THE HYDRA AS A SIMPLE METAZOAN 



253 



begins to form a nematocyst, it becomes a enidoblast cell. The 

 first steps in this process occur in cells lying near the supporting 

 lamella. After the nematocyst is fully developed the enidoblast 

 attaches itself to the supporting lamella, grows toward the sur- 

 face, and thus becomes exposed externally either between the 

 larger ectoderm cells or embedded in their substance {cf. Fig. 122), 

 but the connection with the supporting lamella is retained by 

 means of a delicate stalk (Figs. 124 and 126). Upon reaching this 

 exposed position, the enidoblast develops a trigger-like projection, 

 the cnidocil, which protrudes from the surface and is the part 

 of the enidoblast which is sensitive to the chemical stimuli that 

 normally produce the discharge of the nematocyst. As may be 



Fig. 123. — Formation of a nematocyst in the cytoplasm of an interstitial 

 cell, which thus becomes a enidoblast. 



A to E, the successive stages, ne, nematocyst; nu, nucleus of enidoblast; sc, fluid 

 within nematocyst that is supposed to caure the eversion of the thread by a eudden swelling 

 due tu imbibition of water. (After Hadzi, loc. cit.) 



readily seen under the microscope, this discharge consists of a sud- 

 den eversion, by which the thread is turned right side out and is 

 extended into the surrounding water or penetrates the prey 

 (Fig. 125). When discharged, the nematocyst, therefore, resem- 

 bles a football bladder with an elongated structure like an everted 

 glove finger extending from one end (P^ig. 121 B). 



Although it is impossible to say that enidoblast cells are never 

 controlled by the nervous system, recent investigation indicates 

 that they are what are known as independent effectors (Fig. 129), 

 that is, structures responding like the nervous elements to stimu- 

 lation, but not under nervous control. The enidoblast cells of 

 hydra seem to be little affected by mechanical stimuli, as when a 

 tentacle is rubbed with a fine glass rod, or when the surface is 

 brushed by the movements of the ciliated protozoans, Trichodina 



