284 THE ANIMAL KINGDOM 



stalk 



Statocysfc 

 csLvity 



Limestone 

 Secretion 



Figure 10.3. Detail of a statocyst, located in the margin of the bell between the 

 circular canal on the left and the epidermis on the right. (After Hyman.) 



The temperature and pH of the water may also influence the average 

 depth at which the jellyfish stays. If the temperature or pH increases the 

 medusae move to greater depths, while if the temperature or pH de- 

 creases they rise toward the surface. At night the light is greatly decreased 

 and the pH of the water falls slightly so that the medusae are found 

 closer to the surface than in the daytime. Similar diurnal migrations are 

 performed by many pelagic organisms, and are especially marked in 

 certain crustaceans that migrate several hundred feet vertically. 



Sense receptors for temperature and pH are probably scattered 

 diffusely around the margin of the umbrella, a region known to be sen- 

 sitive to certain chemicals. The activity of Gonioneinus increases mark- 

 edly when the juice of food organisms is added to the water. It swims 

 horizontally as well as upward, keeping its tentacles extended in a ran- 

 dom search for the prey. 



Many other medusae have behavior patterns like that of Gonio- 

 nemus. Gonionemus and its close relatives can attach themselves to 

 marine plants by means of adhesive pads on the tentacles (Fig. 10.2). 

 They live primarily in shallow water where rooted vegetation is abun- 

 dant, and are often found in the daytime attached by a few of their 

 tentacles with the rest outstretched in the netting position. This adapta- 

 tion to shallow water is exceptional, and most medusae remain afloat 

 all of the time. 



61. Gonionemus: Feeding and Digestion 



Nematocysts. When a small organism brushes against an out- 

 stretched tentacle it is stung, and in its violent reaction to being stung 

 it may throw itself against more tentacles. Further stinging paralyzes 

 the prey, which is tightly held by the tentacles. Each tentacle has numer- 

 ous rings of projecting stinging cells visible under the microscope (Fig. 

 10.4/i). Within each of these is a shiny oval body, the nematocyst (Fig. 

 10.5), shaped like a tiny balloon with a very long tubular neck, the 

 nematocyst thread. As the nematocyst develops within the stinging cell 



