Ill] 



AUREL1A 



77 



ever, as we have already mentioned, sense tentacles similar to those 

 of the Scyphozoa are found. In the Trachy medusae the edges of 

 the hood often join so as to form a sac enclosing the organ, whence 

 the name tentaculocyst (9, Fig. 27, II). 



It has been proved experimentally that the ordinary stimuli 

 which cause the rhythmical pulses of the bell proceed from these 

 tentaculocysts, so that they act like minute brains. The co-ordina- 

 tion of the stimuli proceeding from the eight centres is effected by 

 a very thin diffuse sheet of nerve fibrillae on the under surface of 

 the bell. These nerve fibrillae are outgrowths of star shaped cells 

 very like the nerve-cells of Hydra. 





A. 



A C 



FIG. 34. Strobilisation of Anrelia aurita. From Sars. 

 Hydra-tuba on stolon which is creeping on a Laminaria. The stolon is 

 forming new buds at 1 and 2. B. Later stage or Scyphistoma x 4. The 

 strobilisation has begun. C. Strobilisation further advanced x 6. 



D. Free swimming Ephyra stage x 7*5, seen from below. E. The same 

 seen in profile x 7- 5. 



It has been mentioned above that the reproductive organs are 

 swellings of the endoderm. The central space or "stomach" is a 

 wide sac occupying the centre part of the bell and not, as in the 

 Hydromedusae, confined to a large extent to the oral cone. In 

 Aurelia this space is produced into four lobes, and in the floor of 

 each lobe is one of the reproductive organs. From the edges of the 

 stomach a number of branching canals lead out into the circular 

 canal (4, Fig. 33), all these tubes being, as it were, burrows in a 

 continuous sheet of endoderm cells, which is formed in the same way 

 as the endodermal lamella of the Hydromedusae by the development 



