80 LABORATORY STUDIES IN ZOOLOGY 



free life. Gamic reproduction prevails in this type of individual 

 and the sexes are distinct. 



The medusae of Obelia are small. The medusa of another 

 hydrozoan, Gonionenms murbachii, has been chosen for the study 

 of a typical medusa because of its fair size. It is found in con- 

 siderable numbers through the summer in the vicinity of Woods 

 Hole, Mass. 



Structure. — Study a specimen in a watch glass with a dissecting 

 microscope. Use great care in turning the animal over for 

 different views. Never use forceps. A camel 's-hair brush serves 

 the purpose much better. 



Notice the umbrella-shaped body with tentacles hanging free 

 from the margin and the hypostome hanging down within the 

 space formed by the concave surface of the umbrella. The 

 hypostome is the same structure already observed in Hydra and 

 Obelia. The convex surface of the umbrella is called the 

 exumhrella (or aboral surface), while the concave portion is 

 termed the suhumbrella (or oral surface), 



A circular muscular shelf, the velum, extends inward from the 

 edge of the umbrella. By gentle contractions of the umbrella 

 (or bell) and of the velum, water is driven out of the subumbrella 

 cavity, thus effecting locomotion. 



To understand the relation of parts comprising the food- 

 distributing system place the animal with aboral side uppermost. 

 Note the quadrangular area visible through the transparent 

 tissues; this area marks the location of a relatively large saclike 

 structure at the base of the manubrium, the stomach, from the 

 angles of which four delicate, equidistantly spaced radial canals 

 lead to the edge of the umbrella where they open into the circum- 

 ferential canal following a circular course about the margin of the 

 bell. These canals serve as a food-distributing system and 

 together with the stomach represent, as far as spaces are con- 

 cerned, the gastrovascular cavity of the nutritive polyps. 



On the subumbrella side of the radial canals note the corrugated 

 reproductive organs or gonads which may be either spermaries or 

 ovaries. 



Observe the mode of distribution of nematocysts on the 

 tentacles and compare in this respect with tentacles of Hydra 

 and Obelia. An abrupt bend near the distal end of the tentacles 

 of Gonionemus indicates the location of a cuplike adhesive 



