246 PHYLUM ANNELIDA. 
ments of cilia on their anterior end produce the appearance of a rotating 
wheel. The food seems to consist of small organisms and particles 
caught in the whirlpool made by the lashing cilia. The little animals_ 
are tenacious of life, and can survive prolonged drought. If they are 
left dry for long, however, they die, though the ova may survive and 
subsequently develop. 
The body is usually microscopic, and is sometimes (¢.g. in Melicerta 
and Foscularéa) sheltered within an external tube. There is no 
internal segmentation, but there are sometimes external rings, and the 
attaching outgrowth or ‘‘ foot” is sometimes segmented. The anterior 
nd bears, on a retractile ridge, the ciliated ring or ‘‘ trochal apparatus.” 
The nervous system is a single dorsal ganglion with a few nerves. 
-\n unpaired eye and some tufts of sensory hairs are usually present. 
The food canal extends along the body in a well-developed ‘‘ ccelom,” 
and the fore-gut contains a mill, in which two complex hammers beat 
upon an anvil, The canal ends posteriorly on the dorsal surface 
between the body and the foot, and, as the terminal portion also 
receives the excretory canals and the oviduct, it is called a 
cloaca. 
There is no vascular system, but a nephridial tube of a primitive type 
lies on each side of the body, and opens posteriorly into the cloaca. 
The sexes are separate ; the reproductive organs are simple. Except 
in the marine parasite Sezson, in Rhinops vitrea, and two or three other 
forms, the males are dwarfed and degenerate, destitute even of a true 
food canal, and often ‘‘little more than perambulating bags of 
spermatozoa.” In many cases the sexual union (effected by a 
penis) seems to be ineffective, and there is no doubt that many, if not 
most, Rotifers are pathenogenetic. No males have as yet been found 
in Philodina, Rotifer, Callidina, or Adineta. The females lay three 
different kinds of eggs, according to their conditions and constitution— 
either small ova, which become males, or thin-shelled ‘‘ summer ova,” 
or thick-shelled ‘‘ resting or winter ova,” the two last developing into 
females. The so-called winter eggs may occur at any season, and 
seem usually to have been fertilised. Many species, however, are 
viviparous. We include the Rotifers beside the Annelids proper, be- 
cause it seems possible to regard them as derived from ancestors 
somewhat like Annelid larvze. 
Rotifers living in fixed tubes or envelopes,—Melicerta, Floscularia, 
Stephanoceros. 
Free Rotifers,—Notommata, Hydatina, Brachionus, 
Parasitic on the marine crustacean Mebalza,—Seison. . 
Pedalion occupies a unique position ; it has hints of appendages and 
a peculiar jumping motion, 
At this stage it may be mentioned that there are several sets of 
small worm-like animals of which we know very little. It is quite 
possible that some of them, may become of great interest to the 
systematic zoologist, but we do not yet understand what places in 
the system they should occupy. Moreover, as they are small, un- 
familiar, and unknown to myself, I shall simply refer to more complete 
works for an account of the Gasterotricha, Echinoderidze, Demosco- 
lecidze, and Cheetosomide. 
