Phascolion (Isomya) gerardi Rice, 1993
This species lacks holdfast papillae, which are common in other species of Phascolion. The trunk of P. gerardi is completely covered by prominent papillae, the anus opens on the mid-introvert and the tentacles are elongated.
Phascolion tuberculosum is similar to P. gerardi in the arrangement and prominence of papillae, but differs by the presence of flattened holdfast papillae and and the opening of the anus in the anterior trunk (Rice, 1993).
External anatomy: Introvert up to three times the trunk length, sometimes with a bulbous region posterior to the tentacles (depending on the state of specimen contraction). Scattered hooks are present in the bulbous region. Hooks are brown, simple, and curved, averaging 30 µm in basal width and 20 µm in height. Trunk from cylindrical to spherical, less than 10 mm long. Prominent papillae covering the entire trunk, but lacking holdfast papillae. Three types of papillae: the largest ones at the anterior and posterior end are elongated, often cone-shaped with two-or three pores opening on apical protrusion; papillae at mid-trunk are tapered or rounded with a single apical pore; and small, sharply tapered papillae are closely distributed over the introvert. Tentacular crown consists of 9-24 digitiform tentacles with grooves on the inner surface leading to the mouth. Nuchal organ is a distinctive dorsal band, with numerous irregular folds or longitudinal corrugations, extending half way around the base of the tentacles, becoming narrower laterally. Anus situated dorsally in the mid-introvert. (From Rice, 1993; Cutler, 1994). Internal anatomy: Retractor muscles originate on the trunk wall at the posterior end of the trunk. Esophagus is attached by mesenteries for the length of the introvert. Contractile vessel ends in a prominent bulge along the posterior esophagus. Nephridium (right) originates at the anterior end of the trunk. Gonads, at the base of the retractor muscle, developed in lobules and in some specimens extend as a strand between the two muscles posterior to the end of the ventral nerve cord.
Total body length average is 17 mm (Rice, 1993).
Bahamas, Belize and the Yucatán coast of Mexico (Cutler, 1994).
This species inhabits cavities in coralline limestone (Rice, 1993).
The eggs are quite large (the average diameter from measurements in 50 specimens is 257 µm), spherical and opaque and white, enclosed by a thin egg envelope (Rice, 1993).