472 ACALEPELE ; MEDUSA. 



be due to animals not larger that the head of a pin, in close 

 apposition to each other, the vast amount of organic life which 

 ordinarily escapes our notice, can scarcely fail to strike us with 

 astonishment, not unmingled with pleasure at the thought, that 

 each of these little beings is passing a life of enjoyment, and is 

 performing an allotted function in the great economy of Nature. 

 1031. The structure of a large proportion of the Acalephse 

 is not yet sufficiently understood to permit a very satisfactory 

 arrangement of the Class ; and we must be satisfied with a 

 description of some of the principal forms which it includes. 

 The Medusa (Fig. 611) presents to the eye, when it is floating 



A 



FIG. 611. MEDUSA; A, under surface, showing the mouth in the centre, surrounded by 

 the tentacula, and the ovarial chambers exterior to the origins of these ; B, side 

 view, showing the tentacula hanging down in their natural position. 



in its native element, an umbrella-shaped disk, from beneath 

 which hang down four tentacula or arms. The central part of 

 the concave side of this disk is occupied by the stomach, in the 

 middle of which is the mouth, opening downwards, and sur- 

 rounded by four leaf-like tentacula. Around the stomach are 

 four ovarial chambers, with separate orifices. These organs 

 occupy the space inclosed, as it were, within the frame- work of 

 the umbrella; but the delicate membranous disk projects con- 

 siderably beyond them, and floats freely in the water. It 

 appears to have some muscular power ; for it is by its gentle 

 undulations, which are performed with great regularity, that 

 these animals are propelled through the ocean. This free por- 

 tion of the disk is traversed by numerous canals, which are to be 

 regarded as prolongations of the digestive cavity. Eight of these 

 pass directly outwards, and terminate in a corresponding number 



