HYDROiDS 



481 



in which there are hydra-like nutritive individuals, without well- 

 developed tentacles, and reduced prehensile individuals devoid 

 of mouths, and looking like large tentacles richly provided with 

 nettle-cells, and branched in the 

 case of Millepora. Each nutritive 

 polype is surrounded by a num- 

 ber of these modified individuals, 

 the function of which is to secure 

 food. There are also egg -pro- 

 ducing members of the colony, 

 comparable in function, though 

 not in form, to the medusa-stage 

 of Obelia. 



It will have been gathered 

 from the preceding that there is 

 often a division of physiological 

 work between the different mem- 

 bers of a hydrozoan colony, just 

 as in the complex body of a higher 

 animal there is a similar division 

 between the various tissues. This 

 phenomenon is carried to an 

 extreme in the free -swimming 

 marine forms which are grouped 

 together under the name of Com- 

 pound Jelly -Fish {Siphonophord) 



(fig. 294). Each colony com- Fig. 294--A compound Jelly-Flsh (.yaraV^) 



prises individuals of the most TH;^ may be regarded as a medusa (of which the large 



r bell or umbrella is seen at upper end of figure) with a very 



various nature, and just as a ^^"S mouth-stalk (manubrium), on which smaller indi- 



, viduals are formed as buds. 



hydroid zoophyte arises by the 



budding of a fixed hydra-like individual, so here we must suppose 

 that buds of different kinds have been produced on a modified 

 jelly-fish or medusa, in some cases upon the under side of the 

 umbrella and in others upon the elongated mouth-stalk, which 

 has been compared to the handle of the umbrella. 



Order 2. — Splitting Hydroids (Scyphomedusse) 



To this order belong the large jelly-fish which are often 

 seen in great numbers in British seas during the warmer parts 



Vol. I. 



31 



