CHAPTER II 



THE CENTRAL NERVOUS SYSTEM OF MEDUSAE. 

 EXPERIMENTS ON SPONTANEITY AND CO- 

 ORDINA TION 



i. Experiments on Medusae or jelly-fish afford 

 us an excellent opportunity for analysing the con- 

 ditions for spontaneity and coordination, and for 

 deciding whether or not these phenomena are depend- 

 ent upon ganglion-cells. The subumbrella of the 

 Medusae has a very thin layer of muscle-fibres which 

 contract rhythmically. The contraction diminishes 

 the size of the swimming-bell, and forces the water 

 out. By means of the recoil the animal moves for- 

 ward. In regard to the nervous system, we must 

 discriminate between two classes of Medusae : first, 

 the Hydromedusae (Hydroidea, Fig. i), and, second, 

 the Acalephae, one representative of which (Aitrelia 

 aurita, Fig. 2) is familiar to many laymen. The 

 nervous system of the Hydromedusae consists of a 

 double nerve-ring along the margin of the umbrella 

 (d, Fig. r). The upper nerve-ring forms a flat layer 

 in the ectoderm, and consists of thin fibres and gan- 

 glion-cells. The lower nerve-ring has thicker fibres 



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