SENSATION. 



139 



thin layer of the latter passes over it so as to separate it from direct contact with the surrounding 

 water. In Ti/aropsi.s alone does a similar definite pigment mass occupy a different position, hi 

 this genus it is situated not at the root of a 



tentacle, but at the base of a lythocyst, in ■*^"^- '■• • 



the interval between two neighbouring 

 tentacles (woodcut, fig. 57). I cannot, 

 however, regard the pigment-shot in T^a- 

 ropsis as tlie equivalent of the ocellus in 

 other medusa; ; for besides its connection 

 with the lithocyst rather than the tentacle, 

 it is imbedded in a thickening of the endo- 

 derm of the circular canal instead of being, 

 as in the true ocellus, an exclusively ecto- 

 dermal structure. 



When the pigment of which the ocellus 

 is composed is examined under a high power 

 of the microscope, it is seen to possess a 

 more definite structure than the granular 

 coloured matter which is secreted in the 

 walls of the somatic cavity. The ocellus is, 

 in fact, composed of an aggregation of very 

 minute cells, each filled with a homoge- 

 neous coloured matter. Such, at least, is 

 the stnicture of the ocellus in Sijncoryne cximia, and it is probable that in other cases a similar 

 structure may be demonstrated. 



As just said, a transparent body, capable of refracting the light, may be sometimes seen 

 immersed in the pigment. A very distinct refracting body, of a lenticular shape, has been 

 described by Quatrefage as imbedded in the outer side of the pigment in Eleidheria. In the 

 nearly allied Clavcdella I have found a minute, spherical, transparent, and refracting body imbeddeii 

 in the same way in the pigment of the ocellus (PI. XVIII, fig. 5) ; and Krohn and Claparede 

 had already made a similar observation. It is, however, here of soft consistence ; it seems to be 

 easily broken down, and I have occasionally failed in detecting any trace of it. A similar refracting 

 lens-like body may be seen in the ocellus of Cladonema (PI. XVII, fig. 6). In every other case 

 with which I am acquainted the ocellus consists merely of a mass of pigment-cells, without any 

 structure which can serve as a refracting medium, unless the transparent layer of ectoderm whicli 

 is continued in front of it may be regarded as contributing to the functions of the ocellus by its 

 refractive action on the rays of light. In Ehuthcria and Clavafella, indeed, the ectoderm presents 

 at this spot an abruptly prominent convex surface, which Quatrefage has compared in Eleutheria 

 to a cornea. 



We have no means of forming anything like a certain conclusion as to the proper function of 

 the ocellus. The universal presence of a definite pigment, and the occasional occurrence of a 

 refractile, lens-like body, have suggested a comparison with an organ of vision, and against the 

 justice of this comparison no sufficient argument has been yet adduced. 



The L'dliocyd. — Like the ocellus, the lithocyst is invariably developed on the margin of the 



Piirt of the umbrella-margin, with basal bulb of margiual tentacle, 

 iu Syncorijne exlmla. 

 rt. Umbrella wall ; i, distal extremity of a radiatin^^ cannl ; — 

 vibrutile cilia maybe seen clothing its walls; c, part of circular 

 canal ; rf, endoderm of bulbous dilatation of base of marginal ten- 

 tacle ; — it is thrown into prominent lobes, which project into the 

 cavity of the bulb ; e, cushion-like thickening of ectoderm loaded 

 with thread-cells;/, marginal tentacle ; ^, ocellus, imbedded in a 

 thickening of the ectoderm. 



