208 THE MET AZOA DIVISION OF LABOR 



Medusa. Among the most interesting of all the coelenterates inhabit- 

 ing the salt water are the jellyfishes or medusae. These animals vary 

 greatly in size from a tiny umbrella-shaped animal little larger than the 

 head of a pin to huge jellyfish several feet in 

 diameter. 



Development. Many species of medusae 

 pass through another stage of life. As medusae 

 they reproduce by eggs and sperms, that is, 

 sexually. The egg of the medusa segments, 

 forming ultimately a ball of cells (the blastrula) 

 which swims around by means of cilia. Ulti- 

 mately the little animal settles down on one end 

 and becomes fixed to a rock, seaweed, or pile. 

 The free end becomes indented in the same 

 manner as a hollow rubber ball may be pushed 

 in on one side. This indented side becomes a 

 mouth, tentacles develop around the orifice, and 

 we have an animal that looks very much like 

 the hydra. This animal, now known as a hy- 

 droid polyp, buds rapidly and soon forms a 

 colony of little polyps, each of which is con- 

 nected with its neighbor by a hollow food tube. 

 A hydroid colony of six The hydroid polyp differs from its f res h-water 

 polyps : /, feeding polyp ; GQ ^ ^ Q h d b n ^^ possessing a tough 

 r, reproductive polyp ; m. . , . , . 



a medusa; , young polyp. covenn S Wmch 1S not allVe ' 



Alternation of Generations in Coelenterates. 



The lives of a hydroid and a medusa are seen thus to be intimately 



connected with each other. A hydroid colony produces new polyps by 



budding. This we know is an 



asexual method of reproduction. 



There come from this hydroid 



colony, however, little buds which 



give rise to medusae. These me- 



dusae produce eggs and sperms. 



Their reproduction is sexual, as 



was the reproduction by means of 



eggs and sperms from the prothal- 



lus of the fern. So we have in 



animals, as well as in plants, an 



alternation of generations. 



Sea Anemone. Those who 

 have visited our New England 



coast are familiar with another 

 coelenterate called the sea anem- 

 one. This animal gets its name 



Sea anemone. About one half natural size. 

 The right-hand specimen is expanded. 

 Note the mouth surrounded by the ten- 

 tacles. The left-hand specimen is con- 

 tracted. From model at the American 

 Museum of Natural History. 



