732 COMPARATIVE PHYSIOLOGY. 



respond to external stimuli. In Sponges, the transmission of 

 stimuli is effected by the general protoplasm of the cells — 

 little division of labour being apparent—though here and 

 there so-called nerve cells have been described. 



Among the Ccelentera, we find in Hydra special nerve 

 cells, but, as proved by the familiar regeneration experi- 

 ments, these are all similar and equivalent. On the other 

 hand, among the "jellyfish," we find nerve centres and 

 nerves quite distinctly differentiated. As we should expect, 

 the nerve physiology differs in the Craspedota and the 

 Acraspeda. 



In the Craspedote forms the nervous system consists of a 

 ring round the margin of the bell, giving off nerves which 

 form a plexus among the muscles, and furnished with shght 

 thickenings — the marginal bodies — at the bases of the 

 tentacles. The ring controls the movements of the swim- 

 ming bell ; if it is totally destroyed the movement ceases, 

 but the retention of a very small part is sufficient to main- 

 tain the movement. The parts of the ring are apparently 

 equivalent to each other, any part being capable of trans- 

 mitting motor impulses to the whole of the muscles effecting 

 movement. The thickened areas of the ring seem to have 

 a sHghtly more powerful effect than the undifferentiated 

 parts, but the difference is not very marked ; the marginal 

 bodies are, however, distinctly sensitive to fight. If a strong 

 beam of light be thrown upon a swimming bell, it responds 

 by more active contractions, and as the organisms are more 

 active in light than in darkness, we may conclude that light 

 (along with heat) acts as a constant stimulus. If the nerve 

 ring is totally destroyed, the animal becomes motionless, 

 and does not recover itself; if stimulated electrically or 

 mechanically, it responds by a single contraction, or occa- 

 sionally, in very vigorous specimens, by several. 



In the Acraspeda the eight separate nerve centres preside 

 over the swimming movements ; if these are all destroyed, 

 the movements cease. If the specimen is vigorous, how- 

 ever, it not infrequently, after a period of rest, resumes its 

 movements, sometimes only feebly, sometimes with a speed 

 quite comparable to that of an uninjured specimen. If 

 stimulated during the latent period, the Medusa usually 

 responds with more than one contraction, thus being again 



