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 1-40th ofa 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 Taumantias 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 Aguorea violacea,* two 
hemispheric or oval vesicles on each side of each marginal tubercle, and containmg two or 
three spherical corpuscles. Kolliker 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 7kawmantias; 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 gymnopthalmatous Medusz 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 7haumantias, for instance, presenting purple, orange, 
yellow, 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 Slabberia, 
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 Luphysa we have 
curiously particoloured ocelli; also in Willsia, though not so defined. In Lizzea, and 
especially in Bougainvillia, we have compound ocelli, 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 Medusz. In them crystals are present, as was first 
pointed out by Gaedé. 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. 
wo 
