PHYSALIA, AURELIA 55 



following structures: (a) pneumatophore, {b) dactylozooids, 

 (c) gastrozooids, (d) gonodendrons, (e) tentacles. It will be 

 well to refer to a textbook to find the positions and functions 

 of each of these. 



Bigelow: The Siphonophorae. Mem. Mus. Comp. Zool., Harvard, 38, 

 1911. 



SCYPHOZOA 



AURELIA 



This form is one of the common jellyfishes, and is found 

 floating freely in the water. It is frequently washed up on 

 shore. To be appreciated these medusae should be seen as 

 they occur at the surface of the sea, before they have been 

 handled or injured. Frequently vast numbers may be seen 

 together, all gently pulsating and thus keeping near the sur- 

 face. The movement is very different from that of most 

 hydrozoan medusae, being very deliberate and graceful. 



If living material is offered, study the method of locomo- 

 tion and compare it with the locomotion of Gonionemus. 

 Like the latter, the discoid animal exumbrellar (aboral) and 

 subumbrellar (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 exumbrellar surface presents little of interest. In the 

 live specimens, however, you should 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 subumbrellar sur- 

 face: 



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

 circular or regularly indented? Are all of the marginal por- 

 tions similar? 



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



