
MOLLUSCS, VERMES AND HYDROZOA 

aquarium will appear to be a living wriggling mass of threads that a 
when disturbed. ‘These worms construct cases of the 
mud and of mucus secretions which they deposit, in 
which they live, consuming organic substances in the 
water and mud and bringing the residue to the aN 4 
surface. To dispose of them is difficult as the dipping- FIG. 192. Common fresh- 
water Tubicular Worms 
tube or siphon will not be effectual. A better method fimnodrilus claparadii. 
is to introduce one or more mussels, which cause Greatly enlateed: 
their disappearance, or removing the other fishes introduce Sticklebacks, 
which, if not otherwise fed, will in a few days clear the aquarium of the 
pests. The Tudifex of Europe is sparsely represented in the United States. 
FRESHWATER Potyps. These belong to the order Hydrozoa, two 

classes of which, the Hydrida and Corynida, have frequently occurring 
freshwater forms, the Hydra and Cordylophora. 
Hypra. The freshwater hydra consists of a cylindrical body ex- 
panded into a single foot at one end, by which the animal attaches itself 
to any object, and having a mouth and a circle of from 5 to 12 tentacles 
at the otherend. ‘Those having short tentacles are of the species H. viridis 
and those with long filamentary tentacles H. fusca. The body contains a 
single large cavity, the rejected food being ejected from the mouth. Hydra 
possess the power of repairing mutilations and of multiplying artificially, 
and if cut into any number of pieces, each will develop into a perfect polyp. 
Reproduction is both sexual and by gemmation; the buds often develop- 
ing and throwing out new shoots before they detach themselves from the 
parent. In the sexual method of reproduction, ova are developed and en- 
closed in sacs, which when mature are expelled through the body wall, 
the embrio appearing as a free-swimming tiny thread-cell, which attaches 
itself to foreign bodies and develops into the complete animal. Hydra 
occur in all waters and multiply rapidly when the temperature reaches 60° 
F., though cold water does not appear to injure them. 
Hydra viridis. This is the most frequently met with form. Its 
power of changing its appearance is wonderful; thoseshown in Fig. 192, Nos. 
1, 2, 3, 4 and 5, are all the same individual and the changes took place in 
less than one hour. The process of gemma- 
tion may be noticed, together with the growth 
of the young and its separation from the parent. 
The tentacles were not of the same form or 
length for even an instant, and seemed to ooze 
from and return to the body constantly. 

FIG, 193. Shee Palyon Gay : : 
enlarged. 1 to 5. Hydra viridis, Hydra fusca. This polyp, Fig. 193; Nos. 
6 and 7. Hyd ae ; : ath 
es a 6 and 7, is more stalklike than A. viridis, and 
247 
