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MANUAL OF ELEMENTARY ZOOLOGY 



assist by coiling round bristles upon the body of the prey. 

 The third kind of nematocysts is of use in attaching the 

 tentacles of the animal, either to its prey or to other objects 

 when necessary, by the stickiness of their threads. The 

 cnidoblasts arise from the interstitial cells by the formation 

 of a vacuole and its gradual modification into a nematocyst. 

 They are formed in the upper 

 region of the cylinder and migrate 

 thence to various parts of the 

 body, where they take up their 

 position in the outer layer. The 

 germ cells also arise in the 

 ectoderm from the interstitial 

 cells by a process which we shall 

 describe later. Lastly, the 

 ectoderm contains, in the region 

 where the interstitial cells lie, a 

 mesh-work of branching nerve 

 cells which are said to be con- 

 nected with tall, narrow sense 

 cells that reach between the 

 musculo-epithelial cells to the 

 surface. Thus Hydra possesses 

 a nervous system, but this is in 

 the most rudimentary condition 

 possible, consisting of a con- 

 tinuous subepithelial plexus of 

 conducting cells without a cen- 

 tral nervous system, while the 

 cell bodies from which the 

 afferent fibres arise are placed 

 among the cells which cover 

 a surface, as in the olfactory 

 epithelium of the frog, not re- 

 moved from it like those of most of the afferent fibres 

 in the latter animal (Fig. 48). 



In the endoderm the cells are all tall and columnar. 

 Certain of them, especially numerous in the 

 oral cone and absent from the tentacles, are 

 glandular. They have a narrow stem and a wide end, 

 turned towards the enteron and containing granules of a 



Fig. 105. — A small portion 

 of a transverse section 

 of Hydra. 



ect.t Ectoderm; end. , endoderm! 

 f.p., food particle, ingested 

 by an endoderm cell ; int.c, 

 interstitial cells ; m.e.c, 

 musculo - epithelial cell ; 

 ntc. t nematocyst; st.l. t 

 structureless lamella ; vac, 

 vacuoles in endoderm cells ; 

 vac'., vacuoles in ectoderm 

 cells. 



Endoderm. 



