300 HYDRA CHAP. 



externally by the mouth (nitti) at the apex of the hypostome (hyj>). Between the 

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

 In the ectoderm are seen large (ntc) and small (ntc 1 ) nematocysts ; some of the 

 endoderm-cells are putting out pseudopods (flsd), others flagella (JT). Two 

 buds (&/. 1 , hdp) 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). 



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

 derm-cells (ecf) and interstitial cells (int. c) ; two cnidoblasts (cnbf) enclosing 



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

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

 endoderm-cells (end) -with large yacuoles and nuclei (nu), pseudopods (pscT), 

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

 and all enclose minute black granules. 



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



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

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



K, 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.) 



fibre of Vorticella (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 

 contraction. 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 an 

 immediate contraction of the whole body. 



Imbedded in and between some of the large ectoderm-cells 

 are found plear, oval sacs (n/c\ with very well-defined walls, 

 called "thread-cells" or nematocysts. Both in the living 

 specimen and in sections they ordinarily present the ap- 

 pearance 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 con- 

 sists of a tough sac (Fig. 77, A), one end of which is turned 



