AURELTA. 37 



HYDROCORALLINA. 



To this group belong forms that have heavy calcareous exo- 

 skeletons. While material is generally not at hand to study 

 the polyps, it is desirable to study and sketch the characteristic 

 forms of colonies such as Millepora and Stylaster, and to note 

 the difference in the distribution of pores. Later you will see 

 how decidedly these differ from the ordinary stony corals. 



SIPHONOPHORA. 



Examine living or preserved specimens of Physalia, and 

 sketch the type with reference to showing, if possible, the follow- 

 ing structures: (a) pneumatophore, (h) dactylozooids, (c) gastro- 

 zooids, (d) gonodendrons, (e) tentacles. It will be well to refer 

 to a text-book to find the positions and functions of each of 

 these. 

 Bigelow: The Siphonophorse. Mem. Mus. Comp. Zool., Harvard, 38, 1911. 



SCYPHOZOA. 



AURELIA. 



This form is one of the common jelly-fishes, 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 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 suh- 

 umbrellar (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. 



