SCOLECIDA. 125 
Of the free Nematode worms, which are never parasitic at 
any time of their lives, about 200 species have been described, 
most of which inhabit fresh water or the shores of the sea. 
One of the most familiar is the so-called “ vinegar-eel” (Amguzl- 
lula aceti, fig. 54, A). 
ORDER VII. RoTIFERA.—The foéifera, or ‘ wheel-animal- 
cules,” derive their popular name from the fact that the anterior 
end of the body is furnished with one or two circlets of cilia 
Fig. 55.—Rotifera. A, Diagrainmatic representation of Wydatina senta (gene- 
ralised from Pritchard): a@ Depression in the ciliated disc leading to the 
digestive canal; 4 Mouth; c Pharyngeal bulb with masticatory apparatus; 
@ Stomach ; e Cloaca; /Contractile bladder; ¢¢ Respiratory or water-vascular 
tubes; 4 Nerve-ganglion, giving filament to ciliated pit (4); o Ovary. B, 
Melicerta ringens (after Gosse). > ie 
(fig. 55) which, when in motion, vibrate so rapidly as to produce 
the illusory impression of a quickly-rotating toothed wheel. 
The Rotifera are all aquatic, and are mostly inhabitants of fresh 
water. They are all microscopic in size, none attaining a 
greater length than 1-36th of an inch. . In the females there is 
a distinct mouth, intestinal canal, and anus. .A nervous system 
is also present, consisting of ganglia placed near the anterior 
extremity of the body and sending filaments backwards. There 
is, finally, a well-developed water-vascular system. ee 
