reous concretion, a so-called otolith. A thin supporting 

 membrane separates the entoderm from the ectoderm. At the 

 distal extremity of the rhopalium the ectoderm is a thin cu- 

 boidal epithelium, while over the rest of the surface it is a 

 thick sensory epithelium resting on a thick network of fine 

 nerve fibres. This, in turn, rests on tlie supporting mem- 

 brane. I have observed no ganglion cells in this layer of 

 nerve fibres, which is continued under the epithelium of the 

 rhopalial ridge to the central wall of the niche where it be- 

 comes imperceptible. There are no thickened bands of these 

 fibres running to ciliated pockets such as T found (1890) in 

 Dactylometra, and these fibres probably spread out finally in- 

 jto a thin network underlying the general epithelium of the sub- 

 umbrella. 



I The one feature in which this rhopalium differs from what 

 is found i^- Pelagia is the presence of the pigment spot, already 

 mentioned as lying on the aboral side of the rhopalium imme- 

 diately c...;Ove the centre of the mass of concretions. This is 

 an area sensitive to light and only differs from the rest of 

 the sensory epithelium in that here the superficial cells are 

 deeply colored by a yellowish bro'.vn pigment. A more careful 

 exai-nination would undoubtedly show the histology of this strnic- 



ture to be similar to what Schewakoff (1889) has found in Aure- 

 lia. 



14 



