246 COMPARATIVE ANATOMY 



body is defined by an imaginary line passing through the 

 mouth and the centre of the dome of the umbrella. The 

 secondary axes are disposed radially with regard to the chief 

 axis, and we may recognise : (i.) The perradial axes defined 

 by the four radial canals. At the extremity of each canal is 

 a tentacle, the base of which is swollen and covered by a 

 thickened patch of ectoderm containing pigment and nervous 

 cells. These patches are supposed to be sensitive to light, and 

 are called ocelli. (2.) The interradial axes are defined by 

 four imaginary straight lines bisecting the angles formed by 

 the perradii. At the end of every such imaginary line is 

 found an interradial tentacle having an ocellus at its base. 

 (3.) The adradial axes are defined by eight imaginary lines 

 bisecting the angles between the interadii and perradii. At 

 the end of each adradius is a tentacle having not only an 

 ocellus but also a little hollow vesicle containing calcareous 

 concretions at its base. The walls of this vesicle are formed 

 by modified ectoderm cells having fine hairs projecting from 

 their inner ends into the cavity of the vesicle. As these 

 vesicles are analogous to structures found in the auditory 

 organs of higher animals, the function of hearing has been 

 ascribed to them, and they are therefore called otocysts. It 

 is, however, much more probable that they are organs of balance 

 and not of hearing. The perradial, interradial, and adradial 

 tentacles are always developed at the time the medusa is 

 liberated from its blastostyle, and even before that time, and 

 always during subsequent growth, new cycles of tentacles are 

 formed, one in each interspace between tentacles already 

 existing. These newly-added tentacles have ocelli but not 

 otocysts at their bases. In Obelia, then, every tentacle has 

 an ocellus at its base, but only eight namely, the adradials 

 have otocysts in addition to ocelli. The presence of otocysts 

 on the margin of the umbrella is characteristic of the group of 

 Hydromedusae known as Leptomedusce. In the closely-allied 

 Anthomedusa ocelli are always present, otocysts never. 



Fig. 51, C, is a diagrammatic representation of a median 

 longitudinal section through the medusa of Obelia. It will be 

 seen that the bulk of the umbrella is made up by the greatly 

 thickened jelly or mesoglcea. The ex-umbrella, sub-umbrella, 

 and manubrium are clothed externally by ectoderm, which 

 passes into the endoderm at the lips of the mouth. The 



