82 EMBRYOLOGY 



toward the ventral side of the polyp, whereas the surfaces of 

 the septa which are without longitudinal muscle bands face 

 toward the dorsal side. The bilateral symmetry can also be 

 recognized by the presence of a ventral ciliated groove 

 running along the laterally compressed oesophagus (siphono- 

 glyphe, Hickson), and above all by the condition of the 

 mesenterial filaments. Of these the pair belonging to the 

 dorsal septa diifers from the others in structure, function, 

 and development. The filaments of the dorsal pair of 

 septa exhibit an epithelial band consisting of tall flagellate 

 cells, and produce a powerful upward ciliary current, 

 whereas the filaments of the other six septa are charac- 

 terized by their richness in gland cells, and they play an 

 important role in digestion. E. B. Wilson (No. 97) was 

 able to show that the latter take their origin as simple out- 

 growths of the entodermal epithelium of the septa, whereas 

 the dorsal filaments belong to the ectoderm, and are con- 

 tinued on to the margins of the septa as direct outgrowths 

 of the oesophageal epithelium. 



An observation by Wilson is of general interest : that the develop- 

 ment of these dorsal filaments is retarded in the larvae produced from 

 the egg, whereas in the bud they actually outstrip the other filaments in 

 development. Wilson explains this by the conditions of nutrition in 

 the bud, which requires a powerful upward stream of nutritive fluid for 

 its development. 



Of the various kinds of non-sexual reproduction in the 

 Alcyonaria, budding is the most prevalent ; by means of it 

 extensive colonies (stocks, corms) are developed, owing to 

 the fact that the newly arising individuals remain united 

 with the parent. In the simplest case a lateral " runner " 

 arises from the parent animal and grows out at its end into 

 a daughter individual. The portion remaining between the 

 two as a connective is called a stolon (Fig. 36 A). These 

 stolons, issuing from the base of the polyp, may form a net- 

 work (Cornularia), or fuse into a basal plate (Rhizoxenia). 

 We have seen above (p. 81) how, owing to the formation of 

 a basal skeletal plate upon which an axial skeleton arises, 

 the dendritic stocks of the Gorgonidse can be derived from 

 such flatly extended colonies (Fig. 36 B). In other cases 



