IV 



COELENTERATA 



71 



its development. The nematocysts on the tentacles increase till, 

 by their aggregation, they form warts and finally garland -like 

 thickenings. 



Meanwhile, whilst the number of tentacles is increasing from 

 sixteen to twenty-four, other changes supervene. Four new and 

 larger invaginations of the ectoderm of the oral disc make their 

 appearance, just in the positions occupied by the old septal funnels. 

 These are the rudiments of the four sub-genital pits of the adult. 

 Just under the oral disc a hole, the ostium, appears in each taeniola, 

 so that this structure is transformed from a complete ridge into a 

 pillar, and the four gastric pouches of the hydra-tuba become in 

 this way converted into the so-called ring-sinus (Fig. 51). 



The free edge of the taeniola thickens and grows out into two 



FIG. 49. A, a Hydra- tuba with eight tentacles. 

 (After Friedemauu.) B, longitudinal section of 

 a part of a similar specimen to show origin of 

 septal funnels. (After Hein. ) 



s.f, septal funnel ; s.musc, septal muscles ; st, incipient 

 stolon ; ten, tentacle ; t, taeniola. 



FIG. 50. Oral view of Hydra- 

 tuba with twenty tentacles. 

 (The tentacles are repre- 

 sented as cut off.) (After 

 Friedemann. ) 



sp, stomach pouch ; t, taeniola. 



diverging lips, which are the first rudiments of the gastral filaments, 

 and these are covered in the adult with specially active digestive 

 cells. From the oral side of the first eight tentacles there appear 

 eight bud-like warts on the oral disc, which are the rudiments of the 

 sense-organs of the adult. Beyond the ring-sinus eight Jpbes grow 

 out ; four of these, termed per-radial, are outpouchings of the original 

 spaces between the taeniolae, whilst the other four, termed inter-radial, 

 take their origin from those portions of the ring-sinus which have 

 developed from the perforations in the taeniolae. Thus the oral disc 

 becomes drawn out into eight lappets, and the tentacles are then 

 thrown off. Each lappet contains one of the eight lobes which 

 have grown out from the ring-sinus ; the lappet is forked at its distal 

 extremity, and in the re-entrant angle of each fork is the rudiment 

 of the sense-tentacle. 



The next process which occurs is the separation of the " head " or 

 " crown " of the hydra-tuba from the stalk. This process is initiated 



