18 NERVOUS SYSTEMS AND SENSE ORGANS 



In Lizzia. the Hertwig brothers, '78, found the tentacles 

 jrrouped, and at the base of these the nerve cord is swollen, due to 

 a concentration of ganglion cells. The suggestion has been made 

 that the two nerve rings have different functions; the upper one 

 connected with the sense organs, the lower being near the muscles 

 gives nerves to them. 



Loeb found that if the bell without the nerve ring be placed 

 in five-eighths per cent NaCl or five-eighths per cent NaBr. it goes 

 on beating rhythmically, but small quantities of CaCl, or KCl or lx)th 

 added caused the bell to stop contracting. The bell would beat in 

 sea water if not for Ca or K, and possibly some other ions. 



When two specimens of medusae are grafted together after the 

 nerve rings are removed, the two portions contract as one and not 

 from two centers of contraction. 



Krasinska, '14, in Connarhia finds large and small ganglion 

 cells and two kinds of sensory cells. The ganglion cells are mostly 

 multipolar and in a sub-epithelial region nerve elements are also 

 found in the tentacles; large ganc^lion cells are found in the sub- 

 umbrella and small in the tentacles. The velum is enervated by 

 fibers from the inner nerve ring. She does not decide whether there 

 is a true nerve network because she found but few cases of anasto- 

 mosis. The large ganglion cells of the sub-enithelial plexus are con- 

 sidered to be motor, also the smaller ganglion cells of the tentacles. 



In a hydroid medusa, Tiaiopi^is, Romanes found that the manu- 

 brium reaches over to a spot .stimulated by touch. Romanes found 

 that this movement continued after the margin with the nerve ring 

 was removed. 



Loeb explains the coordinating movements of medusae In- 

 simple irritability and conductivity without attributing other spe- 

 cial functions to the ganglion cells exce|it those which occur in all 

 conducting protoplasm. 



Yerkes, '02, determined that the medusa Guiiiiiucmii.^ has a 

 delicate chemical sen.se. All portions of the lx)dy except the velum 

 and exumbrella are sensitive to chemical and mechanical stimuli. 

 The tentacles are especially sensitive to nhotic stimuli. The inten- 

 sity of the .stimulus determines the (uiickness. duration and extent 

 of a reaction. Stimuli which affect s\ inmetrical ))oints of the body 

 uneoually have a directive influence noon the movements. Yerkes 

 concludes that the reactions of si'ecial Parts of Giinioiicmiis are not 

 dependent for their execution upon the functional activity of the 

 central nervous sy.stem. Irritabilitv is a property of all parts of 

 the animal except the iellv of the bell and the exumbrella surface, 

 bi't it differs widely in different regions. 



As lyoeb suggests spontaneitv is not dependent UPon the central 

 nervous system but upon a hii^h degree of irritabilitv of cei'tain 

 parts of the margin of the bell, Thos" <"e<itiiens with the marginal 



