there is a groove rannin^^ towards the centre for a centimeter 

 or more along the aboral surface of the umbrella, in the bottom 

 of each groove the gelatinous layer is extremely thin. [ 



Structure of the Mar,^ina l Sense Organs . 

 The rhopalia have each a pigment spot on the aboral side 

 near the extremity and each one lies in a deep sensory niche. 

 The dorsal sensory groove common in the Pelagidae, Aurelia, 

 etc. is entirely lacking, althoigh Keller found in C. polypoi - 

 des a slightly depressed thickening of the ectoderm that cor- 

 responds to it. The sensory niche and rhopalium are^with the 

 exception of the pigment spot, similar in all essential partic- 

 ulars to what is found in Pelagia. The rhopalium is the on- 

 ly organ in the sensory niche, Fig. 44. It is a hollow, fin- 

 ger-like projection attached by its base to a low ridge that 

 runs along the roof to the central wall of the niche. This 

 ridge is penetrated longitudinally by the continuation of a ra- 

 dial carnal fi'om the stomach and into the distal end of this ca- 

 nal the lumen of the rhopalium opens. In the distal half of 

 the rhopalium the lumen is nearly obliterated by the increase 

 in thickness of its entodermal lining. Here the entoderm, in- 

 stead of being a columnar epithelium as elsewhere, is a mass 

 of parenchyma-like cells each of which contains a large calca- 



- 13 - 



