28 MANUAL OF ZOOLOGY. 



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 nutritive zooids, collectively called the 

 " trophosome," and another series of reproductive zooids, col- 

 lectively called the " gonosome," the entire series often remain- 

 ing 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 

 margin hang a series of tentacular processes. After a period 

 of independent locomotive existence, the Medusa attains its 

 full growth, when it develops ova and spermatozoa. By the 

 contact of these embryos are produced ; but these, instead of 

 resembling the jelly-fish by which they were immediately gene- 

 rated, proceed to develop themselves into the fixed Hydroid 

 colony by which the Medusa was originally produced. 



Still more extraordinary phenomena have been discovered 

 in other Hydrozoa, as in many of the Lucernarida. In these 

 the ovum gives rise (as in Campanularia) to a locomotive 

 ciliated body, which ultimately fixes itself, becomes trumpet- 

 shaped, and develops a mouth and tentacles at its expanded 

 extremity, when it is known as the "hydra-tuba," from its 

 resemblance to the fresh-water polype, or Hydra. The hydra- 

 tuba has the power of multiplying itself by gemmation, and 

 it can produce large colonies in this way; but it does not 



