Vol. 3] Tornij. — Biological Studies on Corymorplia. 293 



spicuous, and is caused by a local absorption of water by the 

 vacuolated endoderm. About the stem, just below the external 

 constriction, is a thickening- in the ectoderm produced by an 

 elongation of cells. Its lower limit marks the future anterior 

 limit of the perisarc, which about this time retreats from the 

 hydranth. 



The tentacles appear after hydranth and stem regions have 

 been defined. The distal tentacles arise usually in advance of 

 the proximal, with indications of a typical quartette arrange- 

 ment. They pause in their development upon reaching the 

 number of four and are then surpassed in number by the proxi- 

 mal tentacles. These numerical relations are soon again re- 

 versed, the distal ultimately numbering twice as many as the 

 proximal series. 



Histologically, the development of both kinds of tentacles is 

 similar. Its chief features are (1) an increase in number of 

 cells in both endoderm and ectoderm where each tentacle is to 

 appear, (2) a slight evagination of the endoderm and a corre- 

 sponding thinning of the ectoderm, (3) an extrusion of a single 

 column of cells (four to ten) from the evagination, pushing the 

 ectoderm and mesogloea before it, (4) the arrangement of the 

 axial cells in several rows decreasing in number from base to 

 tip, the longitudinal axes of the cells lying perpendicular to the 

 plane of movement of the tentacles. 



The frustules arise similarly in larva and adult, continuing 

 to appear throughout the life of the polyp. Histologically, 

 their origin is similar to the origin of the tentacles, including 

 the incorporation of endoderm and ectoderm cells. Each frus- 

 tule attains a much greater length and attenuation than the 

 tentacles, owing to the active locomotion of its clavate amoeboid 

 tip and consequent strain on the filament. The connection with 

 the stem is ultimately lost and the organ, for a time independent, 

 dies upon the formation of the perisarcal investment which 

 serves as an anchor for the stem. 



In the adult, the frustules are arranged in longitudinal rows, 

 the number of the latter being determined by the number of 

 peripheral canals. 



