220 
T. JEFFERY PARKER. 
dozen different theories on this point, all equally supported by 
appearances. But the matter seems to me to be entirely set at 
rest by thin longitudinal sections of specimens preserved in am- 
monic bichromate, which reagent usually has the effect of causing 
a certain amount of separation between the layers. In such sec- 
tions the ectoderm cells are distinctly seen to taper off towards 
their inner ends ; the fibres to pass from them, at a sharp angle, 
towards the endoderm, or, more correctly, towards the supporting 
lamella ; and, in some cases, the fibres can be distinctly traced 
into the attenuated extremities of the cells. 
As to the true nature and functions of these structures. Dr. 
Kleinenberg calls the ectoderm cell, with its filamentous process, 
a neuro-muscle cell ; M. Korotneff prefers to name it an epithelio- 
muscle cell; Professor 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 arrange- 
ment is perfectly intelligible in a set of specially contrac- 
tile filaments, developed as a means of 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 Pro- 
fessor Huxley points out, it is absolutely necessary to assume 
contractility in the cell proper 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 shorten- 
ing, the general contractility of the cells serving for extension, 
this movement being, as observation of a living Kydra 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 Poster, and illustrated 
in the diagram at the beginning of the third chapter of his 
' Text-book of Physiology.’ These show clearly enough that the 
ectoderm cell of Hydra, with its muscular process, is the equiva- 
lent 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 combining Kleinenberg^s and 
Korotneff’s, and speaking of epithelioneuro-muscle cell; but, 
fortunately, it is unnecessary to employ any such cumbersome 
^ * Anat. of Invert. Animals/ p. 64, 
