26 MANUAL OF ZOOLOGY. 



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 Campanularia. The ovum of Cam- 

 panularia 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, 1 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 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- swimming 

 ciliated bodies, such as the one with which the cycle began. 



In this case, therefore, the 'individual' Campanularia con- 

 sists of a series of nntritive zooids, collectively called the 

 * trophosome,' and another series of reproductive zooids, col- 

 lectively called the ' gonosome ' the entire series remaining 

 in organic connection. 



In other forms nearly allied to Campanularia (such as 

 Coryne) the process advances a step further. In Coryne 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 sufficiently matured 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) under the name of Sarsia. 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 in size. The substance of the disc is 

 penetrated by a complex system of canals, and from its 



