HYDROIDS. 29 



zooid often returns to its hydra-life and reproduces itself by budding 

 in the old fashion, and finally becomes " the progenitor of a new 

 colony, every member of which may in its turn l)ud off a pile of 

 medusa-disks." 



The bodies thus detached have all the characteristics of the fully- 

 developed medusae. Each consists of an umbrella-shaped disk divided 

 along its margin into lobes, generally eight in number, and of a stomach 

 terminating in a probosciform mouth. As the creature grows, the 

 spaces between the marginal lobes fill up ; from its border long ten- 

 tacles are developed, and a fringe of tendril-like filaments sprout 

 forth from the margin. The young medusa eats voraciously, and 

 grows proportionately large ; the Chrysaora, which we have been de- 

 scribing, attaining a diameter of fifteen inches, and the Rhizostoma 

 sometimes reacliing to three feet. These raedusse are familiarly known 

 as sea-nettles. When they have reached full development the genera- 

 tive organs appear in four chambers arranged round the stomach, and 

 are contained in curious fluted membranous ribbons which hold the 

 sperm-cells in the male, and ova in the female. The fertilized embryos 

 repeat the same wonderful cycle just described, developing into a 

 hydroid from which medusa-disks are budded off. 



The relation which late investigations have established between 

 the stationary hydroids, and the medusae, forms one of the most inter- 

 esting cases, yet known, of the curious phenomenon called alternate 

 generation. In the majority of cases we find a non-sexual, plant-like 

 form interposed between the ovum and the directly or indirectly 

 sexual form of medusa, though this is not always the case, as direct 

 development has been observed from ovum to medusa. 



The nearest approach, in the adult form, to special organs are the 

 digestive cavity, and the cnidae. The stomach, however, possesses no 

 true parietal walls, and in one form the fresh-water hydra the stom- 

 ach will do duty for the skin, and the skin for the stomach, if necessary ; 

 they seem to be able to live very comfortably, and digest their food 

 without difficulty when turned wrong-side outward. 



The cnidiB are barbed filaments inclosed in tiny sacs, which they 

 can shoot out at will, for their own protection, or for the capture of 

 their prey, as the case may be. In tlie hydra the sac is ejected, and 

 a central dart is projected into the body attacked. There must be a 

 minute poison-sac in communication with the darts, as it is found that 

 any soft-bodied victim, released from the clasp of the tentacles, is in 

 variably dead, no matter how short the time of its imprisonment may 

 have been. The effects of the cnidae in the medusae are very well 

 known, and have gained for them their popular name of sea-nettles. 

 Many an unlucky swimmer has found himself wrapped in the long 

 thread-like filaments of these transparent, floating bells, and been 

 almost maddened as he found himself inextricably inclosed in what 



