OCELLI. 9 



in the course of the marginal circular vessel in very uncertain number, usually, however, one 

 at each side of the larger marginal cirrhi, and beside the smaller one, only at one side. They 

 are round, measuring l-40thof a line in diameter, and consist of a tolerably thick membrane : 

 they contain from one to nine, and even more, round globules. If there is only one, it is situated 

 exactly in the centre of the vesicle, but if there are several, they are lying together either in 

 two groups or separately joined to each other at the wall of the vesicle. Their size varies 

 from 1-300 — 1-150'". I have never observed them move. Muriatic acid dissolves them, and 

 causes the vesicle to burst." In his Thaumantias leucostyla, he describes the auditory vesicle 

 as "measuring 1-60'", and containing globules of the dimensions 1-200'". They are seated 

 beneath the basis of the marginal fibres on a small projection. They are not present, however, 

 beneath all the marginal fibres." Milne Edwards observed, in his Mquorea violacea* two 

 hemispheric or oval vesicles on each side of each marginal tubercle, and containing two or 

 three spherical corpuscles. KoUiker observed that the otolitic cavities or vesicles in Oceania 

 (as well as in higher forms) were lined with vibratile cilia, and that the otolites vibrated. Frey 

 and Leuckhart, whilst they saw the otolites vibrate distinctly in certain Ciliograda, found them 

 perfectly motionless in Geryonia, even as Will had observed. I have observed the vibration 

 of the otolites distinctly in more than one species of Thaumantias ; so has my friend Dr. 

 Melville. I have seen them also vibrating in their cavities at the bases of the tentacles of 

 more than one species of Oceania, a genus in which they are highly developed. 



The ocelli, from analogy, are regarded as rudimentary eyes, or rather light-perceiving 

 organs. In the gjrmnopthalmatous Medusae they are very rudimentary, and in most species 

 consist only of an assemblage of pigment- cells more or less symmetrically disposed. They 

 vary much in colour, different species of Thaumantias, for instance, presenting purple, orange, 

 yeUow, black, and even variegated ocelli. Yellow, with a red dot, is a common appearance. 

 This dot indicates a higher or more concentrated condition of the organ. It is especially 

 defined in Oceania, and in Turris neglecta, forms at the head of the tribe. In 8lahheria, 

 the resemblance of the ocelli to the coloured bulbs which terminate the tentacula is very 

 striking, but when minutely examined, they are easily distinguished from the latter organs 

 by the presence of a small black dot. In some forms of Sarsia and in Euphysa we have 

 curiously particoloured ocelli ; also in Willsia, though not so defined. In Li%s,ia, and 

 especially in Bougainvillia, we have compound pcelli, formed out of several united, and 

 variously coloured, either entirely black, or entirely yellow, or piebald, black and yellow, or 

 yellow and bright red. In Circe, and some other forms, no ocelli can be observed. 



That these bodies are the eye-spots, there can be no doubt, when we compare them 

 with similar bodies in the higher Medusae. In them crystals are present, as was first 

 pointed out by Gaed6. These crystals were shown by Rosenthal to be silicious, a character 

 by which they are strikingly distinguished from otolitic , crystals, which are always 

 calcareous. 



Though, as we have seen, there are well-marked organs of sense in these animals, the 

 presence of a nervous system has not been clearly made out. For my part, I have not been 

 able to satisfy myself as to the existence of either ganglia or nervous filaments in any of the 



* Ann. Sc. Nat. (2d Ser.) t. xvi, p. 195. 



