HYDRA FUSCA 143 



(Fig. C), and one on the inside called the endoderm (Gr. 

 endon = inside), en. These two layers are found throughout the 

 body, both the ectoderm and endoderm extending into the 

 tentacles to their very tips. Hydra is thus a double sac with 

 no space between its two layers. The layers of ectoderm and 

 endoderm are not in actual contact with each other, but are 

 separated by a thin supporting layer known as a mesogloea 

 (Gr. mesos = middle -+- gloia =glue), mes. By means of this 

 intermediate layer, the ectoderm and endoderm are very firmly 

 attached to form one solid mass, forming a body wall made up 

 of two layers of cells. 



Ectoderm. The ectoderm is made of two chief kinds of cells. 

 The first of these is the epithelio-muscle cells ; Fig. D. These 

 are in the shape of cones, with their broad ends outward and 

 their tapering ends toward the mesoglcea. At the tapering ends 

 some long fibers protrude which extend over the body of the 

 animal next to the mesoglcea. The great contractility of Hydra 

 is due to these fibers. The second type of cells is the interstitial 

 (Lat. inter = between + sistere = to stand) cells. These are 

 found between the first cells and are somewhat smaller than the 

 epithelio-muscle cells. They are chiefly interesting because 

 they produce a very peculiar type of organ possessed by Hydra 

 known as the nematocysts (Gr. nema = thread + cystis = sac). 

 The nematocysts, or stinging cells, are little sacs scattered all 

 over the outside of the body of the animal, especially in the 

 tentacles. Each of these is an oval sac, one side of which is 

 pushed inward like the finger of a glove inverted into its palm; 

 Figs. G, H, and /. This inverted portion is in the form of a 

 long thread, much longer than the diameter of the sac, and is 

 wound up in a long coil inside of it; Fig. H. Besides this 

 thread the sac contains a liquid. The peculiarity of these 

 cells is that under a proper stimulus the minute thread may be 

 inverted from the sac as shown in Figure G. This inverted por- 

 tion when discharged carries with it a small quantity of poison, 

 and thus each thread serves as a little poison dart. The thread 



