Diagnosis: Bdelloura candida (Girard, 1850) can be distinguished from B. propinqua by its larger size, and well-developed caudal disk.
Habitus: Preserved specimens measure up to 11 x 5 mm; preserved animals may even be as large as 15 mm (Girard 1850) or 25 mm (Verrill 1893). According to Wilhelmi (1909) living specimens, whilst moving, are about 15 x 4 mm. The body is lanceolate or oval-shaped; the largest breadth is at about the middle of the body. The hind end shows a broad and rounded adhesive disk, which is separated from the rest of the body by a constriction. The disk or sucker is almost as broad as the body, although its form depends on the state of contraction. The lateral body margins are undulated. The entire body is whitish or yellowish.
The large pharynx is about one-third of the body length. The inner circular muscle layer of the pharynx is well developed and comprises about 45 % of the space between inner and outer epithelium. The outer circular muscle layer is considerably thinner. The mouth is situated at the middle of the pharyngeal pocket.
The anterior ramus of the intestine terminates at a short distance behind the brain. Behind the copulatory apparatus the posterior gut trunks have several commissures. The intestinal trunks give rise to mostly forked and lobulated, lateral diverticula. The anterior ramus shows 10-12 pairs of these diverticula, whereas each posterior ramus gives rise to about 20 lateral diverticula.
Male Reproductive System
The small and numerous testes extend along the lateral body margin from a short distance anterior to the root of the pharynx to behind the copulatory apparatus. In fully mature specimens there are about 85 testicular follicles on either side of the body. The rounded follicles usually do not extend between the tips of the intestinal diverticula. The testicular follicles are distributed between the dorsal and ventral body surfaces, i.e. the testes occur along the dorsal and ventral body surface, as well as in the middle of the body.
The vasa deferentia enlarge to form false seminal vesicles only behind the pharyngeal pocket; they show a strong laterally directed bend at the level of the penis and here the diameter of the ducts increases slightly. Hereafter, the vasa deferentia narrow again, while ascending to the penis bulb. The vasa deferentia may expand strongly after having penetrated the penis bulb, thus giving rise to a kind of seminal vesicle. The ducts narrow gradually before uniting in the intrapenial papilla.
The penis papilla is a broad, vertically oriented cone which may taper gradually to form a rather strong tip, or may be set off from the rest of the papilla by a shallow constriction. The very tip of the papilla may be bent strongly in caudal or frontal direction. The penis is provided with an intrapenial papilla in which the vasa deferentia unite to form a short common vas deferens; the intrapenial papilla projects into the ejaculatory duct.
The lining of male and common atrium is underlain with well developed layers of circular and longitudinal muscles. The musculature of the penis bulb is highly developed, consisting of strong, interwoven muscle fibres. The penis papilla shows a well-developed subepithelial layer of circular muscles, which is overlain by a thinner layer of longitudinal muscles. The musculature of the intrapenial papilla is only poorly developed.
Female Reproductive System
The small ovaries lie at a considerable distance behind the brain. In general, they are situated behind the third pair of intestinal diverticula, somewhat medially to the ventral nerve cords. In running backwards, the oviducts follow the course of the ventral nerve cords. At the hind end of the pharyngeal pocket each oviduct gives off a short branch towards the stalk of the lateral bursa. Thereafter, the oviducts continue their course, and unite to a common duct which meets the female genital duct. Numerous shell glands discharge into this female genital duct, which has an oblique, ventro-dorsal, orientation and opens into the posterior portion of the atrium.
The vitellaria are highly developed and occupy the entire space between ventral and dorsal body surface; the follicles lie medially to the testes.
A lateral bursa is situated on either side of the copulatory apparatus and the gonopore. Each bursa opens ventrally to the exterior through a somewhat obliquely oriented stalk. The stalks receive the short branches from the oviducts, the latter running medially to the lateral bursae; the stalks of the bursae are lined with ciliated cells. The opening of each oviducal branch into the stalks may be constricted, due to the presence of strong circular muscle fibres at the point of communication.
The lateral bursae are usually more or less sac-shaped vesicles which are lined with tall cells. The stalks of the bursae are surrounded by a layer of circular and longitudinal muscles, respectively. Contrary to Böhmig (1906: Pl.18), I found that these muscle layers do not extend on the sac-shaped part of the lateral bursa.
Each eye cup contains two retinal cells; the opening of the cup is closed by a thick corneal membrane; there is no lens.
Karyology: B. candida has a diploid complement of 12 chromosomes. The chromosomes of the first five pairs are sub-metacentric, whereas the sixth pair has very small sub-telocentric chromosomes (Pennypacker 1938, Kawakatsu et al. 1988).
Life Cycle: Cocoons are laid from May to mid-August (Wheeler 1894, Wilhelmi 1909). The cocoons are oval-shaped or elliptical structures set on on pedicel, which by means of an endplate is attached to the substratum. One side of the cocoon is flattened, the opposite side is arched. The cocoons are about 4 mm in length and 2 mm in diameter; the pedicel is about 1 mm long.
B. candida is an ectosymbiote of the Horseshoe Crab Limulus polyphemus. The triclads mainly occur on the last pair of cleaning legs and on the book gills of the crab, but may be found also on the joints of the walking legs. The animals are attached to the substratum by means of the caudal disk and feed on food particles left over from the meals of the Horseshoe Crab (Wilhelmi 1909, Lauer and Fried 1977).
Cocoons are laid exclusively on the inner surface of the gill lamellae. The capsules are attached near the lateral margin of the gill plate, with the apical end directed to that margin (Verrill 1893, Graff 1879, Wilhelmi 1909).
According to Pearse (1949), the average (n=4) rate of infestation is 172 triclads per individual Horseshoe Crab. The same author collected on average (n=4) 575 cocoons from the gills of individual specimens of Limulus polyphemus.
Type locality: Chelsea Beach, Massachusetts, U.S.A. B. candida is an ectosymbiote of the Horseshoe Crab Limulus polyphemus, which is confined to eastern North America, ranging from Maine to the Gulf of Mexico (Pocock 1902, Gosner 1971, Barnes 1980, Kawakatsu et al. 1987).
Z.M.B.: 8410, August 1907, Woods Hole, Mass. USA, whole mounts; 8408, whole mount; W 231, whole mount; W 228, whole mount; 8433 whole mount; 8447a, whole mount; 8447b, whole mount; 8419a-n, Woods Hole, 1907, sagittal sections; 8418a-n, transverse sections; W 233a,b, horizontal sections; 8432, Aquarium, 1906, sagittal sections; 8434, ibid., sagittal sections ( "W" refers to Wilhelmi's original numbers).