120 



AN INTRODUCTION TO ZOOLOGY 



by Hydra when stimulated. Near the middle of each cell, em- 

 bedded in the alveolar cytoplasm, is a nucleus containing one or 

 two nucleoli and a network of chromatin. 



NEMATOCYSTS or stinging capsules (Figs. 54, n ; 56) are present 

 on all parts of the body of Hydra except the basal disk, being 



most numerous on 

 the tentacles (Fig. 

 5 5, A). Each is con- 

 tained in a cell 

 known as a cnido- 

 blast. These in turn 

 are embedded in lit- 

 tle tubercles on the 

 surface which give 

 the animal a rough- 

 appearing outline. 

 The tubercles are 

 ectoderm cells, each 

 of which usually pos- 

 sesses one or more 

 large nematocysts 

 surrounded by a 

 number of a smaller 

 variety. Three kinds 

 of nematocysts are 

 found in Hydra. The 

 largest is .013 mm. 

 long and .007 mm. thick ; before being discharged it is pear-shaped 

 and occupies almost the entire cell in which it lies (Fig. 56, nem.). 

 Within it is & coiled tube (t) at whose base are three large and a num- 

 ber of small spines. Projecting from the cell near the outer end of 

 the nematocyst is a trigger-like spine, the cnidocil (cm.). Nema- 

 tocysts may be exploded by adding a little acetic acid, or better, 

 methyl green, to the water. The tube which is coiled within 

 them is then everted. First, the base of the tube with the spines 



FIG. 55. Nematocysts and their action. A, por- 

 tion of a tentacle showing the batteries of 

 nematocysts ; cl., cnidocils ; B, insect larva 

 covered with nematocysts as a result of cap- 

 ture by Hydra. (From Jennings.) 



