THE HYDRA AS A SIMPLE METAZOAN 



257 



cles. Only the fully developed capsules will be found in this region, 

 along with many cnidocils indicating the presence of cnidoblast 

 cells in their final stage 

 of development. Since 

 it is on the tentacles 

 that most of the ne- 

 matocysts are used, 

 there must be some 

 means by which such 

 fully formed nemato- 

 cysts and cnidoblasts 

 can be supplied to re- 

 place those which are 

 lost. In the body 

 region, on the other 

 hand, there are many 

 developmental stages 

 of nematocysts but 

 few cnidocils. This 

 indicates that while 

 the nematocysts may 

 reach their full de- 

 velopment in the 

 body, few of them 

 reach their final loca- 

 tion in the outer part 

 of the ectoderm of the 

 body where they can 

 become functional. 

 The problem thus 

 presented is illumi- 

 nated when we ex- 

 amine some of the 

 marine relatives of the 

 hydra. 



In many hydroids (cf. p. 274) it has been demonstrated that the 

 nematocysts are originally produced by cnidoblast cells in the 

 region corresponding to the body of the hydra, and that these 

 cnidoblasts migrate in an amoeboid fashion through the spacec 

 between the epithelio-muscular cells of the ectoderm (cf. Figs. 122 



Fk;. 120. — Origin and 

 migration of cnido- 

 blasts. 



When its nematocyst is 

 fully formed {cf. Fig. 123) 

 the cnidoblast may either 

 become attached to the 

 supporting lamella in the 

 body region and grow out- 

 ward until it pierces the 

 surface of the body where 

 it lies with cnidocil ex- 

 posed as shown in the 

 lower part of this figure 

 and in Fig. 122; or the 

 cnidoblast migrates be- 

 tween the bases of the 

 ectoderm cells as shown 

 here until it reaches the 

 tentacles, where it be- 

 comes attached to the 

 supporting lamella and 

 grows outward as shown 

 in the upper part of this 

 figure and in Fig. 122. 

 Such a migration is de- 

 monstrable in hydroids 

 like Tubularia, and prob- 

 ably occurs in hydra, 

 .'ince developmental stages 

 ((f the nematocysts are 

 f<'und principally in the 

 ectoderm of the body, 

 while fully developed cni- 

 doblasts and nematocysts 

 are abundant only upon 

 the tentacles, en, cnido- 

 blasts; en', cnidoblasts 

 migrating; ec, ectoderm; 

 11, nucleus of large ecto- 

 derm cell; S.I., supporting 

 lamella; si, stalk of cnido- 

 blast; St', stalk united 

 with another at base. 

 (Based upon the account 

 by Hadzi, loc. cit.) 



