70 THE BIOLOGY OF HYDRA : 1961 



Neuron types. Structurally there seems to be little difference 

 between neuro-sensory elements and purely nervous elements. Ap- 

 proximately one in eight neurons has a process running up to the 

 surface with a hair projecting externally (Fig. 3, 4, 8, 9), but of 

 those which are entirely sub-epithelial the majority have what seems 

 to be a reduced or rudimentary sense hair projecting into the sur- 

 rounding tissue space (Figs. 4, 7). It is possible that such cells 

 are modified sensory elements that have become or are becoming 

 transformed into neurons in the strict sense. However, this does 

 not exclude the possibility that they retain a sensory function, serv- 

 ing for instance to record deep touch or to give position sense. 

 The fibrous processes or neurites are similar in all these elements, 

 whether the cell has a hair or not. 



Interconnections. The neurites associate freely, running side by 

 side for long or short stretches, but there is nothing to suggest that 

 they regularly form continuous connections. This nervous system 

 is quite unlike the closed system of Velella which shows every sign 

 of being a syncytium. It is much more like the non-syncytial open 

 system of Velella. The only evidence for continuous connections 

 is that here and there one finds binucleate neurons and in some 

 places there are suspicious-looking pairs of neurons which could 

 be the two halves of a binucleate pulled apart, but still in primary 

 connection. This gives me the opportunity to insert a remark 

 about the retention of primary connections between cells which 

 was discussed earlier, following the paper by Hess. Such connec- 

 tions have long been known in a variety of coelenterate cell types 

 including young cnidoblasts, interstitial cells and epithelio-muscular 



Ectodermal nervous system of Cordylophora (Figs. 1-9) and hydra (Figs. 

 10-12) as seen in silver-stained whole mounts. Scale indicates 10 m/^. 

 Fig. 1. area of hydranth wall showing parts of five neurons; Fig. 2. neurons 

 in a tentacle; Fig. 3. neuro-sensory cell; Fig. 4. the three types of neurons; 

 Fig. 5. nerve fibers in contact with young cnidoblast; Fig. 6. bipolar ganglion 

 cell; Fig. 7. well-extended neurons in expanded epithelium; Fig. 8. neuro- 

 sensory cell showing root of hair in cytoplasm; Fig. 9. neuro-sensory ceil; 

 Fig. 10. neurosensory cell in contact with cnidoblast; Fig. 11. nerve fiber 

 tract: only two out of four fibers are in focus; Fig. 12. bipolar ganglion 

 cell. 



Abbreviations: en. cnidoblast; g. ganglion cell; hs. subepithelial hair; 

 n. nerve fiber; p. process of neuro-sensory cell running to surface carrying 

 external hair. 



