58 LABORATORY DIRECTIONS 



9. Examine demonstrations of undeveloped hydranths. 

 These must not be confused with old or injured hydranths which 

 have lost their tentacles. An undeveloped hydranth is a bud, the 

 free end of which is covered by a continuous layer of the perisarc. 



GONIONEMUS 



Classification : Phylum Coelenterata, Class Hydrozoa, Or- 

 der Campanulariae-Leptomedusae. 



Gonionemus is the medusoid or jellyfish form of a coelenter- 

 ate. In fundamental structure it closely resembles the medusa of 

 Obelia and is given as a type of medusoid instead of the medusa 

 of Obedlia because of its greater size which makes it more easily 

 studied. The following study is to be made upon a specimen 

 preserved in formalin: 



i. The convex face is called the ex-umbrella (aboral sur- 

 face) and the concave portion the sub-umbrella (oral surface). 



2. The velum is a membrane on the oral surface which ex- 

 tends from the margin of the body inward toward the center, 

 partially enclosing the subumbrella. In many specimens that 

 have been handled roughly this membrane may be torn. Nor- 

 mally it is perforated by a single circular opening in the center. 



3. Hanging in the center of the sub-umbrella is seen the 

 manubrium, at the extremity of which is the wide-lipped mouth. 



4. From the stomach, at the base of the manubrium, the 

 four radiating, chymiferous tubes lead to the periphery of the 

 disc where they open into the very delicate circumferential canal. 

 What is the use of these canals? Study canals in specimen with 

 ex-umbrella uppermost. 



5. The gonads hang in folds from beneath the chymiferous 

 tubes into the sub-umbrella space. The eggs or spermatozoa are 

 discharged from these directly into the water. 



