80 DISCOPIIOR.E. Part III. 



SECTION VIII. 



PECULIARITIES OF THE AURELID^ AS A FAMILY. 



If form, as determined Id}^ structure, constitutes the essential chai'acter of a nat- 

 ural family in the animal kingdom, it is incumbent upon us to show that our 

 Aurelia has a pattern of its own, to justify us in considering it as the type of 

 a distinct family. This is the more necessary, since Eschscholtz associates it with 

 the genera Sthenonia, Cyanea, Pelagia, and Chrysaora, as a member of the fiimily 

 which he calls Medusidoe. Even the most recent writer on the classification of 

 Acalephs, Professor Gegenbaur, unites it in the same way with other Discophorse, 

 which, in my estimation, belong to different fomilies. 



What jDrominently distinguishes Aurelia as a family, is the even curve of the 

 outer surface of its disk, while the lower surface is excavated in its central portion 

 by four large genital pouches, between which hang four stout arms, closing upon 

 one another in the centre, so as to form a rectilinear ojiening, prolonged in undu- 

 lating curves or folds between the lower margins of the arms. The whole edge 

 of this opening, to the extremity of the arms, is set with uniform, minute fringes. 

 The whole margin of the disk is evenly provided with comparatively small ten- 

 tacles, except where the eight eyes occupy comparatively slight indentations, which 

 give the outline the appearance of an eight-lobed disk, the lobes of which are 

 evenly arched outside, with a slight depression in the middle. All these peculiarities 

 in the form of our Aurelia depend upon structural features. The absence of undu- 

 lations on the outer surflice of the disk, which are so characteristic of Cyanida?, 

 arises from the even diminution in the thickness of the whole disk, from the centre 

 to the periphery. The four triangular excavations of the lower surface are owing 

 to the peculiar widening of the interambulacral system of radiating tubes, near their 

 base, and the corres^ionding thickening of the lower floor under these pouches, in 

 consequence of which an open space is circumscribed below them, which communi- 

 cates with the surrounding medium through large, cii'cular apertures. The stoutness 

 and comparative rigidity of the arms, when contrasted with the long, pendent and 

 flowing folds of the oral appendages of Cyanea, are owing to the manner in which 

 the primitive oral tube thickens at its base, while its outer edges, extending hori- 

 zontally, fold respectively with their margins against each other, and to the circum- 

 stance that the margins grow wider than the arched back, in consequence of which 

 they are drawn in folds around the whole oral rim; for the aperture which leads 

 into the main cavity is not limited to the 023ening immediately below the digestive 



