Chap. 23 COELENTERATES SIMPLE MULTICELLULAR ANIMALS 



479 



HYORANTH 



TENTACLES 

 MOUTH 



PLANULA 



THE ASEXUAL GENERATION 



Fig. 23.10. The structure and life cycle of Obelia, a marine colonial hydroid. 

 The mature colony is about one inch high with swollen joints from which the 

 branches, vegetative and reproductive individuals, are given off alternately. A, a 

 mature colony, the asexual generation. B, the minute jellyfish or medusa (greatly 

 enlarged), a free swimming sexual individual, of which there are males and females 

 developed in different colonies. C, section through a vegetative individual (hy- 

 dranth) showing the gastrovascular cavity that extends throughout the colony. D, 

 the early development of a colony, from the fertilized egg, through the free swim- 

 ming ciliated planula, to the young attached colony. (Courtesy, MacDougall and 

 Hegner: Biology. New York, McGraw-Hill Book Co., 1943.) 



produces new individuals (feeding and reproductive polyps) by budding. The 

 next generation, medusae, is sexual and by the fusion of sex cells produces the 

 first polyp of a new colony. In this cycle, an individual is the image of its grand- 

 parents but looks like a stranger to its parents. 



Habitats. Various species of Obelia live on both coasts of North America. 

 Colonies of them, an inch or two high, grow by millions on the long ribbons of 

 kelp and other seaweeds. Attached and branched as they are, even a good 

 observer might well take them for plants just as Aristotle did. They can be 

 examined satisfactorily only with a strong lens. 



The Colony — Its Form and Way of Living. The colony is held fast by 

 the root-shaped hydrorhiza and from this springs the upright branch (hydro- 



