OF THE ACALEPHvE OF NORTH AMERICA. 345 



the periphery or opposite the mouth ; at least not in the Naked-eyed Medusfr, though 

 the fluid discharged from the digestive cavity is circulated througli tubes into the pe- 

 riphery and around it, and the refuse matters, retracing their course, are emptied through 

 the mouth. AVhetlier any Medusae liave peripheric openings tlnough wliich the refuse 

 matters are discharged, as Ehrenberg maintains, I have been unable to ascertain. How- 

 ever this may be, so much is plain, — that, in Medusae and Polypi, the whole digestive 

 apparatus is in direct broad communication with the circulatory apparatus; that the fluid 

 circulated is simply chyme mixed with water, carried through all parts, which either re- 

 traces its course or is discharged through particular openings of the circulatory apparatus; 

 and that there is no continuous alimentary canal with an anterior and posterior opening, 

 and no distinct circulatory system deriving its fluid through lym[)hatics from the alimen- 

 tary cavity, but two closely connected systems, one presiding chiefly over the function 

 of digestion, and the other circulating the whole mass of digested food, (hat is, chyme 

 mixed with water. It will, therefore, be more proper to call the upper sac the di- 

 gestive sac, the central cavity with its branching tubes the chymiferous sac, and the 

 vessels chymiferous vessels ; and to consider the circulation, not as a blood-circulation, 

 but as a chymous circulation, and, in some degree, the centres of this circulation, uliich 

 act in antagonism to each other by their alternate contractions, as a sort of chymiferous 

 heart. 



If we next consider the oblong area and its cavity, I am able to state that the 

 hollow space below, which extends forwards and backwards from the two cloacal bulbs, 

 is a direct prolongation of the cavity of the bulbs, lined equally with vibrating cilia, 

 and in which the fluid accumulated in the bulbs moves also to and fro. (Plate V. Fig. 9.) 

 The ridge which circumscribes the outline of these tubular sacs is very definite, and 

 slightly prominent upon the surface, though smooth. But in some Beroid Medusae, 

 such as the true Beroe, this ridge is slightly fringed, and it may be that these fringes 

 constitute rudimentary gills, and that the marked outline of the area in our Pleuro- 

 brachia is a rudimentary development of such fringes. 



The narrow tubes alluded to above, as converging towards the centre of the circum- 

 scribed area, can be traced from the lower extremity of the vertical rows of locomotive 

 fringes to the immediate vicinity of the black speck in the centre of the lower end 

 of the body. These tubes are direct prolongations of the vertical or ambulacral chym- 

 iferous tubes, tapering gradually towards the lower extremity of the animal, and ex- 

 tending beyond the rows of fringes proper, but reduced in their diameter so much as 

 to appear now as very slender tubes converging from the lower summit of the locomotive 

 fringes to the centre of the lower surface of the animal. These tubes are eight in 



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