XX. 



CHARACTERISTIC DEEP-SEA TYPES. AC ALEPHS. 



CTENOPHOR^E AND HYDROMEDUS^E. 



As with fishes, a number of the deep-sea medusae are occa- 

 sionally taken at the surface, and undoubtedly many of the rarer 

 of our jelly-fishes are deep-water forms which have accidentally 

 found their way to the surface. To these probably belongs one 

 of the most graceful of our jelly-fishes, Ptychogena lactea (Fig-. 

 422), which swims at a considerable depth below the surface. 



The action of the light, and 

 the increase of temperature 

 at the surface, suffice to kill 

 the animal in a short time. 

 As soon as it reaches the 

 surface, the disk loses its 

 transparency, the genital or- 

 gans become dull, and the 

 medusa is soon completely 

 decomposed, showing that 

 the new conditions are to- 

 tally unlike those under 

 which it habitually thrives. 



From the character of 

 their development we may 

 either find medusae on the 

 bottom in their fixed younger hydroid stages, or we may collect 

 them alive from the surface in an older stage. Others again are 

 always pelagic, swimming freely on the surface in all their stages 

 of growth, while a limited number of the so-called deep-sea 

 medusae perhaps inhabit the intermediate depths far below the 



Fig. 422. Ptychog-ena lactea. j. 



