126 INVERTEBRATE ANIMALS. 
Most of the Rodéfera are free-swimming active little animals 
(fig. 55, A), but some are permanently fixed, as in AZe/zcerta (fig. 
55, B) or in the crown-animalcule (Stephanoceros). ‘They are 
usually simple, but they are sometimes composite, forming col- 
onies. As a rule, the male and female Roézfera differ greatly 
from one another, the males being smaller than the females, de- 
void of any masticatory or digestive apparatus, and more or less 
closely resembling the young forms of the species. The males, 
in fact, merely lead a transient existence, and die as soon as 
they have succeeded in fertilising the females. The body in 
most cases is very distinctly ringed or annulated (fig. 55, A), but 
is not composed of distinct rings separated by partitions. The 
integument is usually provided with bundles of muscular fibres 
taking a longitudinal and transverse direction. In the free 
forms the anterior ciliated disc acts somewhat like the propel- 
ler of a screw-steamer in driving the organism through the 
water—in all cases it has the action of producing currents in 
the water by which particles of food are brought to the mouth. 
The posterior end of the body is usually developed in the free 
forms into a kind of tail or foot (fig. 55, A), which may take the 
shape of a kind of pincers or of a little suctorial disc. 
As regards their internal anatomy, in the females of almost all 
the Rotifera there is a well-developed alimentary canal, which 
is completely shut off from the general cavity of the body. The 
mouth (fig. 55, A 2) opens into a dilated chamber or “ pharyn- 
geal bulb” (c), which contains a complicated apparatus of horny 
teeth. The pharynx opens into a capacious stomach (@), con- 
tinued into an intestine which terminates by a chamber known 
as the ‘“‘cloaca” (¢), which forms the common outlet for the 
water-vascular and generative systems. In both sexes there is 
a well-developed water-vascular system consisting of a contrac- 
tile chamber or bladder (/), opening into the cloaca, and giving 
origin to two complicated tubes which are known as the “ res- 
piratory tubes” (gg), and which terminate near the anterior 
end of the body, apparently by blind extremities. The nervous 
system is in the form of a large double ganglion placed above 
the gullet, and having one or two eye-specks placed upon it. 
The ovaries (0) constitute conspicuous organs in the female 
Rotifera, but in summer the young Rotifers appear to be pro- 
duced by the females without having access to the males. 
The Rotifera were long confounded with the Jwfusorza, in 
