36 MANUAL OF ZOOLOGY. 



vided with reproductive organs, and is destined for the per- 

 petuation of the species. In the former case the produced 

 zooids all resembled each other, and the parent organism 

 which gave rise to them; in the latter case, the produced 

 zooids are often utterly unlike each other and unlike the 

 parent, since their functions are entirely different. 



The simplest form of the process is seen in certain of the 

 Hydroid Polypes, such as Hydractinia. The embryo of Hydrac- 

 tinia is a free - swimming ciliated body, which, after a short 

 locomotive existence, attaches itself to some submarine object, 

 develops a mouth and tentacles, and commences to produce 

 zooids like itself by a process of continuous gemmation. These 

 remain permanently attached to one another, with the result 

 that a compound organism is produced, consisting of a num- 

 ber of zooids, or " polypites," organically connected together, 

 but enjoying an independent existence. None of the zooids, 

 however, are provided with sexual organs ; and though there 

 is theoretically no limit to the size which the colony may reach 

 by gemmation, its buds are not detached, and the species would 

 therefore die out, unless some special provision were made for 

 its preservation. Besides these nutritive zooids, however, other 

 buds are produced which differ considerably in appearance 

 from the former, and which have the power of generating the 

 essential elements of reproduction. These generative zooids 

 derive their nourishment from the materials collected by the 

 nutritive zooids, but only live until the ova are matured in their 

 interior and liberated, when they disappear. The ova thus 

 produced become free-swimming ciliated bodies, such as the 

 one with which the cycle began. 



In this case, therefore, the " individual " consists of a series 

 of nutritive zooids, collectively called the " trophosome," and 

 another series of reproductive zooids, collectively called the 

 " gonosome," the entire series remaining in organic connection. 



In other Hydroid Zoophytes allied to the preceding (such 

 as Clytia\ the process advances a step further. In Clytia, the 

 generative buds or zooids do not produce the reproductive 

 elements as long as they remain attached to the parent colony; 

 but they require a preliminary period of independent existence. 

 For this purpose they are specially organised, and when suffi- 

 ciently mature they are detached from the stationary colony. 

 The generative zooid now appears as an entirely independent 

 being, described as a species of jelly-fish (or Medusa). It con- 

 sists of a bell-shaped disc, by means of which it is enabled to 

 swim freely; from the centre of this disc depends a nutritive pro- 

 cess, with a mouth and digestive cavity, whereby the organism 



