VI.] THE FRESH-WATER POLYPES. 355 



y. Examine, in detail, a nematocyst in relation to its 

 parent-cell. The cell-protoplasm will be seen to 

 form a delicate envelope, thickened only around 

 the nucleus, which stains deeply and is to be found 

 in close apposition with the nematocyst. 



8. The cnidoblasts ; recognizable by the presence of a 

 stiff spinous process or cnidocil^ usually projecting 

 from one corner of the cell-body. 



Note the variable size of the cnidocils (cf. Sect. 

 3). Nematocysts may or may not be present; 

 where they are it will generally be found that the 

 longer cnidocils are associated with the smaller 

 thread-cells and vice-versa. 



c. Look for nerve-cells; small, stellate, and with a 

 deeply-staining round nucleus. They are rare and 

 only to be found with difficulty. 



t,. Immature nematocysts ; occasionally to be met with 

 in the isolated state. Bottle-shaped and bearing a 

 short blunted filament, which subsequently becomes 

 invaginated (be careful not to confound this with 

 the cnidocil). 



d. The larger cells of the endoder?n. Examine cells freshly 

 isolated from specimens killed with osmic acid vapour. 

 Note, in addition to the characters already observed 

 (Sect. 5). 



a. The cell-base ; not unfrequently expanded and pro- 

 longed out to form contractile processes, identical 

 with, but shorter and less marked than, those of 

 the ectoderm cells. 



^. The nucleus ; variable in position with the state of 

 distension of the cell ; generally marginal. 



23—2 



