l82 THE INVERTEBRATA 



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 definite positions on the mesenteries. This may be seen in 

 the sections of a polyp in Figs. 140 and 141. It must in the first place 

 be explained 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 ipesenteric filament is seen, and this consists of 

 different elements in the different mesenteries. One pair of mesen- 

 teries, 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. 140 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 



