224 HYDRA LESS, xxi 



(ent. cav), which, as well as the two layers, is continued (ent. cav') into 

 the tentacles, and opens externally by the mouth (mtk) 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 1 ) nematocysts : some of the endoderm cells 

 are putting out pseudopods (psd), others flagella (fl\ Two buds (&/', 

 bd?) 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 ectoderm cells (ect) 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 mesoglcea (msgl) : endoderm cells (end) with large 

 vacuoles and nuclei (mi), pseudopods (psd), and flagella (y?). 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 (mi) and muscle- 

 process (m. pr). 



D, an endoderm cell of H. viridis, showing nucleus (nil), numerous 

 chromatophores (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. 



(D after Lankester : F and G after Howes.) 



are necessarily left between their inner or narrow ends, and 

 these are filled up with the second kind of cells (int. c\ small 

 rounded bodies which lie closely packed between their larger 

 companions and are distinguished as interstitial cells. 



The inner ends of the large ectoderm cells are continued 

 into narrow, pointed prolongations (c, m. /r), placed at right 

 angles to the cells themselves and parallel to the long axis of 

 the body. There is thus a layer of these longitudinally- 

 arranged muscle-processes lying immediately external to the 

 mesoglcea (B, m. pr). They appear to possess, like the axial 

 fibre of Vorticella (p. 129), 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 



