80 HUXLEY, ON CLASSIFICATION. 
of this, and, as it were, stuck on to it, is a little rounded body (Fig. 4, 
B d’), which has received the name of the “nucleolus.” The animal 
swims about, driven by the vibration of its cilia, and whatever nutri- 
ment may be floating in the water is appropriated by means of the 
current which is caused to set continually into the short gullet by 
the cilia which line that tube. 
But it is a singular circumstance that these animals have an alli- 
mentary canal consisting of a mere gullet, open at the bottom, and 
leading into no stomach or intestine, but opening directly into the 
soft central mass of sarcode. The nutritious matters passing down 
the gullet, and then into the central more fluid substance, become 
surrounded by spheroids of clear liquid (Fig. 4, A d), consisting ap- 
parently of the water swallowed with them, so that a well-fed Para- 
mecium exhibits a number of cavities, each containing a little mass 
of nutritious particles. Hence formerly arose the notion that these 
animals possess a number of stomachs. It was not unnaturally 
imagined that each of the cavities in question was a distinct stomach ; 
but it has since been discovered that the outer layer of the sarcode 
is by means of some unknown mechanism, kept in a state of constant 
rotation; so that the supposed stomachs may be seen to undergo a 
regular circulation up one side of the body and down the other. And 
this circumstance, if there were no other arguments on the same 
side, is sufficient to negative the supposition that the food-containing 
spaces are stomachs; for it is impossible to imagine any kind of 
anatomical arrangement which shall permit true dilatations of an 
alimentary canal to rotate in any such manner. Fecal matters are 
extruded from an anus, which is situated not far from the mouth, but 
is invisible when not in use. It is an interesting and important 
character of the Infusoria, in general, that, under some circum- 
stances, they become quiescent and throw out a structureless cyst 
around their bodies. The Infusoriwm then not unfrequently divides 
and subdivides, and, the cyst bursting, gives rise to a number of 
separate Infusoria. 
The remarkable powers of multiplication by fission and gemma- 
tion which many of the group exhibit are well known; but within the 
last few years the investigations of Miller, Balbiani, Stein, and 
others, have shown that these minute creatures possess a true process 
of sexual multiplication, and that the sexual organs are those which 
have been denominated “ nucleus” and “nucleolus.” The nucleus is 
the true ovary—the nucleolus, the testis, in Parameciwm. At par- 
ticular times, the latter increases very much in size, and its substance 
is broken up into rod-like bodies, which represent spermatozoa. Two 
Infusoria, in this condition, become conjoined, and the nucleolus 
(now converted into a spermatic capsule) of each passes into the 
