268 DEYELOPMENT OF BHABDITE-CELLS IK CEPHALODISCTJS. 



rods and sometimes the whole " cell " may be found lying out- 

 side the battery. No two cells are ever structurally identical, and 

 their position varies from one in close proximity to the basal 

 cells to one in which they lie completely outside tbe battery. 

 Besides the secretion-spheres and their accessories, they contain 

 a granular substance in varying quantity identified as plasma or 

 a modification of that substance. In all essential respects they 

 are comparable to the rhabdite-cells of the Turbellaria (and are 

 hence of the same nature as the pseudo-rhabdites of Nemertea), 

 since their prime object is to secrete rods and then to discharge 

 them on to the exterior. 



EXPLANATION OF PLATE 17. 



All figures drawn with Zeiss's 1'5 mm. apochromatic lens with the 

 oc. compens. 12. 



Fig. 1. Portion of a median longitudinal section through the rhabdite-battery 

 and stem of a branchial plume, showing the structure of a piece of the 

 wall of the battery. Reduced, 



1. Peripheral membrane. 



2. Peripheral nuclei situated in the superficial layer of the wall of 



the battery. 



3. Ehabdite-" cells " (semi-diagrammatic ; note position). 



4. Basal cells. 



5. Stratum of protoplasm placing all the basal cells in communi- 



cation proximally. 



6. Nucleated filaments from basal ceUs to periphery. 



7. „ „ from 5 to periphery. 



8. Layer of longitudinal fibres (here appearing transverse). 



9. Fibres of 8 snapped in fixation. 



10. Cavity of rhabdite-battery (continuous with that of stem). 

 Figs. 2 to 7. Stages illustrating the development of the rhabdite-rods 

 (arranged as far as possible in order). Tlie "cells" are in all cases 

 represented in the same position, ^. e., 1 is the peripheral membrane 

 and above it is the exterior. The "cell" shown in fig. 5, therefore, 

 lies practically outside the battery, and those of fig. 6 lie immediately 

 under the surface. 



