Vol. 3] Torrcy. — Biological Studies on Corymorpha. 271 



tiated cells of the ectoderm, do not show themselves until the 

 tentacle anlage has become clearly delineated. Both endoderm 

 and ectoderm cells are smaller, lower and relatively broader than 

 in the adult. The larval endoderm and ectoderm, then, which 

 contribute to the formation of the original tentacles of the polyp 

 are much less specialized than the mature tissues which may be 

 present in the regenerating zone of an adult. 



The further development of the tentacles involves special his- 

 tological changes in the tissues now already established. The 

 proximal tentacles elongate much more rapidly than the distal. 

 Both acquire very early a number of nematocysts in the termi- 

 nal ectoderm (Fig. 18) which soon thickens into a knob. The 

 knobs are recognizable on the distal tentacles even after sixteen 

 proximal tentacles— more than one-half the total number— have 

 appeared, and when six to eight of the characteristic peripheral 

 canals of the adult have been fashioned out of the primitive 

 enteron. At this stage the proximal tentacles have lost com- 

 pletely their capitate tips, which were present when they num- 

 bered eight. 



With the increasing size of the tentacles, the cells of the ten- 

 tacular endoderm alter their shape. Originally placed one upon 

 another in a single row after the manner of a roll of coins and 

 in accord with their arrangement in the vast majority of hy- 

 droids, which have smaller tentacles than Coryynorplia, the axial 

 cells retain this arrangement in polyps with eight to ten proxi- 

 mal tentacles. In one polyp with six distal and twelve proximal 

 tentacles, the former have but one row of cells throughout, while 

 in the latter there are two rows in the middle third and three 

 rows in the basal third. In larger polyps, with fourteen to six- 

 teen proximal tentacles, the axial cells have abandoned the simple 

 arrangement in the basal halves of both proximal and distal 

 tentacles. At this stage, there are two rows of axial cells in the 

 basal half of each distal tentacle. These rows dovetail slightly, 

 each cell covering a little more than half the cross section of the 

 endodermic axis, with its longest diameter horizontal.* The 

 basal halves of the proximal tentacles contain at this time about 



* That is, parallel with the transverse diameter of the vertical polyp. 



