SECT. in. COVERED-EYED MEDUSA. 



97 



are in the substance of the proboscis. Neither of these 

 kinds have more than four ovaries, but some other kinds 

 have eight ovaries and eight radiating canals. Most of 

 the canals are simple, but in one genus they are branch- 

 ing. All are furnished with tentacles, some of them 

 having stings, others none. 



The covered-eyed group consists only of two natural 

 divisions the Rhizostoma, or many-mouthed medusae, 

 and the Monostoma, or one-mouthed medusae. In both 

 the coloured eye -specks at the margin of the umbrella 

 are larger and more numerous, than in the naked-eyed 

 group, and they are covered with a hood. The proboscis 

 of the one-mouthed order terminates in a square mouth, 

 the four angles of which are prolonged into tentacles 

 with a solid hyaline axis. They have a fringed mem- 

 brane along their under-surface, containing numerous 

 stinging thread-cells. Sixteen canals, connected with 

 the stomach or cavity of the proboscis, radiate over 

 the flattish, cup-shaped umbrella; eight of these are 

 branched, and terminate in the circular canal which runs 

 round its fringed edge, and they form the nutrient and 

 respiratory system of the animal, while the eight simple 

 and alternate canals terminate in eight openings at the 

 rim of the umbrella, through which the refuse or indi- 

 gestible part of the food is discharged, thus forming an 

 exception to the other pulmograde medusae, and indeed 

 to the Hydrozoa in general, which eject it at the mouth. 

 All the canals are lined with cilia, whose vibrations 

 maintain the circulation of the fluids, and perform the 

 duties both of a heart and respiratory apparatus. Dr. 

 A. Krohn has observed that in three species of the genus 

 Pelagia belonging to the covered-eyed medusae, the young 

 are at once developed as medusae without the interven- 

 tion of the hydra form. 



The disk of the Rhizostoma, or root-mouthed medusae, 

 is rather flat, and the large proboscis is unlike any 



VOL. II. H 



