ROTATORIA OF THE UNITED STATES. 
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two large sacs (figs. 3, 4, m. r.). These sacs open one at the base of each toe, and 
discharge the mucus out upon the surface of the toe. Thence it trails behind the 
animal as a long thread, by means of which the rotifer attaches itself to various 
external objects and hangs in the water, as a spider by its thread. The mucus 
passes out of the sac between the substyles and the main toes, and the four substyles 
serve to direct its course out along the surface of the toe. 
But the two toes in Diurella tigris Muller are not placed exactly side by side, 
as in most rotifers, bnt. t hey partake of the prevailing asymmetry of the animal. 
The attachment of the toes to the foot is oblique, like that of the foot to the body, 
so that the right toe lies at a higher level than the left. The arrangement will be 
Fig. 1. — Dorsal views of the toes in a number of species of Rattulidce , showing gradual reduction of the right toe. 
(j) B. longiseta Schrank; (fc) B. scipio Gosse; ( l ) B. carinatus Lamarck; (m) B. mult icr inis Kellieott; ( n ) R. pusil- 
lus Lauterborn; (o) B. bicristatus Gosse (base of toe only). 
best understood if one conceives it to have been brought about as follows: The toes, 
originally concave downward, have been twisted at their attachment to the foot, so 
that their concavity now faces to the right (pi. I, fig. 1), and the right toe lies above 
the left, as the animaT creeps along the bottom. The toes and foot can therefore 
now bend only to the right, not toward the ventral side, as in most rotifers. 
Now, as a result of the condition above described, the two toes no longer have 
the same relation to the environment as they have in a bilaterally symmetrical 
animal. This similar relation to the environment is usually assigned as a reason 
for the similarity of paired organs, and the lack of such similar relation to the 
environment may become an equally good ground for the loss in similarity of two 
