iv COELENTERATA 79 



form an inner sac which spreads till it completely displaces the food 

 material in the blastocoele, which is used up in nourishing the 

 growing cells. Both ectoderm and endoderm then co-operate in 

 producing a gelatinous secretion, the so-called jelly, mesogloea, or 

 supporting lamella. The cavity of the gastrula becomes the 

 coelenteron of the adult. 



When the opening of the invagination has become narrowed so 

 as to form a slit-like blastopore, the ectoderm acquires cilia and the 

 embryo rotates within the egg-capsule, which shrinks and becomes 

 more transparent. A little later the embryo escapes from the capsule 

 altogether and swims about as a larva, but the swimming is not very 

 vigorous and the larvae do not rise far from the bottom. 



stem. 



FIG. 59. Two stages in the development of the larva of Urticina crassicornis. 

 (After Appellof.) 



A, free-swimming larva. B, stage just before fixation, ect, ectoderm ; end, endoderm ; m, mesenteries ; 

 p.o, post-oral region ; o, month ; stom, stomodaeum ; ten, tentacles. 



The larvae are of ovoid shape with a broad aboral and a pointed 

 oral end. The aboral end is directed forwards in swimming, but the 

 reduced blastopore, which persists as the mouth, is not situated 

 actually at the oral end but a little to one side of it, so that there 

 is a small post-oral projection of the body. The ectodermal lips of 

 the blastopore grow inwards and form an inwardly-projecting tube 

 which is the stomodaeum (Fig. 58, D). 



The mesenteries now make their appearance. The eight so-called 

 primary ones are formed about the same time. Each originates as a 

 fold of the endoderm which projects inwards into the gastric cavity : 

 the cavity between the limbs of the fold being occupied by a layer of 

 supporting lamella secreted by the cells forming the fold. The 

 mesenteries correspond exactly to the taeniolae which occur in the 

 Hydra-tuba of the Scyphozoa, and which are irregularly developed 

 even in the polyps of the Hydrozoa. The eight mesenteries are 

 arranged in four pairs, or, as it is usual to term them in this case, 



