468 EVOLUTION OF ANIMALS Part V 



in tufts on rocks and seaweeds (Fig. 23.1) and the hydrocorallines, among 

 them the "stinging corals." The class also includes the Siphonophora, the 

 Portuguese man-of-war, and others that live in the open sea and have no 

 sessile stage. 



ScYPHozoA. Larger medusae or jellyfishes with notches in the margin of 

 the umbrella, as in the common jellyfish, Aurelia. 



Anthozoa. These are either solitary or colonial coelenterates, with a great 

 development of the polyp and no medusoid stage. Figure 23.1 suggests the 

 form of the sea anemones, the brown anemone, Metridium, and the true corals. 



Hydra — A Representative of Simple Multicellular Animals 



Hydra is a link between older and newer ways of living. It digests its food 

 partly by the old method of the ameba, partly by the newer methods of the 

 grasshopper, frog, and man. Many of its characteristics are like those of higher 

 animals, but simpler. 



All hydras live in fresh water. They look like bits of coarse thread frayed 

 out at one end, are semi-transparent and, except the green ones, are almost 

 colorless. Their movements are visible to the naked eye and they are easily 

 examined with the microscope. They are also common, widely distributed, and 

 easily kept in aquaria. Only when they are undisturbed in considerable space 

 do they display the deliberate grace of their searching tentacles and their sud- 

 den capture of minute water animals. 



Ecology. Hydras live in sunlit pools, hanging from submerged plants and 

 decayed vegetation. With the help of a gas bubble at the base of the body they 

 are often buoyed up against the underside of the surface film (Fig. 23.2), 

 Enormous numbers occasionally appear in lakes as they have done at Douglas 

 Lake, Michigan, when the seines spread for fishes have been weighed down 

 by the millions of hydras clinging to them. Under certain peculiar conditions, 

 they may turn red, especially toward fall, and large patches of pond surface 

 may be colored by them. Other aquatic organisms do this; the redness of blue- 

 green algae gave the Red Sea its name. 



Hydras reach their full activity in summer and then they frequently produce 

 buds asexually. They usually reproduce sexually toward the end of the season 

 on a lowering temperature, down to 50° F. They make a definite adjustment 

 to winter temperatures. Brown and green hydras collected in winter from ponds 

 in which the temperatures were 46° to 56° F. and placed in pond water at 

 35° F. contracted into balls and stayed so for two weeks, as long as the water 

 was kept at the same degree of cold. When it was warmed to 46° F. they 

 stretched out and began feeding. Active and semiactive hydras are certainly 

 not confined to summer conditions. Various species with flourishing growths 

 of buds have been found thriving beneath the ice. 



Food. Hydras are carnivores that forage freely on protozoans and crusta- 



