REPRODUCTION 



173 



Medusae are bell-shaped individuals (right in figure) which after maturity 

 become separated from the colony and swim freely in the water by means 

 of rhythmic contractions of the bell. Each medusa produces eggs or 

 spermatozoa. The fertilized egg develops into a ciliated free-swimming 

 embryo which eventually attaches itself by one end to a rock and develops 

 into a polyp. This polyp puts forth buds and thus a new colony is 

 formed. Though polyps and medusae are so different in gross form as to 

 have been regarded as different species before the production of one 

 by the other was known, yet the general plan of their bodies is the same. 

 In Fig. 143, by turning the polyp upside down, and introducing an 

 imaginary form between, the scheme of structure is showm to be ahke 



OVARY 



-MEDUSAE 



TESTIS 



HYPOSTOME 

 GONOTHECA 

 MEDUSA BUD 



Fig. 144. — Diagram of life cycle of Obelia, illustrating metagenesis. 



in both. The cavity or enteron does not always enter the tentacles; 

 in some polyps and medusae the tentacles are solid chains of cells. 



Obelia forms a colony somewhat resembling Bougainvillea. In 

 Obelia, however, the medusa buds are produced by budding from the 

 gtalks of certain individuals (blastostyles) which, unlike polyps, have no 

 tentacles. Each blastostyle is enclosed in a swollen chitinous sheath, 

 the gonotheca. Blastostyle, attached medusa buds, and gonotheca 

 together are often designated the gonangium. Obelia is thus composed 

 of three types of individuals, two of which are sessile and incapable of 

 sexual reproduction, w^hile the other is a sexual free-swimming form 

 (Fig. 144). 



