. 
Pleurobema. 
Quadrula. 
Tritigonia. 
Unio. 
It is thus seen that the hookless type is by far the commoner one, and 
furthermore, practically all of the commercial species of mussels belong to 
the genera included in this class. 
The type of glochidium is constant for the genus, so far as our observations 
have gone, except in the genus Lampsilts, one species of which, L. levissimus, 
produces glochidia having hooks, while all of the remaining species that we 
have examined have the hookless forms. The glochidia of L. levissimus, more- 
over, are of quite a different form from those of any other species we have 
encountered, being shaped like an axe head when seen in side view. 
The hookless glochidia are usually much smaller than those having hooks, 
and also differ characteristically from the latter in shape. 
We have repeatedly determined the fact that the hooked glochidia form 
permanent attachment as a rule only to external parts of the fish, as the margins 
of the fins, operculum, anus, and mouth, the barbules, or even to scales and 
along the fin rays occasionally. Wherever a free border is presented that may 
be grasped between the valves of the glochidium, attachment is possible, the 
hooks holding securely as soon as the valves have closed. Hooked glochidia, 
furthermore, are not infrequently found on the gill arches and rakers, as these 
places furnish the requisite conditions for a firm grasp. Glochidia of this type, 
