IV 



PHYLUM CCELENTERATA 



153 



zooids (), each group containing a polype (n) with its tentacles ('), 

 a medusoid (g), and a large enveloping bract (t). The stem often 

 breaks at the internodes, and the detached groups of zooids then 

 swim about like independent organisms. 



Porpita is formed on a different type, and has a close general 

 resemblance to a medusa. It consists (Fig. Ill) of a -discoid 



Parker's Bioloijn, after Gegeubaur.) 



body, enclosing a chambered chitinoid shell (sli) containing air, and 

 obviously corresponding with the float of Physalia. The edge of 

 the disc is beset with long tentacles (t), and from its lower surface 

 depend numerous closely set feelers or dactylozooids (hy'} and blasto- 

 styles bearing medusae, while in the centre is a single polype (7w/), 



