94 NAKED-EYED MEDUSA. PART in. 



Mr. M'Cready has discovered nerve-centres behind each 

 tentacle, and under each marginal coloured speck in 

 several species of the open-eyed medusse, which places 

 this group of Acalephse in a higher grade than any of 

 the preceding orders. The medusse swim by the mus- 

 cular energy of their umbrellas : at each rhythmical con- 

 traction the water, which enters by the mouth and fills 

 the great central cavity within the umbrella, is forced 

 out again through an orifice at the other end, and by its 

 reaction the medusa is impelled with considerable ve- 

 locity in the contrary direction, so that the top of the 

 umbrella goes first, and all its tentacles are dragged 

 after it. 



The medusse are dioecious : in the males four repro- 

 ductive cells full of reddish or purple granular matter 

 surround the cavity of the stomach, and appear like a 

 coloured cross through the top of the gelatinous um- 

 brella. In the females, at a point just before the four 

 radiating canals enter the marginal canal, the flesh on 

 the exterior of the umbrella swells out into bulbs, con- 

 taining vessels full of clear eggs with minute globular 

 yolks. These eggs, when fertilized, are hatched, and 

 the young are developed within these ovaries, so that 

 they come into the water as a kind of infusorial ciliated 

 animalcule destitute of a mouth. One end of the crea- 

 ture acquires a suctorial disc, fixes itself to an object, 

 and uses its cilia. The other end opens into a mouth, 

 round which tentacles like fishing lines spring forth ; the 

 central part is converted into the cavity of the stomach, 

 and thus a perfect hydra is formed, capable of being 

 propagated naturally by budding, or artificially by being 

 cut in pieces, each piece becoming a perfect hydra, dif- 

 fering in no respect from a common simple fresh- water 

 Hydra. 



From one of these, numberless successive generations 

 of simple hydrse may be produced by budding, all catch- 



