136 Prof. H. James-Clark on the Spongie ciliate 
fixed state (fig. 3), and the other a free and motile stage (figs. 1, 
2,4). During its sedentary life, it may be found in great 
abundance on the old stems of Myriophyllum, Potamogeton, 
Ceratophyllum, and other aquatic phenogamous plants which 
inhabit quiet waters, and are more or less thickly covered by a 
floccose overgrowth of various minute Confervee, Diatomaceze, 
&c. In its free state it swims with either a sort of hitching, 
wriggling motion, or, gliding along smoothly, revolves at an 
inconstant but never rapid rate upon its longer axis, of which 
the flagellum (fig. 2,7), which always precedes it, may be said 
to be a prolongation. ‘This is the condition in which it is 
most frequently to be found after it has been kept a few days 
in an aquarium. It then gathers in swarms about decomposing 
matter, and thus affords frequent opportunity of seeing its mode 
of collecting and swallowing its food. 
The form of the body in a fixed state (fig. 3) may be com- 
pared to a flattened heart, of which one summit is prolonged 
into a broad, conical, transparent beak (/p), and at the opposite 
end the apex is attached to a slender, flexible pedicel pa), 
which frequently is equal in length to four or five times the 
antero-posterior diameter of the body. In a free condition 
(fig. 2) the posterior end is rounded and about as broad as the 
front, but still it presents the same lateral flattening as the 
fixed form. ‘The prevailing colour is a faint olive or yellowish 
reen. 
The flagellum (fl) is the only cilium-like organ which this 
creature possesses. It is attached to the front, close to the 
proximal side of the conical beak (dp), and consequently lies 
in the axial line of the body. In a quiet state, which it most 
frequently assumes during the fixed condition, it appears like 
an arcuate bristle, and extends from near its base to its apex 
in one uniform, slightly but distinctly curved line, and termi- 
nates without any very sensible diminution in thickness. The 
plane of its curve is in direct extension of the plane of the 
greater diameter of the body, and at the same time passes 
through the conical beak. During natation the flagellum 
takes precedence and vibrates with an undulating whirlin 
motion which is most especially observable at its tip, an 
produces by this mode of propulsion the peculiar rolling of the 
body which at times lends so much grace to its movements as 
it glides from place to place. During the fixed state of the 
body the chief design of the movements of the flagellum is the 
prehension of food; and this is accomplished by a peculiar 
abrupt deflection of the end of this organ towards the front, 
by means of which particles of various kinds are made to im- 
pinge upon the region immediately at the proximal side of the 
