170 



THE BIOLOGY OF HYDRA : 1961 



Fig. 1. Reconstruction in wax of a segment of the fishing tentacle of 

 Physalia. The extension of the gastro-vascular cavity into the tentacle and 

 the relationship between the cavities of the batteries and of the tentacle are 

 clearly shown. 



Regularly spaced between cnidoblasts bearing the large nemato- 

 cysts occur small cnidoblasts whose capsules range from 7 to 15 

 microns in diameter. In favorable preparations each of the cni- 

 doblasts may be seen to be provided with a cnidocil, projecting 

 through the cuticular layer of the epithelium into the ambient water. 

 The light microscope reveals a perinuclear basketwork of elastic 

 fibers within the cnidoblast, which appears to surround the nema- 

 tocyst. Other than this perinuclear network and the nematocyst 

 capsule, the cytoplasm of the definitive cnidoblast appears to pre- 

 sent very little structural specialization. 



Cnidoblasts are regularly distributed throughout the epidermis 

 of the external battery hemisphere, and there is also a repeating 

 pattern of internal structure in adjacent nematocysts. The internal 

 coiled thread, characteristic of the nematocysts of all Cnidaria, in 

 each of the nematocysts originates at about the same point in the 



