Ill] 



HYDROMEDUSAE 



63 



the upper and lower layers, respectively, of the velum, constitute an 

 upper and a lower nerve ring. Each thread is to be regarded as the 

 tail of an excessively small ectoderm cell. This nervous system 

 differs from the nerve cells which were described in Hydra in that 

 the processes of the cells are longer and finer in proportion to the 

 size of the body of the cell and that the cells are more numerous and 

 that their processes run more or less parallel to one another. Certain 

 of the nerve cells have their bodies still wedged in between neigh- 

 bouring ectoderm cells : in this case the nerve process issues from 

 the base of the cell and the cell is termed a sense cell. 



II. ,6 



FIG. 27. I. A. Eye of Lizzia koellikeri seen from the side, magnified. B. The 

 same seen from in front. C. Isolated cells of the same. From 0. & R. 

 Hertwig. 



1. Lens. 2. Pigment cells. 3. Percipient cells. 



II. Radial section through the edge of the umbrella of Carmarina hastata 

 showing sense organ and velum. 



1. Velum. 2. Jelly. 3. Circular muscles of velum. 4. Upper nerve 

 ring. 5. Lower nerve ring. 6. Nematocysts. 7. Radial vessel 

 running into circular vessel, both lined by endoderm. 8. Continuation 

 of endoderm along aboral surface. 9. Sense organ or tentaculocyst. 



10. Auditory .nerve. 



In Hydra we found the earliest appearance of sense hairs ; and 

 the cells of which they are processes, viz., the cnidoblasts, may be 

 called sense cells, although they possess no nerve processes or fibres. 

 In the Medusa we meet with definite collections of sense cells 

 aggregated so as to form sense organs. These are found close to 

 the position of the nerve ring, either on the velum itself or immedi- 

 ately outside it at the bases of the tentacles, so that the stimuli 

 which they receive are easily transmitted to the nerve ring. Two 



