62 



Mr. T. J. Parker. 



[Dec. 18, 



and using various methods of preparation, might readily frame a 

 dozen different theories on this point, all equally supported by ap- 

 pearances. But the matter seems to me to be entirely set at rest by 

 thin longitudinal sections of specimens preserved in ammonic bi- 

 chromate, which reagent usually has the effect of causing a certain 

 amount of separation between the layers. In such sections (fig. 1) 

 the ectoderm cells (ec.) are distinctly seen to taper off towards 

 their inner ends ; the fibres (m.p.) to pass from them, at a sharp angle, 

 towards the endoderm, or, more correctly, towards the supporting 

 lamella ; and, in some cases (e.g., the fibre to which the line from 

 m.p. points), the fibres can be distinctly traced into the attenuated 

 extremities of the cells. 



As to the true nature and functions of these structures, Dr. Klein- 

 enberg calls the ectoderm cell with its filamentous process, a neuro- 

 muscle cell; M. Korotneff prefers to< name it an epithelio-muscle cell; 

 Prosessor Huxley* considers that the fibres " are solely internuncial in 

 function, and therefore the primary form of nerves." This last view is 

 rendered, to say the least, decidedly improbable, by the great number 

 and the regular disposition of the fibres. It seems, a priori, unlikely 

 that an animal devoid of all muscular tissue should have a layer of 

 close-set longitudinal nerve-fibres throughout its whole body, while 

 such an arrangement is perfectly intelligible in a set of specially 

 contractile filaments, developed as a means ©f rapid retraction of the 

 body. 



The term " neuro-muscular" implies, as Kleinenberg explains, 

 that the process only is contractile ; the function of the cell itself being 

 merely to receive and transmit impressions. But, as Prof essor Huxley 

 points out r it is absolutely necessary to assume contractility in the 

 cell prop-er, to account for the lengthening of the body. The fibres 

 merely have a special degree of contractility assigned to them, in 

 correspondence with the obvious advantage accruing to the animal 

 from the power of instantaneous shortening, the general contractility 

 of the cells serving for extension ; this movement being, as observa- 

 tion of a living Hydra shows, a comparatively slow one. The fibres 

 must also be of use in the characteristic " looping " movements of the 

 animal. 



The simplest and most reasonable way of looking at these structures 

 is that adopted by Dr. Michael Foster, and illustrated in the diagram 

 at the beginning of the third chapter of his " Text-book of Physi- 

 ology." These show clearly enough that the ectoderm cell of Hydra, 

 with its muscular process, is the equivalent of what, in the higher 

 animals, becomes sensory cell, sensory nerve, nerve cell, motor nerve, 

 and muscle cell. So that a fairly logical term might be made by 



* " Anat. of Invert. Animals," p. 64. 



