

* 



CHAPTER VIII 



JELLYFISHES 



THE ccelenterates whose neuromuscular organization 

 has been most considerably studied are not the sea-anem- 

 ones, but the jellyfishes. These animals belong either 

 to the class Eydrozoa or Scypliozoa. The hydrozoan or 

 craspedote jellyfishes are relatively small, even micro- 

 scopic animals, characterized by the presence of a thin 

 membrane, the veil, which extends from the edge of the 

 bell partly across its open mouth. Because of their small 

 size they are not very favorable for experimental work, 

 and the best known from this standpoint are the larger 

 forms, such as Carmarina and Gonionemus. The scypho- 

 zoan or acraspedote jellyfishes on the other hand are 

 mostly large, sometimes very large, animals and conse- 

 quently much more experimental work has been done on 

 these than on the craspedote forms. They are charac- 

 terized by a freely open bell without veil. They include 

 such well-known representatives as Pelagia, Cyanea, 

 Aurelia, Cassiopea, and Rhizostoma. 



The neuromuscular system of a jellyfish is limited al- 

 most entirely to the edge and concave or subumbrellar 

 surface of the bell. In Aurelia, for instance, the edge of 

 the bell has upon it at regular intervals eight groups of 

 sense organs, the marginal bodies. Each group includes 

 an eye-spot, a static organ, and several other organs prob- 

 ably of the chemical sense. These receptors represent 

 specialized and thickened portions of the superficial epi- 

 thelium from which bands of nervous tissue spread cen- 

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