300 HYDRA CHAP. 



externally by the mouth (mth) at the apex of the hypostome (hyp). Between the 

 ectoderm and endoderm is the mesoglcea (msgl), represented by a black line. 

 In the ectoderm are seen large (ntc) and small (ntc') nematocysts ; some of the 

 endoderm-cells are putting out pseudopods (psd), others flagella (fl). Two 

 buds (bd. 1 , bd.'t) in different stages of development are shown on the left side, and 

 on the right a spermary (spy) and an ovary (ovy) containing a single ovum (ov). 

 (X 20.) 



B, portion of a transverse section more highly magnified, showing the large ecto- 

 derm-cells (eel} and interstitial cells (int. c) ; two cnidoblasts (cnbl) enclosing 

 nematocysts (ntc) and one of them produced into a cnidocil (cnc) ; the layer 

 of muscle-processes (m. pr) cut across just external to the mesogloea (msgl) ; 

 endoderm-cells (end) with large vacuoles and nuclei (nu), pseudopods (psd), 

 and flagella (fl). The endoderm-cell to the right has ingested a diatom (a), 

 and all enclose minute black granules, (x 100.) 



C, two of the large ectoderm-cells, showing nucleus (nu) and muscle-process (m. pr). 



D, an endoderm-cell of H. viridis, showing nucleus (nu), numerous Zoochlorellas 

 (chr), and an ingested nematocyst (ntc). 



E, one of the larger nematocysts with extruded thread, barbed at the base. 



F, one of the smaller nematocysts. 



G, a single sperm. (From Parker's Biology: D after Lankester; F and G after 

 Howes.) 



(p. 273), a high degree of contractility, the almost 

 instantaneous shortening of the body being due, in great 

 measure at least, to their rapid and simultaneous con- 

 traction. It is probably correct to say that, while the 

 ectoderm-cells are both contractile and irritable, a 

 special degree of contractility is assigned to the muscle- 

 processes, the cells themselves being eminently irritable, 

 the slightest stimulus applied to them usually being 

 followed by immediate contraction of the whole body. 



Imbedded in and between some of the large ectoderm- 

 cells are found clear, oval sacs (ntc), with very well 

 denned walls, called " thread-cells " or nematocysts. Both 

 in the living specimen and in sections they ordinarily 

 present the appearance shown in Figs. 76, B, and 77, 

 A, but are frequently met with in the condition shown 

 in Figs. 76, E, and 77, B, that is, with a short, conical 

 tube protruding from the mouth of the sac, armed near 

 its distal end with three recurved barbs besides several 

 similar processes of smaller size, and giving rise distally 

 to a long, delicate, flexible filament. 



Accurate examination of the nematocysts shows that 

 the structure of these curious bodies is as follows. 

 Each consists of a tough sac (Fig. 77, A), one end of 



