53 



tacles, rather than that tlie tentacles arise, haphazard, 

 wherever there is space enough on the margin to accomodate 

 them (Ilargitt, 1901), Certain it is that the most crowded 

 part oi' the bell-m.argin at any particiilar time is that from 

 which the new tentacles arise. 



XI. HISTOGENE SIS OF MARG INA L ORGAN S. - 

 A. In the Larva, 



The similarity in the appearance of the ru- 

 diments of a tentacle in polyp and in gonosome make it de- 

 sirable to deocrihe both in the same section; for that rea- 

 son the account of the origin of the tentacles in the polyp 

 was reserved for this place. At first the larval tentacle 

 is merely a small roi^nded knob in external appearance, and 

 internally it is made up of a core of two or three endoder- 

 mal cells. When tPie tentacles make their appearance the 

 body v,'ail of the polyp is made up of a doixble layer of 

 cells, the ectoderm and the endoderm, separated by a thin 

 supporting lamella of mesogloea. These three layers are 

 pushed out somev/hat in the growth of the tentacle, the re- 

 gion of greatest activity being in the endodermal layer, 

 where the core of the tentacle is formed by a rapid out- 



