61 



sections cut through this tentacle pad (Figs, 33 and 34) it 

 is seen that the concretions which lie towards the boll- 

 margin are more dense and homogeneous; that further inwards 

 they are somewhat less solid in appearance, spaces appear- 

 ing within their outer walls; and that at the side nearest 

 the circular canal there are great mmibers of nettling 

 cells in various stages of formation. All gradations are 

 present betvfeen the solid concretion and the nettling cell 

 (Fig. 33), It is therefore evident that the two distinct 

 kinds of specialized cell products, the one for the protec- 

 tion of the delicate nerve ring and the other for capturing 

 and paralizing prey, ai"e produced from the same cells and 

 in the same locality ; ^hat they are, in fact, homologous. 

 Fig\^re 33 was drawn by Professor Brooks to show this fact 

 in Gonionema. Whether the cartilaginous colls actually 

 change into nettling cells after they have become fully de- 

 veloped is not clear. At the inner margin of the cartilag- 

 inous pad the nematocysts lie closely packed together (Fig, 

 33). From this breeding place the nematocysts work their 

 way out onto the tentacle along which they migrate until 

 they reach a spot where t?iey are needed. In young tenta- 

 cles, which are still elongating, the nematocysts are car- 



