176 A COURSE ON ZOOLOGY. 



goes new transformations ; series of annular contractions 

 appear a certain distance behind the circle of tentacles ; 

 gradually these contractions become more and more 

 prominent, and the appearance of the animal is like that 

 of a number of superposed disks. The borders of the 

 disks become fringed in their turn ; the contractions or 

 strangulations between them increase, and, finally, the 

 mass breaks up into as many little medusae as there are 



FIG. 104. 



LIFE-HISTORY OF THE COMMON JELLY-FISH. 1, free-swimming embryo (pla- 

 nula); 2-6, the embryo fixed, developing into a "hydra-tuba," which (7-8) 

 divides transversely into a pile of individuals; these in turn (9) are liber- 

 ated and grow (10-11) into jelly-fish. (From Haeckel.) 



disks ; each of these lives freely, grows, and acquires the 

 full and definite form, .and gives birth to ciliated larvsB. 

 Then the cycle of alternate generation recommences in 

 the same manner and in the same order. 



The coralligena include creatures that are often also 

 called polyps, because of the numerous processes or ten- 

 tacles that surround their mouths. Nearly all these pass 

 their existence fixed upon some foreign body. They 

 reproduce their species by eggs and by buds, each of the 



