22 



ZOOLOGY. 



meanwhile budding out from the farther end, and a mouth- 

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

 Hydra, the Actinia, and other polyps, 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 



Fig. 19.— Colony of Hydractinia echinata 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 the Hydroids except Hydra the sexes are separate, 

 and we for the first time in the animal kingdom meet with 

 two sorts of individuals, i.e., males and females. 



The simplest form next to Hydra is Hydractinia, a Hy- 

 droid encrusting shells (Fig. 19). In this form the indi- 

 vidual is composed of three parts, each endowed with dif- 

 ferent functions, and called zooids — namely, a, hydra-like, 

 sterile or nutritive zooids; b and c, the reproductive zooids, 

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

 ternally, having below the short rudimentary tentacles sev- 



