Discussion on : 



Is there a Nervous System 



in Hydra? 



HESS: Electron microscopists say that they can't see a nervous 

 system in hydra. And some of them say that a nervous system is 

 not needed to account for the movements of hydra because the 

 muscle cells in both ectoderm and endoderm contact each other 

 allowing muscle to muscle transmission to take place. 



Dr. George Mackie from the Department of Zoology, Univer- 

 sity of Alberta, has a few slides showing some silver stains of the 

 nervous system. He might have something more convincing to 

 convey about the presence of nerve tissue in hydra. 



MACKIE: This is a brief report on the results of a recent at- 

 tempt to stain ner\'es in the body wall of hydra and Cordylophora 

 using the classical Holmes silver technique. This work is still in its 

 initial stages. I will begin with CorcJi/lophora. 



General topography of the nerve net. There is only one neuron- 

 system in Cordylophora, unlike Velella which has two histologi- 

 cally distinct plexuses. Neurons are abundant in the ectoderm of 

 tentacles (Fig. 2) and hydranth (Fig. 1). We have the following 

 figures for relative abundance of three tissue elements in a hydranth 

 preparation where all showed well: 



Epithelio-muscular cells Neurons Cnidoblasts 

 231 94 64 



Neurons also run in the ectoderm of the stem. In the hydranth the 

 neurons lie external to the muscle fiber sheet, running in the spaces 

 between the stems of the epithelio-muscular cells. They do not 

 follow the cell outlines, seen in surface view. 



6.9 



