CHESTER. — STRUCTURE OF PSEUDOPLEXAURA CRASSA. 749 



Other cells, lying close to the mesogloeii with processes parallel to it, 

 are interpreted as ganglion cells (Fig. 20, el. (jn.). The diflieulty with 

 tiiis inter|)ri-tation is due to the presence in the adjoining niesogloea 

 of the niesogloeal cells, which sometimes resemble the supposed 

 ganglion cells in shape. In preserved material, the fibers of these 

 ganglion cells sometimes possess many minute varicosities. The cyto- 

 plasm is more evenly and closely granular than in surrounding cells. 



The ectoderm of the tentacles (Plate 2, Fig. 5) and pinnules (Fig. 

 6) is relatively thicker than that of the polyp wall, but shows the same 

 superficial and deep layers. It also differs from that of the body wall 

 in the smaller number of the large kind of nematocysts and in the 

 presence of muscle cells in the ectoderm of the oral face of tentacle 

 and pinnule. The deep end of the muscle cell (Figs. 16, 17) is elon- 

 gated into a process which is perpendicular to the axis of the body 

 of the cell, and runs lengthwise of the tentacle or pinnule. Each 

 process contains in its axis a single highly refractive contractile fiber, 

 or myoneme. The nucleus is small and finely granular. Usually 

 the cell body is flattened in the plane of the muscle fiber; but the 

 flattening may be at right angles to that plane. It is only occasion- 

 ally that these muscle cells reach to the surface of the ectoderm and 

 thus present the typical condition of an epithelio-muscle cell. Since 

 all the muscle fibers run lengthwise of the tentacle, a transverse section 

 of the tentacle (Fig. 7) shows, adjacent to the mesogloea of the oral 

 surface and the sides, a very definite row of dots — the cut ends of 

 muscle fibers. In sections dyed in Mallory's stain these fibers, being 

 deeply colored, appear as dark dots, or, if cut somewhat obliquely, 

 as short lines. In the partially contracted condition of the tentacle 

 the dots no longer occupy a plane surface, for the originally plane 

 surface is so folded that the line of dots is very sinuous, and even 

 forms in some regions a series of pinnate figures. 



The ectoderm of the polyp wall passes gradually into that of the 

 coenench^ma, where, between the polyps, the cover cells (Plate 3, 

 Figs. 23, 24) are very long and conical, but never show the indented 

 peltate shape seen in the ectoderm of the polyps. The ectoderm of 

 the coenenchyma shows a thicker subepithelial region, which gradually 

 merges with the mesogloea. As a rule, the interstitial cells are so 

 numerous in the thick mesogloea and the niesogloeal elements are so 

 near the long cover cells that it is hard to find a boundary between 

 the two layers. The nematocysts of the larger kind are arranged in 

 very large and numerous batteries; those of the smaller kind occur 

 individually and are very few. Both kinds of nematocysts have the 



