COELENTEEATA 



69 



and a more pointed end where the mouth opens. Thus by a single 

 process a stage is reached which, in the Hydromedusae, is attained 

 first by a process of the immigration of cells, then by the absorption 

 of the more central cells, and lastly by the formation of an aperture 

 to the exterior. 



The embryo now emerges from the maternal pockets and swims 

 freely about by means of its cilia. The outer cells commence to show 

 traces of the formation of nematocysts, whilst the inner cells develop 

 large vacuoles, as in Hydra. It appears that, as in the blastula stage 

 so also in the gastrula stage, cells migrate from the wall of the 

 stomach into its cavity and are digested. 



After swimming about for four or five days the larvae attach 

 themselves by their broad ends, the ectoderm cells of which secrete an 



o.c 



sp. 



FIG. 47. Two longitudinal sections through two Hydra-tubae of different ages. 

 A (after Hein), through a specimen with four tentacles. The section goes through the origin of 

 a tentacle. B (after Friedemann), through a specimen with eight tentacles, showing the stomach 

 pocket intervening between two taeniolae. o.c, oral cone ; s.p, stomach pocket ; t, tentacle ; t.b, 

 tentacle bases of vacuolated endoderm. 



adhesive secretion. The attached larva becomes gradually some- 

 what flattened, and passes from a cylindrical to a cup shape. The 

 endoderm cells in the neighbourhood of the mouth multiply rapidly to 

 form a slight elevation or oral cone. The ectoderm cells in this 

 region do not multiply but become stretched so as to form a thin 

 flattened layer. The almost obliterated blastopore becomes now 

 widened so as to form the permanent mouth ; it becomes indeed quite 

 a gaping opening. 



Immediately after this, four primary tentacles arise as warts 

 surrounding the mouth. The interior of each is occupied by a solid 

 cord of endoderm, and the ectoderm covering it becomes crowded 

 with nematocysts. Alternating with these tentacles there arise four 

 taeniolae or ridges of the endoderm projecting into the stomach 



