“This is a curved to U-shaped species that can reach a maximum length of 10 cm (4 in), but most individuals are half that size. The body is swollen near the middle and tapered at both ends. In some specimens, the posterior end is drawn out into a short tail that lacks tube feet. The skin is soft, thin, and smooth, and in live individuals it is somewhat slimy. Short, cylindrical tube feet are set on low, conical warts and are somewhat scattered. However, there is a tendency, which is most pronounced on the introvert, for the tube feet to be aligned in double rows along the radii, the rows separated by a narrow strip of naked body wall. The tube feet have well-developed terminal suction disks, and they can retract completely below the surface of the body wall. Dorsally, the tube feet are widely spaced, and ventrally they are close set and numerous. The mouth is surrounded by 20 tentacles, an outer whorl of five large pairs and an inner whorl of five smaller pairs. Between each adjacent pair of large tentacles in the outer whorl, there is a smaller pair situated closer to the mouth. The larger tentacles are four or five times the length of the smaller ones and more richly branched. The anus is surrounded by 10 small
papillae, two in each radius. The body wall
ossicles are tables that have four to eight large perforations and, occasionally, one to six smaller accessory perforations. The tables are delicate, with flat, thin disks and conspicuously
dentate margins. The reduced spires of two short pillars terminate in one to three short, acute spines. The coloration of
P. occidentalis varies from a uniform light or dark brown to yellow or golden brown. The tube feet are transparent, except for a dark ring of pigment surrounding the white suction disk. In many individuals, there are patches of dark brown flecks on the body wall, adjacent to the tube feet. The dark brown or black tentacle stems contrast with the lighter branches, which have heavy accumulations of ossicles. When fully extended, the introvert is transparent, and the internal calcareous ring is visible” (Hendler, Miller, Pawson, Kier 274).
