18 DISCOPHORiE. Part III. 



at one point (PL X. Fig. 1 7/), Avliere they are so densely packed together as to 

 appear quite dark. The Purkinjeau vesicle (^i) is very clear and homogeneous, and 

 is one quarter of the diameter of the egg. The Wagnerian vesicle [w) is a clear cell, 

 which occupies a little less than one half of the diameter of the Purkinjeau vesicle. 

 The yolk sac {Fig. 2 v) of the mature egg is quite thick, a peculiarity before 

 noticed, when speaking of the mature egg of Aurelia (PI. X''. Fig. 23). The yolk 

 is divided into two kinds : an outer, thick layer (//) of very transparent, rather 

 coarsely granular substance, and a central mass {?/^) of densely crowded dark grains. 

 The Purkinjeau vesicle has burst, but the place which it occupied is marked by 

 a clear space (p) in the darl^er yolk mass {g^). 



The Planula of Cyanea. There is a remarkable difference between the mode of 

 develojDment of Aurelia and that of Cyanea, and this, too, from the earliest period 

 after the segmentation of the yolk. The eml^ryo of Cyanea, in its globular state 

 (PL X. Fig. 3), has not more than two thirds the diameter of that of Aurelia. The 

 figure given here was drawn from a specimen magnified five hundred diameters. 

 The vibratile cilia are very short and faint, and difficult to detect when the animal 

 is revolving rapidly. The cells of the exterior are very prominent, so that the 

 surface of the revolving globe appears as if papillated. They are also very trans- 

 parent to a considerable depth; but, although apjsearing like a thick envelope, they 

 do not as yet form a distinct wall apart from the interior mass. The bulk of 

 the body consists of a dark gray mass of cells, in the centre of which is a clear 

 portion, equalling one thiixl of the diameter of the whole body. In this solid state 

 the embryo moves about in the same manner as the young of Aurelia, and gets 

 into the pouches of the proboscis by the same process. 



From the globular state the embryo passes to a more active existence, and, 

 increasing considerably in diameter, changes its form to a broadly ovate shape 

 {Figs. 4 and 4^"), and its cilia grow to more than double their former length, and 

 become quite conspicuous. The outer transparent layer of the cells retains the 

 thickness of the last phase, l)ut the inner dark gray mass changes to a great extent 

 and its peripheric portion becomes very dark orange red, whilst the interior region, 

 constituting two thirds of the whole body, grows very clear, like the peri2:)hery of 

 the embr3'o. The revolutions of the body are now very rapid, and, its axis of 

 rotation corresponding to its greater diameter, the embryo moves iu direct lines 

 from place to place, with the broader end forward. The vibratile cilia incline to 

 the body at different angles at different times ; when the rotation is slow they 

 project nearly at right angles, 1jut when it is rapid they incline, contrary to the 

 direction of the revolution, at an angle of forty-five degrees or even less {Fig. 4"). 

 In the latter instance the cilia appear as if swept backward by a swift current, 

 whereas the movement of each one is completely under its own control, as may 



