582 



THE POPULAR SCIENCE MONTHLY. 



of water whidi is discharged by the next muscular pulsation. 

 The tentacles are so elastic and hair-like that they are held by the 

 resistance of the water, and are drawn out behind the animal into 

 fine glassy threads which are thrown into graceful undulations at 

 each pulsation as it swims through the water, and, when it comes 

 to rest and sinks slowly toward the bottom, they form a web or 

 net which is almost invisible, but far more dangerous than any 

 spider's web, for every thread is covered with the terrible poison- 

 darts. 



Great as the difference is between the sedentary hydra and the 

 swimming jelly-fish, comparative anatomy shows that they are 

 modifications of the same type, and that the jelly-fish, like the 

 blastostyle, the defensive hydra and the root, is a specialized feed- 

 ing hydra. 



In some species of Dysmorphosa the jelly-fish which is set free 

 from the blastostyle is the last stage in the long series, and it 

 quickly acquires reproductive organs, lays its egg or discharges 

 its spermatozoa as the case may be, and dies ; but in other species 



it no sooner begins its own in- 

 dependent life than it pro- 

 duces buds which are ulti- 

 mately set free, as jelly-fish 

 like the parent, each of which 

 soon becomes a mature male or 

 female. The eggs are thrown 

 out into the water, where they 

 are fertilized by union with 

 the male cells, and each egg 

 then begins the process of de- 

 velopment, which is to result 

 in the founding of a new hy- 

 droid colony. The life of the 

 jelly-fish is very short, and 

 simply serves to multiply the 

 species, and to scatter the eggs 



tier layer of cells, Phown in section in the lower far and wide aloUg the shore 



risfht-hnnd quadrant; d, the central cavity; a', /• ,1 i ^i j 



the point where the month is to be formed. Ot the OCean, and tllUS to SC- 



cure the wide distribution of 

 the hydroids. The Qgg hatches, however, neither into a jelly-fish 

 like the parent, nor into a hydra, but into a minute microscopic 

 animal of extremely simple structure, which is known as a planula. 

 Fig. 2, a, which is a highly magnified drawing of the planula of 

 another species, will serve to show what it is like. It has no mouth 

 nor tentacles, and its pear-shaped body is covered with cilia, by 

 means of which it swims slowly through the water for a short 

 time, but, unless its slight locomotor power soon brings it into 



Fig. 8.— Planula of Liriope saitigera, highly magni 

 fled, drawn from Natnre by W. K. Brooks : a, 

 Burface layer of cells, which is shown in section 

 on the right half; b. gelatinous substance ; c, in- 



