CHKSTER. — STRUCTURE OF PSEUDOPLEXAURA CRASSA. 755 



larfic-moslu'd reticuliiiu tluit stains deeply with innci-carniim', and 

 each luis n darkly stainiii^' niieleus. There are oeeasioually found 

 also gland cells (Fig. ")5, (V. (/ni.) similar in character to the granular 

 gland cells of the stoniodaeuni. The feeble staining of the nucleus 

 and the presence of large granules in ttie cytoplasm show the likeness. 

 The cells may be siiorter or even spherical They are not limited to 

 particular regions, but are scattered throughout the endoderm of the 

 polyp wall, the canals and tiie mesenteries. 



The endoderm of the pol^p Avail (Figs. 20, 55) has longer cells and 

 stronger m\'onemes than that of the polyp chamber or canals. How- 

 e\er, in the pinnnles and in the tentacles, except at their very bases, 

 the myonemes and the granular gland cells are lacking (Figs. 5, 6); 

 otherA\ ise the layer is here like that of the body wall. 



The epithelial cells and the muscle elements of the endoderm of the 

 mesenteries (Plate 3, Fig. 33) are specialized, at least in certain 

 regions. On the so-called ventral surface of the mesentery the longi- 

 tudinal myonemes belong to cells that are entirely below the free 

 surface of the epithelium. These muscle cells are long and spindle- 

 shaped (Plate 4, Fig. 38), with a small amount of cytoplasm cnA'elop- 

 ing the fiber (myoneme) and most abundant around the elongated 

 nucleus. The cells are very numerous and the fibers are so arranged 

 that in the cross section of the mesentery they form wavy rows of 

 black dots adjacent to the ventral side of the mesogloea (Fig. 33, 

 my'nm.). Otiier muscle fibers, that run radially on the mesentery, 

 are found in both layers of the endoderm and their cells are epitlielio- 

 muscular, though in some cases the cell body may be slightly sunk 

 below the surface. Where the endodermal epithelium covers the 

 genital cells, the epithelial cells are shorter and bear no muscle 

 fibers. 



The endoderm of the longitudinal canals (Plate 4, Fig. 59) has very 

 long cells as compared with that of the other canals ; they are, however, 

 of the same type, \iz. supporting cells; they are slender, columnar, 

 vacuolated, and slightly separated from one another except at their 

 proximal and distal ends. Xo muscle cells, or myonemes, are found 

 in this endoderm. 



As compared w^ith corresponding structures in other alcyonarians, it 

 may be said, in brief, that the ectoderm of Pseudoplexaura is like that 

 of the other members of this group described by pre\ious authors, in 

 having an epithelial and a subepithelial layer; in the shape and charac- 

 ter of its cover cells it is like Alcyonium (Ilickson, '95, Kassianow, 



