COMPARATIVE ANATOMY. 



279 



cation with one another ; (2) Sexual persons, which undertake the 

 duty of ripening the sexual products and also of planting them 

 out and dispersing them, so that the young brood of Hydroids 

 proceeding from the fertilised egg may attach themselves in new 

 places and form new stocks. l"he sexual persons which are des- 

 tined for a free-swimming life, and which are buds of the Hydroid 

 stock, attain a structure corresponding with this manner of life ; 

 they become young Craspedote Mcdusce, which detach themselves 

 from the stock, swim away, and — often after longer or shorter 

 metamorphosis — ripen the sexual products. 



Fii^. 20. — Bougainville ramosa (after Allman), with budding Medusse. 

 h. Nutritive polyps; vi.k.. Medusa buds; w., Detached young Medusa 



{Margalis ramosa). 



Turning now towards the end of Vol. I., we come to Class 

 Afitennata, sub-class Hexapoda or Insecta, which is thus described : 



"The body of the Hexapoda falls typically into three parts, quite 

 distinct from each other : head, thorax, and hind body (abdomen). 

 The unsegmented head probably originally consisted of four seg- 



