33 
Dr. Reid on the Development of the Medusie. 
apparatus to the microscope, it was observed that these crystals 
depolarized the light. I gave some of the young Medusae to 
Principal Sir David Brewster for examination, and he returned 
me the following report : The small raised portions of the Me- 
dusae named ocelli consist each of six or more similar parts, each 
part having the property of depolarizing polarized light. When 
all the other portions of the animal are absolutely black, the ocelli 
shine with considerable brightness. Upon turning the Medusae 
round in a plane perpendicular to the axis of vision, the individual 
parts of the ocelli disappear and reappear, according to the angle 
which their neutral axes (if they have double refraction), or their 
planes of separation (if they are merely polarizing laminae), form 
with the plane of primitive polarization. If these raised por- 
tions named ocelli are really organs of vision, the probability is 
that their axis of vision is perpendicular to the general surface 
of the Medusa.^^ 
The inner half of the lower surface of the bifid portion of each 
of the marginal lobes (fig. 17) is thinned off to a sharp edge, 
bounded externally by a continuation of the ridge running along 
the middle of the inferior surface already described, so that the 
bifid portion resembles in form a pair of strong scissors. 
A number of larger and smaller filiferous capsules, similar to 
those observed in the larvse, adhere to the outer surface of the 
young Medusae ; and fine cilia are present on the inner surface 
of the lips and stomach, and on the outer surface of the four bifid 
processes floating in the stomach. 
Though the normal number of the marginal lobes or rays is 
eight, yet oecasionally they were as few as four and as many as 
twelve. In a few cases one or more of these lobes were trifid, 
with an oeellus placed in the cleft of each division. 
I was not able to preserve the young Medusae alive more than 
twenty days. During that time the lobes or rays had beeome 
shorter from the expansion of the body, and in a few, small pa- 
pillae were forming in the clefts between the lobes. 
A comparison between the observations of Sars and Steen- 
strup upon the larvae of the Medusa living in the ocean, and 
those made upon them while living in the artificial condition de- 
scribed, elicits some facts of considerable interest. Aecording 
to Sars and Steenstrup, the colonies of these animals living in 
the ocean split up entirely into young ^ledusae each spring, and 
completely disappear, and new ones are founded in September 
from the ova of the adult Medusas ; but while living in the arti- 
ficial state, as was also some years ago remarked by Sir John 
Dalyell *, a certain number only of the individuals of the colony 
* .famiesou’s Pliilosojjliiciil Journal for 1836, 
Ann. ^ Mag. X. Hist. Scr. 2. Vol. i. 
3 
