DISCOPHORA 471 



complete the picture we need only add a few further details. The 

 mouth (as in the fresh-water polyps) leads into the digestive or body 

 cavity, from which numerous canals pass out radially towards the edge 

 of the umbrella. As it leads a free and active life, the jelly-fish is 

 provided at the loled edge of its umbrella with numerous feeler-like 

 threads or tentacles, as well as a number of small bodies which support the 

 organs of sense (marginal bodies, or tentaculocysts). The latter convey 

 visual and probably also auditory impressions. Living in a free con- 

 dition, the jelly-fish is also able to move voluntarily from place to place. 

 By the forcible contraction of the edges of the umbrella the water con- 

 tained under the concavity of the bell is driven out, and the animal, 

 with the convex side of its body turned forwards, swims backwards 

 through the water (like the cuttle-fish). 



This general structural agreement of the jelly-fish and fresh- water 

 polyp is, however, by no means accidental, for we may daily still see 

 how the one of these two animals is developed from the other, for which 

 purpose we shall briefly examine 



D. The Development of the Jelly-Fish. 



The egg gives rise to an oval-shaped larva (planula), which for some 

 time swims about freely in the sea. It then attaches itself to some. fixed 

 object, and assumes the shape of a hydra, or fresh-water polyp (hydra- 

 tuba stage), which, however, by ring-like constrictions is soon separated 

 into a number of discs (strobila stage). The uppermost of these discs 

 finally falls off and becomes a medusa, or jelly-fish, such as we have just 

 described ; the second disc follows suit in the same manner, and so on. 

 Thus, from the egg of the medusa is produced a hydra, or polyp, which by 

 division again produces medusce. The animal therefore appears in two 

 perfectly distinct generations, viz., as polyp and as medusa, which 

 alternate with each other, whence this mode of reproduction has been 

 termed alternation of generations. 



ORDER II. : HYDROID POLYPS (HYDROIDEA). 



IN the Fresh-Water Polyp (Hydra) we have already become acquainted 

 with a species of this extensive group which rendered us familiar with 

 the structure of Ccelenterata in general, and that of the Hydroidea in 

 particular. With the exception of this species and another, which, 

 starting in brackish water, is gradually encroaching more and more into 

 fresh water, all the others are of marine habitat. They are, however, 

 distinguished from the fresh-water polyps by the fact that all the polyps 



