MEGAZOOIDAL INDIVIDUALS 103 



radial canal system. To this whole structure the name 

 " Hydra-tuba " has been given. 



The next stage of the Individual's development is the 

 formation of a stolon from the base of the hydra-tuba, and 

 from this stolon, or " runner," there arises another fixed 

 medusoid like the first. This method of vegetative pro- 

 duction may go on for a long time until a large colony of 

 hydra-tubae is formed, but a time inevitably comes when 

 development proceeds on new lines. 



The hydra-tuba grows longer, and the phenomenon of 

 " strobilation " occurs, a series of circular grooves or inden- 



Fig. 34. — Development of Aurelia, one of the Acraspedote 

 Medusae, a, ciliated free-swimming embryo, or " planula " ; 

 b, hydra-tuba ; c, hydra-tuba in which fission has considerably 

 advanced, and the " strobila " stage has been reached ; 

 d, hydra- tuba in which the fission has proceeded still further, 

 and a large number of the medusoids have been already 

 detached to lead an independent existence. (Nicholson.) 



tations appearing on the tuba- walls. The grooves deepen, 

 and the resulting prominent rings develop little lobes so 

 that the tuba structure (now termed " scyphistoma ") looks 

 like " a pile of saucers " (Nicholson) with concave surfaces 

 upwards. Finally the saucer-like discs are set free one by 

 one, to develop into free-swimming medusoid gonophores 

 which grow to a very large size. They produce sexual 

 elements, and the fertilised ova originate new Individuals 

 in the production of fresh colonies of hydra-tubae (Fig. 34). 

 The Aurelia Individual is of the greatest interest, for 

 it will be shown later that from some such serially-medusoid 



