HYDRA. 



137 



Some of the endoderm cells have muscular roots like those of the 

 ectoderm. They lie on the inner side of the middle lamina, in a trans- 

 verse or circular direction. A few cells near the mouth and base are 

 described as glandular, and the presence of a few stinging cells has been 

 recorded, though some suggest that the last are discharged ectodermic 

 nematocysts which have been swallowed. 



The middle lamina, representing the mesoglrea, is a thin homogeneous 

 plate, on each side of which lie the muscular roots of ectodermic and 

 endodermic cells (Fig. 40, D). 



It is historically interesting to notice the important step which was 



'CI 



\ '*• 



t_ -Az^- 



Fig. 40.^Minute structure of Hydra. (After T. J. Parker 

 and JiCKELl.) 



A. Ect. ectoderm ; mg. mesogteal plate ; st. c. stinging cell ; End. 

 endoderm wilh flagella and amceboid processes. 



B. nc. nerve cell, and St. c. stinging cell together. 



C. Stinging cell with ejected thread ; n. nticleus. 



]}. Mesogteal plate with contractile roots resting on it. 

 E. 7it.c. musctdar cell with contractile roots, c.r. 



made when, in 1849, Huxley definitely compared the outer and inner 

 layers of the Ccelentera with the epiblast and hypoblast which embryo- 

 looists were beginning to demonstrate in the development of higher 

 ammals. Not long afterwards AUman applied to the two layers of 

 Hydroids, the terms ectoderm and endoderm. 



