28 



FIRST LESSONS IN ZOOLOGY. 



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

 body-wallsj the bud enTeloping a portion of the stomach, 

 until it becomes constricted and drops ofE, 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 Hydractijiia, a Hy- 

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



FiG. 23. — Animal of Millepwa nodosa, a, nutritive zooid; 6, tentacnJated 

 zooid; c, lasso- or nettling" thread; d, the same coiled up in its cell; c a third 

 form. (AH highly magnified.) 



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

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

 like, sterile or nutritiye zooids; b and c, 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- 

 dusse of Coryne (Fig. 25). 

 The minute animals of Millepora secrete large coral-like 



