A MEDUSA 141 



Class : Hydrozoa. Order : Trachomedusm 

 A MEDUSA (GONIONEMUS) 



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

 size, than the minute tubularian or campanularian jellyfish. It 

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

 bution 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 jellyfish to be readily turned over. If it is alive, study 

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

 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 gastrovascular space, the entodermal lining 

 of which is colored brown. 



Directly beneath the four radial canals and projecting slightly 

 into the subumbrella space are the four reproductive organs, 

 which are also brown in color and present a corrugated appear- 

 ance. The sexes are separate, but the animals are not dimorphic. 



Observe the number and arrangement of the tentacles, of which 

 an adult medusa possesses from sixty to eighty. Note the spiral 

 arrangement of the nettle cells on each tentacle, and also the ad- 

 hesive pad near its outer end. It is by means of the nettle cells in 



