ALCYONARIA 159 



through the superficial layers of the colony are rather difficult to 

 interpret in consequence. 



There is no oral cone in the actinozoan polyp, but the mouth is an 

 elongated slit and is situated in the middle of a circular flattened area, 

 the oral disc, which is surrounded by the tentacles. It does not open 

 directly into the enteron but into a tube lined with ectoderm, the 

 stomodaeum or gullet, which communicates with that cavity. The whole 

 of the stomodaeum is ciliated, but at one end of it there is a groove 

 which is lined with specially strong cilia which draw water in at the 

 mouth. This is the siphonoglyph, and it is said to occupy a -ventral 

 position, but the student must be warned that there is no homology 

 between surfaces so termed in the coelenterates and in the higher 

 Metazoa. 



Internally the enteron is divided up by eight vertical folds of 

 the body wall, the mesenteries, which project so far into the cavity of 

 the enteron that their upper parts join with the stomodaeum. Below 

 the level of this organ they end in an enlarged free edge, the mesenteric 

 filament. The foundation of the mesentery is the mesogloea, which is not 

 much thicker here than in the body wall but is folded in the muscular 

 region of the mesentery. On both sides it is covered with endodermal 

 epithelium. While in the hydroid polyp there is little differentiation 

 into regions, in the actinozoan polyp the endodermal cells specialized 

 for various functions are arranged in strips of tissue occupying de- 

 finite positions on the mesenteries. This may be seen in the sections 

 of a polyp in Figs. 136 and 137. It must in the first place be ex- 

 plained that the presence of the siphonoglyph and the elongation of 

 the stomodaeum are an indication that on the original radial sym- 

 metry of the polyp a bilateral symmetry has been imposed, and on 

 each side of the axis of the stomodaeum the mesenteries correspond 

 exactly in arrangement. Now the muscular endodermal cells are 

 concentrated on the ventral side of each mesentery and into a narrow 

 part of it to form a longitudinal retractor muscle. In the section below 

 the siphonoglyph the mesenteric filament is seen, and this consists 

 of different elements in the different mesenteries. One pair of 

 mesenteries, which are dorsal in position, are distinguished from the 

 rest in having a filament which is flattened in cross-section, and is 

 covered by very large ciliated cells (Fig. 136 F). They work in concert 

 with the cells of the siphonoglyph to produce a current of water 

 which is drawn in at the mouth and flows right along the ventral side 

 of the tubes through the system, bearing with it oxygen and food for 

 the tissues which are contained in the depths of the colony. The cilia 

 of the dorsal mesenteries are responsible for the return current which 

 makes its way out of the polyp by the dorsal side of the stomodaeum. 

 These two mesenteries are much longer than the rest, as may be seen 



