listed in the bibliography, tells of Trembley himself 

 and of his work with hydras and other microscopic 

 animals of fresh water. Hydras may be readily col- 

 lected in nature or purchased from biological supply 

 companies, and they can be cultured in the home if 

 kept in pond water, maintained at temperatures be- 

 low 75°F., and fed on small crustaceans such as 

 daphnias. Detailed directions for rearing many kinds 

 of invertebrates are given in the Galtsoff book listed 

 in the bibliography. 



THE TRACHYLINE MEDUSAS 



A jellyfish in fresh water once seemed as anoma- 

 lous as did a black swan to the ancients. Yet the 

 black swan eventually turned up in Australia. And 

 in 1880 a fresh-water jellyfish was discovered in the 

 tank in Kew Botanical Gardens, near London, in 

 which were kept the giant water lilies from the Ama- 

 zon. It was named Craspedacusta sowerbyi, and af- 

 terward more were found in other botanical gardens 

 in Europe, confirming the impression that it had been 

 brought with water lilies from Brazil. During recent 

 decades it has turned up on other continents, and is 

 frequently but sporadically reported from all over 

 the United States. Either it is becoming more wide- 

 spread, or more people are aware of its existence and 

 are keeping an eye out for it, especially in the most 

 likely season, from July to October. It seems to favor 



The commonest jellyfish encountered by amateur 

 yachtsmen along the Atlantic coast of America is 

 Gonionemus miirbachii, whose tentacles and dark 

 sense organs around the rim of the bell mark a circle 

 as much as an inch in diameter. The cross-shaped 

 marking in the dome of each bell is produced by the 

 ruffled reproductive organs. (Massachusetts. Lorus 

 and Margery Milne) 



^ 



artificial bodies of water or those of limited size, such 

 as aquaria, ditches, old flooded quarries, impounded 

 streams, and small ponds, perhaps because it does 

 best where plankton is very rich; but it has been 

 found also in small lakes. No written account or col- 

 ored plate of a beautiful or bizarre coelenterate from 

 the ocean deeps can substitute for the thrill of look- 

 ing over the edge of a rowboat, a thousand miles 

 from the nearest salt water, and seeing little trans- 

 parent hemispheres, 1 inch across at most and usu- 

 ally smaller, pulsating in the water. Removed to a 

 large jar or an aquarium, where one can watch them 

 at leisure, they are seen to have several sets of ten- 

 tacles and to hold one group erect or obliquely up- 

 ward, the others horizontal or extending downward. 

 There is a shelflike velum extending in from the um- 

 brella, and four radial digestive canals. From each of 

 these hangs a baglike sex organ that looks like a little 

 pistol holster. Though usually seen in small groups, 

 all of one sex, they may occur by the thousands. 

 They are known to feed on rotifers and various small 

 crustaceans. The polyp stage, originally described 

 and named as Microhydra ryderi. lives on the bottom 

 as a minute ( ' lo to '3 of an inch) colony of several 

 polyps lacking tentacles. A good account can be 

 found in Pennak's Fresh-Water Invertebrates of the 

 United States. Luunocnida, a similar jellyfish, is 

 known from lakes and streams in Africa. 



The fresh-water jellyfishes seem to be allied to the 

 trachyline medusas, a marine group that differs from 

 the hydrozoan medusas mentioned earlier in having 

 a very minute polyp or none at all. It also differs in 

 the nature of the sense organs found around the rim 

 of the umbrella, which are thought to have a balanc- 

 ing function. A well-known marine member is Go- 

 nionemiis, familiar to all students of biology from 

 laboratory experience with preserved specimens. In 

 nature it uses the adhesive pads near the tips of the 

 tentacles to cling to the eelgrass among which it lives. 

 When the light is not too bright or too low, Gonio- 

 itemiis fishes for its food by swimming in rapid pul- 

 sations to the surface, turning upside down, and then 

 coasting slowly downward with its tentacles out- 

 spread in a wide net. 



THE HYDROCORALS 



The hydrocorals are a wholly marine group that 

 includes the "stinging corals," likely to be long re- 

 membered by any inexperienced collector in warm 

 waters who tries to break off what may look like an 

 innocent and beautiful bouquet of light pink branch- 

 ing coral. They used to be lumped with the hydroids, 

 which they resemble in many ways, but hydrocorals 

 secrete massive skeletons of limestone, either erect 

 and branching or leaflike, or low and encrusting. 

 The millepores, which are white or of pale fleshy or 

 yellowish tones, contribute a good share to the for- 



V, 



