Page:Walcott Cambrian Geology and Paleontology II.djvu/88

 and destroyed the remains of the radiating canal system appear as fine, more or less irregular, shiny lines on the dark background.

Alimentary canal.—The alimentary canal is an open spiral located concentrically in the umbrella about midway between the center and margin of the disk (pl. 9, fig. 5). It is clearly shown in over two hundred specimens in the collection. The anterior or oral end opened on the surface of the subumbrella at a point about one-half the distance between the center and the outer margin, and the posterior or anal end opened farther out toward the margin. The coil of the canal was dextral or left to right and probably nearly on the same plane except that the anterior end bent downward from the region of the œsophagus and the anal end may also have been curved slightly downward. The bending of the anterior side is indicated by fig. 5, pl. 9, and text fig. 5.

The canal appears to be more or less corrugated (pl. 9, fig. 3; and pl. 10, fig. 3). Whether this corrugation has anything to do with the radial canals or lobes of the umbrella has not been satisfactorily determined, except that the radial lobation and the divisions formed by the slight constrictions causing the corrugation appear to be more or less in accord in size and position. The corrugations show more clearly on the outer margin of the canal. The canal is beautifully outlined on the dark, smooth shale by the glistening silver-like luster of the stomach section and the less prominent but distinct outlines of the oral and intestinal sections.

The canal is divided into four sections that, compared with the typical holothurian alimentary canal, may be considered as the oral chamber, œsophagus, stomach, and intestine.

The oral chamber is indicated (fig. 5, pl. 9) at the inner end of the spiral alimentary canal. The chamber extends from the outer end to the constriction indicating the œsophagus a little beyond the dotted line leading from the center out to the letters "cr". This chamber is also more or less clearly shown by fig. 3, pl. 8; fig. 1, pl. 9; and text fig. 5. (In this description it must be constantly recalled that we are dealing with specimens flattened in the shale.)

The outer opening of the oral chamber is best shown by fig. 5, pl. 9. A number of specimens show that from each side of the flattened opening there is a short projecting arm which supports a cluster of short tentacles, or, if we interpret the short arm as a strong tentacle, with a disk to which are attached digits, the whole tentacle being retractile and capable of being withdrawn into the oral chamber. Some of the specimens suggest a three-lobed disk (text fig. 5). With