36 COELENTERATA. 



or injured. Frequently vast numbers may be seen together, all 

 gently pulsating and thus keeping near the surface. The move- 

 ment is very different from that of most hydrozoan medusae, 

 being very deliberate and graceful. 



If living material is offered, study the method of locomotion 

 and compare it with the locomotion of Gonionemus. Like the 

 latter, the discoid animal presents ex-umbrellar (aboral) and sub- 

 wnbrellar (oral) surfaces, but the edges of the disk are indented, 

 fringed with very numerous short tentacles, and a velum is 

 wanting. What difference does the velum make in locomotion? 



The ex-umbrellar surface presents little of interest. In the 

 live specimens, however, prove that the animal is sensitive over 

 this area as elsewhere. 



Preserved and hardened material is better than living for the 

 study of the rest of the anatomy of this form. With a specimen 

 in water in a finger-bowl, with a black tile for the background, 

 find the following from the sub-umbrellar surface : 



1. The shape of the animal. Is the margin perfectly circular 

 or regularly indented? Are all of the marginal portions similar? 



2. Four large, fringed oral arms or lips hang from the corners 

 of the nearly square mouth, which is located in the center. No- 

 tice how each arm is similar to a long, narrow leaf, with the sides 

 folded, especially along their margins. Examine the arms for 

 nematocysts. Do you understand how the animal gets its food? 

 If the arm edges appear to be covered with dark specks and 

 granules, examine to see if embryos may not be entangled. 



3. The mouth is found to lead by a short gullet into a rather 

 spacious stomach, which is produced in the region between each 

 two corners of the mouth to form a gastric pouch. In each of 

 the pouches are a number of gastric filaments. Determine the 

 shape of the stomach. 



4. The remaining parts of the digestive (and also in this 

 case circulatory) system include the numerous radial canals 

 and the single circular canal. 



(a) Directly beneath each oral arm a per-radial canal is given 

 off, which, at a short distance from the stomach, gives off a 



