208 



THE MET AZO A— DIVISION OF LABOR 



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

 ing the salt water are the jellyflshes or medusse. 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 medusas 

 pass through another stage of hfe. As medusae 

 they reproduce by eggs and sperms, that is, 

 sexually. The egg of the medusa segments, 

 forming ultim.ately a ball of cells (the blastula) 

 which swims around by means of cOia. Ulti- 

 mately the little animal settles down on one end 

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

 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 Uke 

 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. 

 The hydroid polyp differs from its fresh-water 

 cousin, the hydra, by usuallj' possessing a tough 

 covering which is not aUve. 



Alternation of Generations in Coelenterates. 

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

 coimeeted with each other. A hj-droid 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 medusffi. These me- 

 dusse 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 



A hydroid colony of six 

 polyps : /, feeding polyp ; 

 T, reproductive polyp ; m, 

 a medusa ; y, young polyp. 



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. 



