Diagnosis: Bdelloura propinqua Wheeler, 1894 can be distinguished from its congeners by its small caudal disk and lower number of testicular follicles, which extend between the intestinal diverticula.
Habitus: Preserved, mature specimens are up to about 6 mm long and 2.25 mm wide. Living specimens measure up to 8 mm in length and 1.30 mm in width (Wheeler 1894, Wilhelmi 1909). The body is lanceolate with the largest breadth slightly anterior to the root of the pharynx. From there the body narrows gradually towards the hind end, or the body margins runs more or less parallel to the level of the copulatory apparatus, after which they converge towards the rear end. The posterior end of the body functions as an adhesive disk, but the latter is not set off from the rest of the body by means of a constriction, as is the case in Bdelloura candida. In B.propinqua the body margins taper, without interruption, towards the square or slightly convex hind end.
Live animals are white (Wilhelmi 1909). The position of the eyes is similar to that in B.candida.
The pharynx is rather small, being one-sixth to almost one-fourth of the body length. The well-developed inner circular muscle layer of the pharynx comprises about 25 % of the space between the inner and outer pharynx epithelium. The mouth opening lies slightly posteriorly to the middle of the pharyngeal cavity.
The anterior ramus of the intestine terminates at a short distance behind the brain. The posterior intestinal rami do not meet in the hind end of the body, nor do they have commissures. The anterior ramus gives rise to about 8-12 lateral diverticula, whereas each posterior ramus gives off about 15-19 of these branches. As in B. candida, these diverticula are often forked and have rounded tips.
Male Reproductive System
The testes are less numerous than in B. candida, i.e. there are about 58 follicles on either side of the body. The testes extend from the level of the ovaries to well behind the copulatory apparatus. In dorsal view the testes are more or less oval-shaped; they extend between the tips of the intestinal diverticula. In general, the testes are removed as far from the dorsal as from the ventral body surface. The course of the vasa deferentia is similar to that in B.candida, with the difference that they may already form false seminal vesicles anterior to the pharynx. The male copulatory apparatus is similar to that of B. candida.
Female Reproductive System
The ovaries are generally situated between the third pair of intestinal diverticula, although occasionally they lie between the second or fourth pair of diverticula. The ovaries are situated medially to the ventral nerve cords. The course of the oviducts is similar to that in B. candida. The female copulatory apparatus and the lateral bursae are similar to the same structures in B. candida.
As in B. candida, but number of retinal cells unknown.
Life Cycle: Cocoons are laid on the gills of the Horseshoe Crab Limulus polyphemus from mid-July to mid-August (Wheeler 1894, Wilhelmi 1909). B. propinqua prefers to lay its cocoons at the basal portion of the gill leaf, at some distance from the margin. The coocon is much smaller than that of B. candida, measuring only about 1.25 mm in length and 0.3-0.4 mm in diameter (Wheeler 1894, Wilhelmi 1909). The capsules have a narrow, elongated appearance and possess a rounded tip; they are placed on a pedicel which is about 0.4 mm wide (Wilhelmi 1909) and is set on a small endplate. Probably the cocoons are somewhat flattened on one side, but the literature is not clear on this point.
B. propinqua lives on the Horeshoe Crab Limulus polyphemus.
Type locality: Woods Hole, Massachusetts, U.S.A. For distribution: see B. candida.
Private collection I. R. Ball: C78, Woods Hole, Mass, U.S.A., Febr. 1973, transverse sections on 1 slide; C78, sagittal sections on 1 slide; C78, sagittal sections on 3 slides (labelled SS1, SS3, SS4); C78-1, sagittal sections on 2 slides; C78-2, sagittal sections on 1 slide; C78-4, sagittal sections on 2 slides; C57, Nahant, Mass., U.S.A., August 1973, horizontal sections on 1 slide; C57-1, sagittal sections on 2 slides; C57-3, sagittal sections on 2 slides; C57-5, sagittal sections on 2 slides; C57-6, sagittal sections on 2 slides; C46, Panacea, Florida, laboratory culture University of Waterloo, 11.04.1973, sagittal sections on 3 slides; C46, whole mount; WAT-2, laboratory culture University of Waterloo, sagittal sections on 3 slides.
Z.M.B.: 8440a, 8440b, Woods Hole, 1907, sagittal sections of one specimen on 2 slides; 8438, Woods Hole, 1907, 4 whole mounts on 1 slide; 8435, 5 whole mounts on 1 slide; 8436, 4 whole mounts on 1 slide; W 242a-d, 4 whole mounts on 1 slide; W 246a-d, 4 whole mounts on 1 slide; 8437, 4 whole mounts on 1 slide ("W" refers to Wilhelmi's numbers on the labels).