OBELIA 69 



each of which, from a structural standpoint, may be regarded as 

 comparable to an individual Hydra with a stalk attached, and, sec- 

 ond, the reproductive polyps, or gonangia, which have become 

 greatly modified for the purpose of asexual reproduction and are 

 dependent upon the nutritive polyps for their nutrition. 



The body wall of a hydranth is composed, like that of Hydra, of 

 an outer layer of ectoderm and an inner layer of endoderm. Be- 

 tween these two layers there is a thin layer of the non-cellular 

 mesogloea. Tentacles are present and surround a central mouth 

 opening which leads into the enteric cavity. The living coenosarc, 

 which is continuous throughout the colony, has the same structure 

 as does the body wall of a polyp, and in fact it is to be regarded 

 as a continuation of the ectoderm and endoderm. In other words, 

 the bodies of the hydra-like polyps can be thought of as having 

 become differentiated into an anterior portion, the hydranths 

 proper, and a greatly elongated, posterior portion, the coenosarc. 

 Surrounding the entire colony is the non-living perisarc, formed 

 as a secretion by the ectoderm cells. 



In a young colony of Obelia, all of the polyps are of the nutritive 

 type, but, as a colony gets older, the gonangia are formed, which 

 are specialized for the sole purpose of asexual reproduction. They 

 are club-shaped and consist of an outer covering (gonotheca) 

 which is a continuation of the perisarc, and an inner portion 

 (blastostyle) which is a continuation of the living coenosarc of 

 the stalk. On this central, club-shaped blastostyle, a number of 

 little swellings, the medusa buds, appear, each of which develops, 

 while still attached to the blastostyle, into a tiny jellyfish, or 

 medusa. When fully formed, the medusae become detached from 

 the blastostyle, float out through an opening in the distal end of 

 the gonotheca, and begin an independent existence. 



B. Structure of the Sexual Stage of Obelia 



The sexual medusae of Obelia are very small, and resemble, in 

 shape, a flattened dome or umbrella. The outer convex surface is 

 called the ex-umbrella, or aboral, surface. Attached to the edge of 

 the umbrella-shaped body are numerous tentacles. The under, 

 concave surface of the body is called the sub-umbrella, or oral, 

 surface. The opening on the oral surface is partly closed by a 

 circular, perforated membrane, the velum, which is attached near 

 the base of the tentacles. (W. f. 37, B, C; 38, B.) 



