28 FIRST LESSONS IN ZOOLOGY. 



Hydra; the smaller bud (?) is a simple bulging out of the 

 body-walls, the bud enveloping a portion of the stomach, 

 until it becomes constricted and drops off, the tentacles 

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

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

 Hydra, the Actinia, and other polyps, and in fact all the 

 lower animals, is simply due to the division and consequent 

 multiplication of cells at a special point at or near the out- 

 side of the body. 



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

 droid encrusting shells (Fig. 22). In this form the indi- 



FIG. 23. Animal of Millepnra nnrloaa. ft, nutritive zooid; b, tentaculated 

 zooid; c. lasso- or nettling thread; d, the same coilad'uj) in its cell; e, a third 

 form. (All highly magnified.) 



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

 ferent functions and called zooids. These are, a, hydra- 

 like, sterile or nutritive zooids; l> and r, the reproductive 

 zooids, both being much alike externally, having below 

 the short rudimentary tentacles several round sacs, or 

 "medusa-buds" which produce either male or female 

 medusae. These medusa-buds correspond to the free me- 

 dusae of Coryne (Fig. 25). 



The minute animals of MiUcpora secrete large coral-like 



