108 



HYDEOZOA. 



Fig. 51. 



Cassiopea Borbonica, 



the most ordinary provisions for obtaining nourishment met with in the 

 Pulmonigrada ; we will therefore return to consider the structure of 

 the stomach itself, and of the canals that issue from it and convey the 

 digested nutriment through the system. In Cassiopea Borbonica, 

 which will serve to exemplify the 

 general arrangement of these parts 

 in the whole Order, the stomach 

 (fig. 51, 6) is a large cavity 

 placed in the centre of the inferior 

 surface of the disk, and is ap- 

 parently divided into four com- 

 partments by a delicate cruciform 

 membrane arising from its inner 

 walls. Into this receptacle all 

 the materials collected by the ab- 

 sorbing suckers are conveyed 

 through eight large canals, and 

 by the process of digestion be- 

 come reduced to a yellowish semi- 

 fluid pulpy matter constituting 

 the pabulum destined to nourish the whole body. Erom the central 

 stomach sixteen large vessels arise (fig. 51, c), which radiate towards 

 the circumference of the disk, dividing and subdividing into numerous 

 small branches that anastomose freely with each other, and ultimately 

 form a perfect plexus of vessels as they reach the margin of the mush- 

 room-shaped body of the creature. The radiating vessels are moreover 

 made to communicate together by means of a circular canal (fig. 51, e) 

 which runs round the entire animal, so that every provision is made 

 for an equable diffusion of the nutritive fluid derived from the stomach 

 through the entire system. Now, if we come physiologically to inves- 

 tigate the nature of this simple apparatus of converging and diverging 

 canals, we cannot but perceive that it unites in itself the functions of 

 the digestive, the circulatory, and the respiratory systems of higher 

 animals : the radiating canals, conveying the nutritive juices from the 

 stomach through the body, correspond in office with the arteries of 

 more perfectly organized classes ; and the minute vascular ramifications 

 in which these terminate, situated near the thin margins of the loco- 

 motive disk, as obviously perform the part of respiratory organs, inas- 

 much as the fluids permeating them are continually exposed to the 

 influence of the air contained in the surrounding water, the constant 

 renewal of which is accomplished by the perpetual contractions of the 

 disk itself. 



(277.) The umbrella-like disk of Cyanea aurita, whose anatomy has 

 been most carefully studied by Ehrenberg, is composed of a highly 

 organized gelatinous substance invested by three membranous integu- 



