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 mesoglcea, 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 



Ervd 



f f ! 'f 1- *(' 



FIG. 40. Minute structure of Hydra. (After T. J. PARKER 

 and JICKELI.) 



A. Ect. ectoderm ; ing. mesogloeal plate ; st. c. stinging cell ; End. 

 endoderm with flagella and amoeboid processes. 



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



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



D. Mesogloeal plate with contractile roots resting on it. 



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



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

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

 logists were beginning to demonstrate in the development of higher 

 animals. Not long afterwards Allman applied to the two layers of 

 Hydroids, the terms ectoderm and endoderm. 



