274 GENERAL ZOOLOGY 



In various details of structure Aurelia resembles the 

 medusa of certain hydroids, and also the sea anemone, the 

 coral polyp, and the fresh- water polyp. The mouth opens at 

 the center of the under-surface into a large cavity which 

 branches freely into many tubes running to the circum- 

 ference, where they join the circular canal. Between the 

 ectoderm covering the body, and the endoderm lining the 

 gastro vascular cavity, is a great mass of jelly secreted by 

 the cells of the two layers. Hanging from about the mouth 

 are four broad ribbon-like appendages called "lips." These 

 and the delicate fringe of tentacles at the rim of the bell are 

 covered with nettle cells. 



A very noticeable detail of structure that one observes in 

 watching these animals in the water, especially in the late 

 summer, is the set of four partial rings about the center, 

 yellowish in color in the males and reddish in the females. 

 These rings are the gonads. They are sacs which contain 

 the eggs or sperm until these cells are ready to be discharged. 

 On the rim of the bell, at eight equidistant points, are small, 

 rounded balancing organs. The most important muscle is 

 a circular band near the rim. The nerve cells are grouped 

 near the sense organs, and connect the latter with the 

 muscle band. When the circular muscle contracts, the 

 bell becomes more convex, and the resulting action of 

 the water in the hollow of the bell against the water that 

 is outside sends the animal along in a slow, periodic, pul- 

 sating movement. 



The development of Aurelia is very different from that of 

 the hydroids. The simple, free-swimming larva of Aurelia 

 sinks to the bottom, attaches itself to some fixed object, 

 and takes on a form resembling Hydra. The formation of 

 circular grooves below the tentacles develops a series of 

 saucer-like divisions, one within another. These separate 

 and, swimming away with the convex, aboral surface upper- 



