GONIONEMUS 175 



HYDROZOA 



A TRACHOMEDUSA (Gonionemus) 



This animal is a better form to study, on account of its larger 

 size, than the minute tubularian or campanularian medusse. It 

 is a very common medusa at Woods Hole, but its range of 

 distribution is very limited although it has also been found in 

 Long Island Sound. 



Place the medusa in a small dish of water, which should be 

 set upon a dark background. The water should be deep enough 

 to permit the jelly-fish to be readily turned over. If it is alive, 

 study the pulsations of the bell, by means of which it swims. 

 Note the inverted position of the animal when at rest. With a 

 simple lens or a compound microscope study its form and color. 

 Note the radiate type of structure. Unlike the bilaterally sym- 

 metrical animals, the medusa has no dorsal, ventral, anterior, or 

 posterior side. 



The outer, convex surface of the bell-shaped body is called 

 the exumbrella or the aboral side, and the concave underside is 

 called the subumbrella or the oral side. From the center of the 

 latter extends a large, dark-brown projection called the manubrium, 

 at the distal end of which is the mouth, surrounded by four re- 

 curved lips. At the base of the manubrium is the stomach, a 

 four-sided sac from the four corners of which the four straight 

 radial canals extend to the periphery of the body, where they are 

 united by the ring canal, which runs around the margin of the 

 bell. The radial and ring canals, together with the stomach and 

 the cavity of the manubrium, form the gastro-vascular space, the 

 entodermal lining of which is colored brown. 



