MOLLUSCS, VERMES AND HYDROZOA 



FIG. 192. Common fresh- 

 water Tubicular Worms, 

 Timnodrilus claparadii. 

 Greatly enlarged. 



aquarium will appear to be a living wriggling mass of threads that disappear 

 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 

 surface. To dispose of them is difficult as the dipping- 

 tube or siphon will not be effectual. A better method 

 is to introduce one or more mussels, which cause 

 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 Tubifex of Europe is sparsely represented in the United States. 

 Freshwater Polyps. These belong to the order Hydrozoa, two 

 classes of which, the Hydrida and Corynida, have frequently occurring 

 freshwater forms, the Hydra and Cordylophora. 



Hydra. 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 t2 tentacles 

 at the other end. 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. 

 I, 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. 



Hydra fusca. This polyp. Fig. 193; Nos. 

 6 and 7, is more stalklike than H. viridis, and 



FIG. 193 

 enlarged. 



Freshwater Polyps. Greatly 

 I to 5. Hydra viridis. 

 6 and 7. Hydra fusca. 



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