132 REPRODUCTION OP HYDROID ZOOPHYTES. 



are a number of little cells or bell-shaped chambers, with 

 their mouths upwards, every one of them containing a polype 

 that bears a strong resemblance to the Hydra. Each of these 

 polypes is capable of living independently of the rest, obtains 

 its nourishment by means of its own arms, and digests it in 

 its own stomach ; but all are connected by a canal that passes 

 along the stem and branches, in which a kind of circulation 

 takes place, that strongly reminds us of that of the compound 

 Tunicata ( 114). This plant-like structure extends itself by 

 budding; new branches are formed from those previously 

 existing ; and these are enlarged at a certain point into cells, 

 in which after a time polypes make their appearance. 



125. Besides the cells containing the polypes, however, we 

 find capsules in which are evolved buds of a different nature, 

 that form within themselves the generative products. These 

 buds in some instances assume the form of Medusce, and, 

 becoming detached from the stalk that put them forth, swim 

 about freely, living upon food obtained by themselves, and 

 setting free either sperm-cells or germ-cells, by the concur- 

 rence of whose contents eggs are formed, from which new 

 polype-growths arise. In other instances the Medusoid bodies 

 give forth their generative products, without ever leaving the 

 capsules in which they were themselves developed. And in 

 other cases, again, it does not seem that any Medusoid form 

 intervenes at all, the germ-cells and sperm-cells being evolved 

 from the Zoophytic structure itself. But since it is also 

 known that even the most characteristic Medusan forms are 

 evolved as buds from a Zoophytic stock (Chap, xv.), and since 

 those composite forms of Acalephse whose structure has until 

 lately been most obscure, turn out to be, as regards their 

 essential characters, Hydrozoa organized for floating, there 

 seems to be no longer any sufficient ground for ranking the 

 Acalephse as a separate class. 



126. It is not, however, by animals of this very simple 

 structure, that the massive stony fabrics are built up, which 

 constitute the coral islands of the Pacific Ocean, and of which 

 a large portion of our limestone rocks seems to be composed. 

 These are constructed by animals belonging to the group of 

 Anthozoa, and formed upon the same general plan with the 

 Sea-Anemone, a plan which is higher than that of the Hydra, 

 inasmuch as we find the interior of the body containing other 



