170 THE MECHANICAL FUNCTIONS. 



The whole substance of the body of these medusae 

 is semi-transparent and gelatinous, without any 

 distinct librous structure; yet it has considerable 

 elasticity, and possesses also some degree of con- 

 tractile power. The animal is seen alternately to 

 raise and depress the margin of its hemispherical 

 body, and to flap with the fringed membrane, or 

 mantle, which descends from it, in a manner some- 

 what similar to the opening and shutting of a 

 parasol. This pulsatory movement is performed 

 about fifteen times in every minute, with great 

 regularity : and by the reaction of the water, the 

 animal is sustained at the surface ; or by striking 

 the water obliquely, it may even perform a slow 

 lateral movement. Medusae descend in the water 

 by simply contracting their dimensions in every 

 direction : and sometimes, in order to sink more 

 quickly, they turn themselves over, so that their 

 convex part is undermost. 



Medusae are met with of very various sizes ; 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 

 tinije to the colour of the waves for hundreds of 



* The luminous property of sea water, or its phosphorescence, as 

 it is sometimes called, generally arises from the presence of minute 

 medusae, which are met with in greatest numbers at the surface, 

 being specifically lighter than the surrounding fluid. Humboldt 

 ascribes this phenomenon principally to the Mammaria scintillans. 

 It is also frequently produced by certain species of Infusoria ; more 

 especially the following : Prorocenteum micans ; Peridinium Mich- 

 aelis, micans, fusns, furca, and acuminatum ; and Synchceta 

 baltica. Ehrenberg. 



