22 



ZOOLOGY. 



meanwhile budding ont from the fartlier end, and a mouth- 

 opening arising between them, as at c. Budding in the 

 Hydra, the Actinia, and other polyi)s, and in fact all the 

 lower animals, is simply due to an increase in the growth 

 and multiplication of cells at a special point on the outside 

 of the body. 



The Hydra, exactly as in the vertebrates, including man, 

 arises from an egg which, after fertilization, passes through 



Fro. lf>.— Colonj- of Hydractinia echinnta on a shell tenanted by a hermit crab, 

 natural size. 



a blastula and then a gastrula stage, the germ consisting 

 at first of two cell-layers. 



In all tlie Ilydi'oids except Hydra the sexes ai'c separate, 

 and wo for tlie first time in the animal kingdom meet with 

 two s<)rl;s of individuals, i.e., males and females. 



The sinijile.st form next to Hydi'a is Ili/dractinia, a Hy- 

 droid encrusting shells (Fig. 10). In this form the indi- 

 vidual is composed of three ])arts, each endowed with dif- 

 ferent functiiins, and c;illeil znnids — nameh', a, hydra-like, 

 sterile or nuti-itive zooids; h and r, the reprour/:tive zooids, 

 one male and the other female, both being much alike ex- 

 ternally, liaving below the short rudimentary tentacles sev- 



