58 



secrete the solid concretion which later attains a consid- 

 erable size. The concretion is invested v/ith the thin mem- 

 branous ectodermal covering. In Gonionema the concretions 

 correspond with the composition which has been given for 

 similar structures in other medusae - a lime carbonate de- 

 posit in an organic matrix. Thus it is seen that both ten- 

 tacle and sense-organ consist of an endodermal core which 

 appears first and grows out from the lining of the circular 

 canal. In each case this core becomes invested with a tu- 

 nic of ectoderm which remains associated with it. 



XIII. NEHATOCYSTS . - 



In the hydra stage the earliest appearance 

 of nematocysts was as interstitial cells arising from eith- 

 er tissue-layer. Their growth in Gonionema is much as it 

 is in Cordylophora lacustris . as described by Morgenstern 

 (1901), They are carried out on the tentacles by migration 

 along with the ectodermal layer in which they arr> set. The 

 extreme attenuation of the tentacle as it is fully extended 

 (Fig. 13) gives an admirable chance to study the construc- 

 tion of the cell-elements, especially the nematocysts. The 

 tentacle appears as a delicate rod of translucent substance 



