134 



ANIMAL BIOLOGY 



come to be a division of labor between different -indi- 

 viduals of the colony, some being specialized for catching 

 prey (feeding hydroids) ; others, richly furnished with net- 

 tling cells, are set apart for protection (defensive hydroids) ; 

 while others (the reproductive hydroids) are devoted en- 

 tirely to reproduction. Very commonly the marine 

 hydroids give rise by budding to a free-swimming genera- 

 tion of jelly fish, or medusa. The larger part of a typical 

 medusa consists of a disk, or umbrella, furnished with tenta- 



rc 



FIG. no. Hydroids A and medusa B; m, manubrium; mb, medusa 

 buds; re, radial canals; t, tentacles; v, velum. (After Allmann.) 



cles along the outer margin. Hanging down from the middle 

 of the lower side is the manubrium, at one end of which is 

 the mouth. The latter leads to the stomach from which a 

 number of canals (commonly four) radiate outward where 

 they open into a circular canal near the margin of the um- 

 brella. Jelly fish are usually transparent animals, fre- 

 quently of very delicate and beautiful structure; they 

 swim through the sea by contractions of the umbrella 

 and live upon animals which they catch by means of their 

 tentacles. Jelly fish produce sex cells which are dis- 



