MEDUSA. 143 



a marginal membrane, like the fold of a mantle extending 

 loosely downwards from the circumference; together with a 

 central pedicle descending from the lower surface, like the 

 stalk of a mushroom, and terminating below in several 

 fringed laminae, or processes, which have sometimes been 

 denominated tentacula. 



The whole substance of the body of these medusai is se- 

 mi-transparent and gelatinous, without any distinct fibrous 

 structure; yet it has considerable elasticity, and possesses also 

 some degree of contractile power. The animal is seen al- 

 ternately to raise and depress the margin of its hemispheri- 

 cal body, and to flap with the fringed membrane or mantle, 

 which descends from it, in a manner somewhat similar to 

 the opening and shutting of a parasol. This pulsatory move- 

 ment is performed about fifteen times in every minute, with 

 great regularity: and by the reaction of the water, the ani- 

 mal is sustained at the surface; or by striking the water ob- 

 liquely, it may even perform a slow lateral movement. They 

 descend in the water by simply contracting their dimensions 

 in every direction. Sometimes, in order to sink more quick- 

 ly, they turn themselves over, so that their convex part is 

 undermost. 



Medusae are met with of very various ^izes; the larger 

 abound in the seas around our coast; but immense numbers 

 of the more minute and often microscopic species occur in 

 every part of the ocean.* In some parts of the Greenland 

 seas they swarm to such an extent that they give a visible 

 tinge to the colour of the waves for hundreds of miles. The 

 total number of these animals dispersed over that space sur- 

 passes the utmost stretch of the imagination. In these si- 

 tuations a cubic foot of water, taken indiscriminately, was 

 found by Mr. Scoresby to contain above 100,000 of these 

 diminutive medusae. 



Belonging to the tribe of Medusaria is a singular genus, 



• The luminous property of sea water, or its phosphorescence, as it is some- 

 times called, generally arises from the presence of minute medusec, which 

 are met with in greatest numbers at the surfiice, being spccifially lighter than 

 the surrounding fluid. 



