HISTORY OF THE ROTIFERA, OR WHEEL ANIMALCULES. 483 
rotate, but the black teeth only, and these change then* form 
in certain parts of their revolution, becoming confused, and then 
again bursting into distinctness. 
It is almost impossible to believe that you do not see an 
actual rotatory movement of the parts, that the black spots are 
not real solid organs, they are so palpable, so well-defined. Yet 
it is manifest on a moments reflection, that such a motion, 
continued without intermission for hundreds of revolutions, 
would be perfectly incompatible with the necessary conditions 
of an animal body. In reality you do not see parts at all ; the 
black spots are only waves in the cilia : an optical illusion pro- 
duced by the cilia being brought momentarily closer together 
at certain regular points, causing opacity, and alternating with 
correspondent separations, causing transparency. These waves 
run ceaselessly round, but the cilia themselves do not change 
their place : they merely bend and straighten themselves in 
rhythmic alternation. 
After we have somewhat satiated our sense of sight with this 
beautiful spectacle, we have leisure to look at the tubular case 
in which the animal dwells. It is not, hke that of the Crown- 
wheel and the Floscules, and, indeed, like those of most of the 
species of this family, transparent and simply gelatinous, but 
quite opaque ; so that, with the exception of those upper parts 
that are protruded from its protection, the body is altogether 
concealed. We can discern that its surface is composed of round 
bead-like objects set in a regular symmetry in a kind of mosaic 
pattern, and that these globules are of a dark-reddish or yel- 
lowish-brown hue. In truth, the foundation of this Melicerta’s 
case is a tubular layer of gelatinous mucus, thrown off from the 
surface of its body, tenacious and transparent, as in other kin- 
dred cases, as may be distinctly seen in the creature’s infancy, 
when it begins to construct its house ; but there is superadded 
to this an outer layer of stiff’ globules, which are imbedded indi- 
vidually in the gelatinous substance, imparting to it firmness 
and opacity. The preparation and deposition of these build- 
ing-stones form one of the most interesting chapters in the 
history of the class. 
In carefully watching your specimen, it may be that you will 
be fortunate enough to detect him in the very act of building’ 
his house, — a process which is not performed all at once, but step 
by step, at long intervals, a little at a time, as the owner’s 
growth requires commensurate elbow-room. Now, as he is the 
judge of this necessity for labour, we cannot force him to work 
when he has not a mind to it ; and so you may watch a number 
of individuals, and tantalizingly fail in ever seeing the process. 
But it may be that you will be more fortunate, as I have been 
