White Colonial Phoronid

Phoronis ijimai is a species of marine horseshoe worm in the phylum Phoronida.

Body except lophophore 2.40–16.83 mm in length (avg. 5.87±4.04 mm, n = 34; average of topotypes 9.55±4.78 mm, n = 12); 0.49–0.90 mm in diameter at ampula (avg. 0.64±0.11 mm, n = 34; average of topotypes 0.59±0.12 mm, n = 12); white and translucent in living state (Figs 2A, 2B, 3), yellowish white after fixation (Fig. 2C). Lophophore horseshoe-shaped, without significant coiling (Fig. 4); 0.87–3.11 mm in length (avg. 2.17±0.55 mm, n = 34; average of topotypes 1.66±0.49 mm, n = 12), 0.27–0.99 mm in diameter at its base (avg. 0.61±0.17 mm, n = 34; avg. of topotypes 0.43±0.09 mm, n = 12); tentacles 106–151 in number (avg. 129±18, n = 7; avg. of topotypes 110±5, n = 3). Inhabits a transparent cylindrical tube either encrusting or burrowing in hard substrates (Fig. 2C). Nephridium 162.00–204.00 μm in height (avg. 183.00±29.70 μm, n = 2), with straight nephridial papilla and curved ascending branch (Fig. 5A, 5B). Descending branch absent. Ascending branch with single chamber. Nephridial papilla situated beside anus, 294.24–324.91 μm in length (avg. 309.57±21.69 μm, n = 2); nephridiopore situated on nephridial papilla opening above (in living orientation) anus level (Fig. 5A, 5C). Ascending branch offset along body axis near intestine, with its lower end extending toward esophagus (Fig. 5B, 5D); 277.55–323.49 μm in length (avg. 300.52±32.49 μm, n = 2). Two nephridial funnels present; anal funnel larger than oral funnel. Anal funnel large (avg. 69.00±4.24 μm in height, 45.77±3.15 μm in width at base, 111.94±16.48 μm in maximum width at tip; n = 2), its aperture located at lower end of ascending branch. Oral funnel small (avg. 20.01±1.40 μm in diameter, n = 2), its aperture opening on lateral surface of ascending branch, situated slightly lower than anal funnel. Body-wall longitudinal muscles of generally bushy type (Fig. 6A, 6B) but sometimes feathery in lower part of body; 45–50 in number, arranged in following formula (Selys-Longchamps 1907): Composite formula Mean formula (n = 7 sections from 3 individuals) Left and right lateral mesenteries present (Fig. 6A). Two giant nerve fibers present; left giant nerve fiber 3.16–10.61 μm in diameter (avg. 6.72±3.27 μm, based on eight sections from different parts of the body, from two individuals), situated at base of left lateral mesentery (Fig. 6C); right giant nerve fiber 2.47–7.81 μm in diameter (avg. 4.55±2.15 μm, based on nine sections of different parts of the body from two individuals), situated at base of right lateral mesentery. Esophageal valve absent. Hermaphroditic; early-stage ova and spermatocytes found beside lateral blood vessel. Brooded eggs observed in specimens from Hiroshima (Fig. 7A, 7B, 7C); embryos of various developmental stages brooded on basal nidamental glands on lophophore (Fig. 7C).