194 Prof. H. James-Clark on the Spongie ciliatee 
gradually reverses the are and, assuming a much stronger bend 
in the opposite direction, terminates abruptly, and far beyond. 
the edge (61) of the collar, with about the same thickness as at 
its base. Itis avery conspicuous organ, and therefore its whole 
sigmoid length may be studied with any amount of detail that 
could be wished for. The plane of this sigmoid curve is a 
direct continuation of that which passes through the opposing 
longer and shorter curves of the obliquely oval body. Calling 
to mind now what has been said in regard to the direction of 
the curve of the flagellum of the respective individuals of the 
colony, it will be seen that if these planes are projected in- 
wardly and downwardly, at the same time passing along the 
pedicels (fig. 8, pd*) of each body, they will all meet at the 
main stem (pd). 
Besides being used as an organ of prehension, the flagellum 
is occasionally devoted to other purposes,—for instance, to act 
as a scavenger by whirling in a gyratory manner, and thus 
clearing the area within the collar of feecal matters which have 
been ejected from the anus at a point near to, or perhaps co- 
incident with, the mouth (m).. At other times it acts as an 
organ of propulsion during the act of natation (fig. 23), when 
one of the resultants of self-division breaks loose from the 
colony and seeks another point to settle down upon and secrete 
its stem. During this wandering life of the Monad it swims, 
at times very rapidly, with its basal end (fig. 23) preceding it 
in the direction of its course, and the flagellum (fl) following 
behind and vibrating in rapid undulatory and gyratory curves, 
as if it were the screw propeller of some subaqueous vessel. 
That the mouth (figs. 23, 24, 248, m) is situated near the 
base of the flagellum (7) is rendered certain by the fact that 
particles of food are thrown by that organ directly against the 
area (fr) upon which it is based, and are taken within the 
body somewhere in that region ; but, on account of the minute 
size of these morsels, and the rapidity with which they are 
swallowed, it has not been possible to determine precisely at 
what point. The position of the anus, which, as I have al- 
ready suggested, may possibly be coincident with the mouth, 
is easily determined, even to the narrowest limits, as the feecal 
matter is discharged in large, highly refractile pellets (fig.24#,d) 
close to the base of the flagellum. The digestive vacuoles are 
quite conspicuous, and frequently very large; but they never 
have been observed to be so numerous as to obscure the view 
_ of the interior of the body. | 
The contractile vesicles (cv, cv) are two quite conspicuous 
globular organs, which-lie close to the surface and in the pos- 
terior third of opposite sides of the body. Occasionally three 
