i68 



THE INVERTEBRATA 



Stage of its digestion. Physalia can catch and devour a full-grown 

 mackerel, and the poison of its nematocysts is so virulent as to en- 



dac/-~- 



Fig. 125. Examples of Siphonophora. A, Velella. Altered from Haeckel. 

 Vertical section, showing the cavity of the pneumatophore (stippled) and 

 produced into branching gas tubes, the tracheae (tra.), and a network of 

 endodermal tubes (black), which arise from the cavity of the gastrozooid and 

 gonozooids (black); ?ned. medusa buds. Other letters as in Fig. 124. B, Phy- 

 salia showing the "drift net" arrangement of the tentacles of the dactylo- 

 zooids. 



danger human life. In Velella (Fig. 125 A) the disc-shaped colony 

 has a superficial resemblance to a single medusa. The pneumato- 



