A STUDY OF HYDRA 23 



its outer end with a trigger-like process, the cnidocil. The 

 long thread-like tubes that are shot out when the nematocysts 

 are exploded carry a poison called hypnotoxin, which paralyzes 

 or kills minute animals that are stung by them. 



The endoderm lines the digestive cavity. Some of the endo- 

 derm cells are provided with fine threads, or flagella, whose 

 lashings set up currents that waft the food in and out. Other 

 cells are furnished with blunt processes which may surround 

 and engulf some of the food particles, and digest them. 



ovy 



FIG. 7. Diagram of a longitudinal section of Hydra, bd, buds, the 

 upper one just beginning to develop; ect, ectoderm; end, endoderm; hyp, 

 hypostome; mth, mouth; net, nematocysts; ovy, ovary; spy, spermary. 

 (After Parker and Haswell.) 



Some of the endoderm cells have contractile processes like 

 those of the ectoderm. The middle, non-cellular layer is a 

 thin homogeneous layer on each side of which lie the con- 

 tractile roots of the cells of the ectoderm and endoderm. 



Hydra may produce new individuals either asexually or 

 sexually. In some individuals small swellings or buds may be 

 observed, usually near the base. At first these are simply 

 evaginations of the body-wall, but later they develop tentacles 



