THE HYDROIDS AND HYDROMEDUS^ 93 



Exercise 5. — Occurrence and Habits of a Large Hydromedusa. 



(h) The meduste of the obelia are very small and not easily studied. 

 The larger hydromedusa, Gonionemus murhachii, which is almost iden- 

 tical in structure and essentially like the obelia in its life-history, is a 

 more favorable specimen. The preserved medusa may be studied, 

 under water in a watch glass, with handlens and lowest power of 

 microscope. The appropriateness of the popular name jellyfish will 

 now be appreciated. Handle the specimen carefully, and return it 

 uninjured at the close of the period. 



(i) This medusa lives in the shallow water of protected inlets. It 

 originates by detachment from a simple hydroid colony which is at- 

 tached on the bottom. Compare with the life-cycle of the obelia. In 

 life the gonionemus is often seen ''fishing," by swimming to the surface, 

 turning mouth side uppermost, and slowly settling to the bottom with 

 tentacles widely extended. If a small fish or similar animal comes in 

 contact with the tentacles, it is quickly paralyzed by the nematocysts 

 and drawn to the mouth. Individual medusae sometimes show the 

 enteron above the hypostome greatly distended with food. Compare 

 with the hydra. What may be the advantage of such a free-living 

 stage in the life-cycle of an attached animal? 



Exercise 6. — Structure of a Large Hydromedusa. 



(j) The medusa is umbrella-shaped, with the hypostome in the 

 position of a short, thick handle. The margin bears many tentacles 

 which are well supplied with stinging cells. The velum is a circular 

 shelf projecting inward from the margin of the medusa, so that it 

 partly closes the subumbrellar cavity. Notice the four much-convo- 

 luted gonads, ovaries or testes according to the sex. The hypostome is 

 perforated by the mouth which communicates with a stomach, from 

 which extend four radial canals, one above each gonad. The radial 

 canals communicate with the circumferential canal, at the margin of 

 the disk. At the base of each tentacle is a colored eye-spot. Between 

 these organs, the statocysts, or organs of equilibrium, can be seen with 

 the compound microscope as clear vesicles. The animal is covered on 

 the outside with ectoderm, and the cavities entered through the mouth 

 are lined with endoderm as in the hydra. Between the ectoderm and 

 endoderm is a thick mass of gelatinous material, the mesoglea, which 

 corresponds to a much-thickened supporting lamella such as is found 

 in the hydra. Make a drawing ( X 4) of the animal as seen from the 

 oral or concave surface. Construct a diagrammatic vertical section 

 ( X 4) in the plane of two opposite radial canals. 



