56 



tentacle formation is the thickening of the ectodorm at the 

 point where the tentacle is to appear. This is contrary to 

 the condition which we have in Gonionema * But to continue 

 our description; along with the growth of the endodermal 

 prpcess v/hich is to be the core of the tentacle the ecto- 

 derm also increases rapidly, and constitutes an investment 

 which contains within it numerous nematocysts and concre- 

 tions Avhich were scattered throughout the ectoderm at the 

 margin of the umbrella. After the tentacle has grown out 

 for a little distance beyond the bell-margin the cells on 

 the upper or aboral surface become modified to form an ad- 

 hesive organ. (Fig. 27, 2S) . The cells over a disk-shaped 

 area become elongated until they have t>ie form of a thick 

 pad. The tissue immediately around the pad grows out in a 

 flange so that the organ becomes a vacuum-cup strongly mus- 

 cular around the edge (Fig. 28) . 



After the tentacle has grown out to a length of six 

 to eight mm. and has increased in diameter considerably, 

 the cavity of the circular canal is drawn into it. The en- 

 dodermal cells, arranged radially about tlie axis, thicken 

 until they are forced away from the centre, and a tubular 

 cavity is left(Fig. 2). As this process takes place first 



