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Wheeler, 1894

Overview
Diagnosis: Syncoelidium pellucidum Wheeler, 1894 can readily be distinguished from other ectosymbiotic planarians by its small size, rounded front and hind end, the apparent lack of a caudal disk, and by the single intestinal ramus behind the copulatory apparatus.
Habitus: Live animals up to 4.5 mm long and 0.8 mm wide (Wilhelmi 1909); preserved specimens are up to 2.5 mm in length and 0.95 mm in width. The body is lanceolate with rounded front and hind end. The largest breadth of the body is at about the hind end of the pharyngeal cavity. The caudal disk is not set off from the rest of the body.
The paired eyes are set close together at a considerable distance from the anterior margin. The animals are unpigmented.

Alimentary System
In preserved specimens the pharynx is one-sixth to one-fifth of the body length. The inner circular muscle layer of the pharynx is considerably thicker than the outer layer of circular muscles. The mouth opening lies at the middle of the pharyngeal pouch.
The anterior ramus of the intestine terminates at a short distance behind the brain and gives off 5-6 pairs of lateral diverticula. The posterior rami fuse behind the copulatory apparatus, thus giving rise to a single backwards extending branch, which gives off about two pairs of lateral diverticula. Before fusion each of the posterior rami gives rise to 7-9 lateral diverticula and also to several short, medially directed branches.

Male Reproductive System
The number of testes are few: 10-17 on either side of the body. The follicles extend from a short distance behind the ovaries to well behind the copulatory apparatus; they are situated between the tips of the intestinal diverticula and occupy most of the space between the dorsal and ventral body surfaces.
The vasa deferentia expand to form large false seminal vesicles but their diameter decreases abruptly where they penetrate the penis. As thin ducts the vasa deferentia run through the proximal portion of the penis papilla, but in the distal part of the latter the ducts expand again to form accessory seminal vesicles. Hereafter the vasa deferentia narrow again and open separately at the tip of the intrapenial papilla.
The penis is a rather plump, cone-shaped structure which is provided with a well-developed layer of circular muscles, which is strongly developed at the base of the penis papilla. The penis is provided with a rather large intrapenial papilla.

Female Reproductive System
The paired ovaries are situated medially to the ventral nerve cords at about half-way (or somewhat less) the distance between the brain and the root of the pharynx. The oviducts arise from the ventro-lateral surface of the ovaries and initially the ducts run medially to the ventral nerve cords. However, at about the level of the lateral bursae the oviducts run dorsally to the nerve cords, after which they curve towards the midline of the body and unite to a common oviduct. The latter communicates with the female genital duct, which receives the secretion of shell glands and opens into the atrium.
The vitellaria are well developed, extending from anteriorly to the ovaries to well behind the copulatory appartus, and occupying the entire space between dorsal and ventral body surfaces.
At the level of the penis papilla a lateral bursa is situated on either side of the body. These bursae are large sac-shaped vesicles filling most of the space between dorsal and ventral body surface. They are lined with large vacuolated cells and may contain sperm. Each lateral bursa opens ventrally to the exterior through a very narrow stalk which is surrounded by a thick layer of circular muscles. The opening of this stalk to the exterior lies laterally to the ventral nerve cords.

Eyes
The eyes are situated directly on the brain and are enclosed by the glial covering of the latter. There is no lens; the number of retinal cells could not be determined.

Reproduction
Life Cycle: Cocoons are laid from mid-July to mid-August (Wheeler 1894, Wilhelmi 1909). Almost without exception the cocoons are deposited along the apical outer margin of the gill leafs of the Horseshoe Crab. The small and oblong cocoons measure 0.6-1.0 mm in length and 0.15-0.35 mm in diameter (Wheeler 1894, Wilhelmi 1909, Material Examined), and they are attached to a pedicel which has a length of 0.15-0.5 mm (Wheeler 1894, Material Examined). According to Wheeler (1894) one side of the cocoon is flattened and turned towards the gill leaf.

Ecology
S. pellucidum is an ectosymbiote of the Horseshoe Crab Limulus polyphemus. According to Wheeler (1894) the triclad only occurs on the gills and is never to be found on the cephalothoracic legs of the crab.

Distribution
Type locality: Woods Hole, Massachusetts, U.S.A. S. pellucidum is known only form Horseshoe Crabs collected in Massachusetts, viz. Woods Hole (Wheeler 1894, Wilhelmi 1909, Material Examined) and Pleasant Bay (Material Examined). However, the range of the triclad probably coincides with that of its host species.

Material Examined
Z.M.B: 8488, Woods Hole, 1907, one whole mount on 1 slide; 8490, 7 whole mounts on 1 slide; 8492, 4 whole mounts on 1 slide; 8491, 3 whole mounts on 1 slide; 8489, 6 whole mounts on 1 slide; 8493, 2 whole mounts on 1 slide; Wilhelmi collection no. 262a-f, 6 whole mounts on 1 slide; 8487a+b, Woods Hole, 1907, transverse sections of one specimen on 2 slides; 8494, sagittal sections of one specimen on 1 slide; 5401-1, Woods Hole, 08.1907, sagittal sections on 1 slide; 5401-2, sagittal sections on 1 slide.
Private collection L. F. Bush: C999-1, Woods Hole, Jan. 1982, sagittal sections on 1 slide; C999-3, sagittal sections on 1 slide; C999-5, horizontal sections on 1 slide; C1004-1, Pleasant Bay, Massachusetts, 13/14.09.1982, sagittal sections on 1 slide.

Type Material
No type specimen available.

Syncoelidium pellucidum