kept an aquarium to interest the boys he was tutoring, and he himself became 

 deeply engrossed in some tiny, thread-like, tentacled creatures which appeared 

 there. It is said that, in order to determine whether they were plants or 

 animals, he attempted to grow more of them from cuttings, just as we do 

 geraniums and other house plants. The creatures did grow from cuttings, 

 but Trembley observed enough of their activities while watching them to leave 

 little doubt that they were animals. He later published a book describing 

 the hydras and their marvelous powers of reduplicating or regenerating lost 

 parts. 



Hydras may usually be found attached to water plants, such as Elodea, 

 in cool and quiet water. When the plants are lifted from the water, the 

 animals contract into tiny points of jelly, hut, if they are put into a glass con- 

 tainer with pond water, they will be seen to expand again in a short time. 

 Hydras may often be collected in great quantities by bringing in a bucket full 

 of water plants and putting them into a container with just enough pond water 

 to cover them. As the mass becomes foul, which it usually does within two 

 or three days, the hydras float up to the surface in search of more oxygen and 

 may be easily taken out with a pipette. To remove them from a plant requires 

 quick action because, if given any warning, they grip their perch so securely 

 that the suction of a pipette has little effect upon them. 



A hydra in a drop or two of water with other small organisms well repays 

 observation. It reaches out for its prey with its tentacles, which shoot out 

 tiny poisoned arrows or nematocysts. The stupefied victim is grabbed by 

 these tentacles, pushed into the mouth which lies among them, and forced 

 on into the body, often distending it most grotesquely. Hydras are sometimes 

 reported to be a source of loss in fish hatcheries. Although the young fish are 

 usually too big to be swallowed by the hydras, they are often poisoned by the 

 nematocysts. 



Hydras ordinarily reproduce by budding, a protrusion from the side 

 gradually developing into a complete animal that eventually becomes detached 

 from its parent. In autumn or when living conditions become unfavorable, 

 sex organs, spermary and ovary, may appear. The fertilized egg develops into 

 a group of cells called a planula, whi:h rests over until conditions become 

 favorable for its development into a complete hydra. Many marine Coelenter- 

 ates have a peculiar method of reproduction by which the offspring resembles 

 the grandparent rather than the parent. A hydra-like animal gives rise to a 

 medusa, a bell-shaped "jelly -fish", which in its turn gives rise to more hydra- 

 like forms. Such metagenesis or alternation of generations is rare in fresh- 

 water Coelenterates, but occurs occasionally. Craspedacusta, such a medusoid 

 form, has been found several times in different places in America and Europe. 



The third group of animals under consideration may be either moss-like 

 or jelly-like. They are called Bryozoa, meaning ''moss animals", and are 



50 



