CHAP. IV NEMATOCVSTS 295 



externally by the mouth {iitth) at the apex of the hypostome {hyj>). Between the 

 ectoderm and cndoderm is the mesogloea {insgl\ represented by a black line. 

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

 endoderm cells are putting out pseudopods {psd), others flagella {Jl). Two 

 buds ij}d.^ bd^.) in different stages of development are shown. on the left side, and 

 on the right a spermary {spy) and an o\say{o7'y) containing a single ovum {oz'). 



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

 derm cells {cct) and interstitial cells (jfit. c) ; two cnidoblasts {cnbP) enclosing 

 nematocysts {ntc), and one of them produced into a cnidocil {cue) ; the layer 

 of muscle-processes (»r. pr) cut across just external to the mesogloea {iiis^C)\ 

 endoderm cells (t'?;^/) with large vacuoles and nuclei (?;//), pseudopods {psd) 

 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 {tiii) and muscle-process (w./?). 



I), an endoderm cell of H.vzridis, showing nucleus («w), numerous chiomato- 

 phores ichr), and an ingested nematocyst {ntc). 



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



F, one of the smaller nematocysts. 



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

 Howes.) 



fibre of Vorticella (p. 276), a high degree of contractiHty, the 

 ahnost 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 clear, oval .sacs {ntc), with very well-defined walls, 

 and called thread-celh or nematocysts. Both in the li\'ing 

 specimen and in sections they ordinarily present the ap- 

 pearance shown in Figs. 74, b, and 75, a, but are frequently 

 met with in the condition shown in Figs. 74, e, and 75, 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 si/e, and 

 giving rise distally to a long, delicate, flexible filament. 



Accurate examination of the nematoc)sts shows that the 

 structure of these curious bodies is as follows. Each con- 

 sists of a tough sac (Fig. 75, a), one end of which is turned 



