TYPES OF CCFJ.ENTERA- -HYDRA. 141 



doubled in as a pouch, usually bearing internal barbs, and prolonged 

 into a long, hollow, spirally coiled filament or lasso. This lasso is 

 bathed in a fluid, presumably poisonous. On its free surface the sting- 

 ing cell usually bears a delicate trigger hair or cnidocil. Under stimulus, 

 whether directly from the outside or from a nerve cell, the cnidoblast 

 explodes and the nematocyst is thrown out. Besides the ordinary 

 stinging cells, there are others of small size which do not seem to 

 explode. 



ni& 



:-' 



.' . ' ', < 



i i 



| 



B 



C : 



\ 



D E 



FIG. 65. Minute structure of Hydra. After T. J. Parker and Jickeli. 



A. Ect., ectoderm ; tug., mesogloeal plate; st.c., stinging cell ; End., endo- 



derm with flagella and amoeboid processes. 



B. nc., nerve cell, and sf.c., stinging cell. 



C. Stinging cell with ejected thread ; ., nucleus. 



D. Mesogloeal plate (mg.) with contractile roots resting on it. 



E. in.c., muscular cell with contractile roots, c.r. 



(3) Scattered about there are minute nerve cells, with fine connec- 

 tions, especially with the muscular and the stinging cells (Fig. 65, B). 



(4) Small interstitial or indifferent units fill up chinks in the ecto- 

 derm, and seem to grow into reproductive, stinging, and other cells. 



(5) Granular glandular cells on the basal disc or "foot" probably 

 secrete a glutinous substance. They are also said to put out pseudopodia, 

 and so move the animal slowly. 



The inner layer or endoderm is less varied in structure, as is to be 

 expected from the fact that it is not, like the ectoderm, exposed to the 



