224 Elementary Biology. 



of labour. As in the plant we found leaves nutritive in 

 their function and leaves reproductive (sporophylla), leaves 

 protective (sepals) and leaves attractive (petals), so in the 

 Hydrozoa we have zooids performing nutritive, protective, 

 and reproductive functions ; only in the case of the plant 

 we had differentiations of organs, in the Hydrozoa differen- 

 tiation of persons or zooids. 



If we now return to the consideration of Obelia, we 

 find that the various zooids are becoming more closely, i.e. 

 organically, united, but physiologically less dependent on 

 each other. All the zooids (the protective zooids are want- 

 ing) are, moreover, aggregated or gathered together on one 

 branch ; a manifest gain, for not only is a far smaller 

 amount of stolon required, but the colony being closely 

 packed can maintain itself in healthy life, with less expendi- 

 ture of energy, and is moreover not so liable to injury in 

 its compact form. If space permitted, we might instance 

 Hydrozoa that showed still further aggregation. We must 

 content ourselves by a reference to Hydra only. In this 

 form we have not only the functions performed by one 

 person, but we have degeneration and simplification taking 

 place as well. Hydra does not form colonies, and the in- 

 dividual consists of one zooid similar to the alimentary zooid 

 of the Hydractinia, but with the power of producing sexual 

 organs, which latter are of a far simpler type than those 

 possessed by Obelia or even Hydractinia. 



We must now leave general questions and devote our- 

 selves to the study of the social economy of the colony, and 

 the life-history through which it passes. 



As already mentioned, the alimentary zooid of a Hy- 

 dractinia or an Obelia does not materially differ from a 

 Hydra. It will, therefore, be convenient to describe the 

 histological structure and mode of life of Hydra as an ex- 

 ample of an alimentary zooid. One point of difference we 

 must, however, note, viz. that the perisarc is absent in Hydra. 

 The body and tentacles, eight in number, are eminently 



