TAPE-WOKMS OF THE SUB-FAMILY AYITELLININ.F. 345 



tion of a single section (fig-. 21) shows that dorsally and 

 venfcrally the following changes have taken place. The 

 original cuticula, together with the subcnticula, has shifted its 

 position from parallel to the longitudinal axis to one at right 

 angles to the long axis, pivotting on the posterior border of 

 the segment, the mai-gin originally anterior having become 

 the outer margin. The space between the outer margin, the 

 anterior margin and the posterior border of the segment has 

 filled with parenchyma ; the new surface from the anterior 

 border to the outer margin is clothed with a thin cuticula, 

 under which there is no subcuticula. This cuticula may have 

 been derived by stretching* of the already present cuticula, or 

 it may be of new origin ; the fact that the cuticula covering 

 the subcuticula now on the posterior surface is enormousl}^ 

 thickened (by contraction ?) would suggest that the cuticula 

 covering the new surface is of new origin. The new cuticula 

 only measures I'b /n to l"7/t that on the posterior surface is 

 much folded, and measures up to 9 fi. The subcuticular muscles 

 are also much more evident than usual on the posterior surface, 

 the whole giving the impression that the cuticula and muscles 

 have been compressed laterally (i.e. in the plane of the 

 surface of the cuticula), thus causing the increase in 

 thickness. 



The Scbcuticula. Figs. 19-21. 



The subcuticula is present everywhere in a single layer 

 close under the cuticula, except in the suckers and places 

 mentioned above. It consists of a stratum of elongate cells, 

 standing vertically to the cuticula. Its component cells are 

 widely separated from each other, or densely congn-egated, 

 according to the state of contraction of the worm. In shape 

 they usually taper slowly towards the cuticula and are 

 rounded or tapering towards the parenchyma (figs. 19, 20). 

 Before reaching the cuticula several branches are usually 

 formed, so that each cell touches the cuticula in more places 

 than one. The outline of the cell is extremely distinct in 

 properly fixed and hardened material ; a thin membrane is 



