THE PHYLA COBLiNTBRATA AND CTENOPHORA 



183 



are usually shortened by the contraction ot muscle fibers running 

 lengthwise through them. 



Downward movement is passive, for the jellyfish is slightly heavier 

 than sea water and sinks slowly if it does not swim. As the velar and 

 subumbrellar muscles relax completely the medusa opens wide. The 

 muscle fibers in the tentacles relax and the tentacles slowly elongate. 

 Probably as a result of its shape, the jellyfish usually turns over as it 

 falls. The tentacles may trail behind, or be held out to the sides. By 

 swimming up and drifting down in this way Gonionemus "nets" for 

 food. 



The medusa needs an orienting mechanism if it is to swim upward, 

 rather than at random. The statocysts (Fig. 10.3) are sense structures 

 that determine the direction of giavity. Each is a small concretion of 

 calcium carbonate suspended on a flexible stalk in a cavity. The pressure 

 of the stone against the cells in the wall of the cavity apparently pro- 

 vides the basis for orientation. Many statocysts are embedded in the 

 margin of the medusa between the bases of the tentacles. 



Although Gonionemus lacks eyes and does not orient its body to 

 light, it sinks when the light is strong and rises when it is weak. Other 

 species of medusae have eyespots, some of which provide directional 

 information so that the jellyfish can swim toward or away from the light. 



•Tentacle 



G astro dermis 



Stomach' 



Circular canal 



Exumbrellar 

 V epidermis"^ 



-Mesoglea 



' Radial canzd 



Nerve ring- 



Velum 



Mouth V 

 • Subumbrelleir epidermis 



Figure 10.2. Gonionemus. Above, side view of whole animal, with many of the 

 tentacles incompletely drawn. (Redrawn from Mayer.) Below, diagrammatic hemisection 

 showing tissue layers; tentacles and gonads omitted. 



