GONIONEMUS 59 



6. Examine the tentacles. How are the nematoc\sts ar- 

 ranged on them? Note the sucking disc some distance from the 

 end of each tentacle. 



7. Two kinds of sense organs can be made out on the edge 

 of the medusa. See demonstration of : 



(a) Light-percipient organs. These are large, round, pig- 

 mented bodies at the bases of the tentacles. 



(b) Otocysts are small, transparent, ovoid bodies located 

 between the tentacles. In Gonionemus each otocyst consists of 

 a large vesicle surrounding the true sensory organ which is lo- 

 cated at the end of a short stalk extending into this vesicle. The 

 rounded structure at the end of the stalk contains a cavity, the 

 secondary vesicle, within which a small calcareous body called 

 the otolith is located. 



8. Make a drawing (a) from the oral surface, 70 mm. 

 (without tentacles), filling in outline one complete tentacle and 

 bases of one-third of remaining tentacles, (b) of an optical verti- 

 cal section through the radial tubes, (c) a portion of a tentacle 

 much enlarged, showing sucking disc, (d) an otocyst much en- 

 larged. 



9. Make a brief word diagram showing in a circle the life 

 history of a hydrozoan. 



REFERENCES 



Perkins, H. F., 1903. The Development of Gonionemus mur- 

 bachii. Proc. Acad. Nat. Sci. Phila. 54 750-790. 



Thomas, L. J., 1921. Morphology and orientation of the oto- 

 cysts of Gonionemus. Biol. Bull., 40:287-296. 



