STINGING-CELLS IN CRASPEDOTE MEDUS/E. 777 



pouches at the interradii (PI. 43^ fig. 7, r.p.e}). The ring- 

 canal is formed by the fusion of the distal ends of the radial 

 pouches at the bases of the bulbous swellings referred to 

 above. 



Up to this point the histology of the two layers has been 

 quite constant; the superficial ectoderm has retained its 

 original character and remains crowded with interstitial cells 

 of all kinds, in striking contrast with the small-celled regularly 

 arranged tissues derived from the entocodon. The endoderm 

 lining both the cceleuteron and the radial pouches consists 

 of large clear cells, with somewhat indefinite outlines and 

 containing numerous large nutritive spheres, which stain 

 deeply with iron-hsematoxylin ; a few irregularly shaped 

 interstitial cells are to be found, most numerous between the 

 endoderm cells lining the manubrium. 



In the last stage of the development described above we 

 found all the organs of the adult medusa already well defined, 

 with the exception of the tentacles. From this point onwards 

 the more important changes are to be found in the structure 

 of the umbrella, whicli now grows rapidly, especially in the 

 region between the ocellar bulbs and the base ot" the manu- 

 brium, so that the superficial ectoderm loses its characteristic 

 features, as noticed above, and gives rise to a low, small- 

 celled epithelium covering the external surface of the bell. 

 The endoderm behaves in a somewhat similar fashion. The 

 ocellar bulbs, however, remain unaltered ; the endoderm still 

 consists of large irregular cells with nutritive spheres ; the 

 ectoderm is still crowded with interstitial cells, thread-cells, 

 and nematoblasts, the latter increasing rapidly and forming- 

 new nematocysts, both large and small (Text-fig. 5). 



The ocellar bulbs give rise to the tentacles, their main 

 function being obviously that, already mentioned on p. 768, 

 of supplying these organs with stinging-cells. 



In the preceding pai-agraphs I have tried to emphasise the 

 fact that beyond an increase in the actual number of cells, 

 the ectoderm of this region has remained practically un- 

 changed throughout the development of the gonophore. The 



