Dr. Reid on the Development of the Medusre. 33 



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 Medusse ■ 

 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 visicm 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 ofi" 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 larvae, 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 occasionally 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 ocellus 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 become 

 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 larva? of the Medusa living in the ocean, and 

 those made upon them while hving in the artificial condition de- 

 scribed, elicits some facts of considerable interest. According 

 to Sars and Stecnstrup, the colonies of these animals living in 

 the ocean split up entirely into young Medusae each spring, and 

 completely disappear, and new ones are founded in September 

 from the ova of the adult Medusse ; 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 



* Janiieson's Philoso])liiciil Jmunal for 1836. 



Ann. «(• Ma(/. N. Hist. Ser. 2. Vol. i. 3 



