IV 



COELENTEEATA 



ment is only temporary, it is some considerable time afterwards that 

 a permanent attachment is effected and the pedal disc of the adult 

 formed. Before this occurs the rudiments of the first eight tentacles 

 make their appearance. Each arises as a simple outpouching of one 

 of the eight chambers into which the coelenteron is divided (Fig. 59, b), 

 and all appear to arise about the same time ; those belonging to the 

 dorsal and ventral chambers and to the lateral chambers are larger 

 than those belonging to the other four chambers. 



The thickened edges of the mesenteries, where they end freely 

 in the coelenteron below the stomodaeum, are known as the 

 mesenterial filaments. In the adult Uriicina each filament is 

 composed of a median strip of cells "containing gland cells and 

 cnidoblasts, flanked by two strips of cells carrying long cilia. The 

 mesenterial filaments appear on the ventro-lateral mesenteries long 

 before they appear on the others. They first appear in the neigh- 

 bourhood of the stomodaeum and grow downwards as simple streaks 

 of columnar epithelium ; then the central portion shows the 

 glandular modification of its cells, and much later the lateral portions 

 develop cilia. The other filaments make their appearance in the 

 same manner much later, when tentacles have already been developed. 

 According to Appellof, and 

 here he has the support of 

 other authors such as Gardiner 

 (1902), the filaments are com- 

 posed of ectoderm which has 

 grown down from the stomo- 

 daeum. 



In Alcyonaria where, as in 

 the young Urticina, there are 

 eight mesenteries, two only 

 of these, the so-called dorsal 

 mesenteries, bear filaments 

 which carry cilia, — the other 

 filaments being purely gland- 

 ular in character whilst the 

 ciliated filaments are devoid of 

 gland cells. Following Wilson 

 (1883), who is the best 

 authority on Alcyonarian de- 

 velopment, the two dorsal fila- 

 ments are usually said to be of 

 ectodermal origin whilst the 

 others are stated to be endodermal, but the evidence which he adduces 

 in favour of this view is, however, neither thorough nor convmcmg. 



As the filaments are stated to close round the body of ingested 

 prey and to form a sort of temporary alimentary tube within the 

 coelenteron, and as their cells are stated to be the cells which secrete 

 the digestive ferment, one would at first sight naturally expect them 



VOL. I ^ 



Fig. 61. — Trausverse section through a larva of 

 Urticina crassicornis hi which the tentacles 

 have just been developed. (After Appellof.) 



Lettera as in preceding ligure.s. In addif;ion, j, jelly 

 or supporting lamella ; m.s, mesenterial stomata ; muse, 

 muscular thickening on mesentery. 



