30 MANUAL OF ZOOLOGY. 



organism which gave rise to them ; in the latter case, the pro- 

 duced 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 ovum 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 pro- 

 duce 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 

 number of zooids, or " polypites," organically connected to- 

 gether, 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 con- 

 siderably 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-swim- 

 ming 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 con- 

 nection. 



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 reproduc- 

 tive 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 sufficiently 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 consists of a bell-shaped disc, by means of 

 which it is enabled to swim freely ; from the centre of this 

 disc depends a nutritive process, with a mouth and digestive 

 cavity, whereby the organism is able to increase considerably 



