HYDRO1DS 



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 (Siphonophora) 



(fig. 294). Each COlony COm- 



prises individuals of the most s "^ regarded M a med . usa F ( f wh j ch . t j c Iarge 



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



VariOUS nature, and jUSt aS a lo "g 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. 



Fig. 294- -A Compound Jelly-Fish (Sarsia) 



Order 2. SPLITTING HYDROIDS (Scyphomedusae) 



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 



