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(Ijima and Kaburaki, 1916)

Overview
Diagnosis: Ectoplana limuli (Ijima and Kaburaki, 1916) can be distinguished from all other ectosymbiotic marine triclads living on Horseshoe Crabs, by the conical penis papilla, the presence of a copulatory bursa, and by the outbulgings of the bursal canal.
Habitus: Fully extended live specimens up to 4-6 mm long and 0.7-1 mm wide. The body has an elongate-lanceolate shape, the body margins converging gradually towards the obtusely pointed front and hind end. The eyes are set close together and lie at a considerable distance from the anterior margin. Pigment is absent.

Alimentary System
The anterior ramus of the intestine extends as a single branch anterior to the eyes; behind the eyes it gives rise to about 8-10 lateral diverticula. Each posterior ramus gives off about 14-16 lateral diverticula, while the rami meet in the hind end of the body.
The pharynx is between one-fifth and one-fourth of the body length. The inner circular muscle layer of the pharynx is thicker than the outer circular muscle layer. The mouth opening is at the hind end of the pharyngeal pocket.

Male Reproductive System
There are about 18 ventrally placed testes on either side of the body, extending from directly behind the ovaries up to the root of the pharynx.
In the penis bulb the vasa deferentia unite to a common duct which, subsequently, expands to form a small seminal vesicle. From the latter arises the ejaculatory duct which opens at the tip of the penis papilla. The ejaculatory duct is surrounded by a well-developed layer of circular muscles, which is continuous with a thinner layer surrounding the short common vas deferens at the base of the penis bulb. Penis glands, which lie outside the penis, discharge their secretion into the ejaculatory duct.
The penis consists of a large bulb and a broad, stubby papilla. The latter is covered with an infranucleate epithelium which is underlain with a well-developed, subepithelial layer of circular muscles and a layer of longitudinal muscles. The male atrium is lined with flat, infranucleated cells, excepting the postero-ventral section of the atrium, which is lined with tall, nucleated cells.

Female Reproductive System
The paired ovaries are situated directly behind the brain. The oviducts arise from the lateral surface of the ovaries and run laterally to the ventral nerve cords. At about the level of the gonopore the oviducts turn dorso-medially and open separately into the large lateral expansions of the dorsal portion of the bursal canal. The oviducts are penetrated by openings of shell glands just before communicating with the bursal canal.
Apart from the two large lateral expansions, the dorsal portion of the bursal canal also shows a well-developed, anteriorly directed extension and one that is directed posteriorly, the latter communicating with the copulatory bursa. The ventral section of the bursal canal shows various, laterally directed expansions which, however, show bilateral symmetry. The bursal canal is lined with an infranucleate epithelium which is underlain with a well-developed subepithelial layer of circular muscles and a layer of longitudinal muscle fibres; on the dorsal, expanded section of the bursal canal these two muscle layers are somewhat intermingled. The parenchyma around the bursal canal is traversed by muscle fibres which run from the canal outward into the surrounding parenchyma. The bursal canal, including its expansions, is penetrated by numerous openings of unicellular glands.
The irregularly shaped copulatory bursa is surrounded by a thin layer of muscles.

Eyes
The eye cups contain three retinal cells and an oval lens.

Reproduction
Life Cycle: Kaburaki (1922) assumed cocoon-laying to take place in July, since he found half-grown animals in early August. Because these young triclads were attached to the gills, Kaburaki also assumed that cocoons are deposited on the gill lamellae.

Ecology
E. limuli is ectosymbiotic on the Horseshoe Crabs Tachypleus tridentatus, T. gigas, and Carcinoscorpius rotundicauda.

Distribution
Type locality: Ectoplana limuli was originally collected at Ajino, Province Bizen, Japan (34°28'N 133°48'E) from the Horseshoe Crab Tachypleus tridentatus. The species is known from its type locality and another sampling locality in the vicinity of that place (Kaburaki 1922). Kawakatsu et al. (1987) and Kawakatsu and Sekiguchi (1988) reported E. limuli from the Gulf of Siam, Thailand and from two new localities in Japan, viz. Tamano and Hakata Bay. When the range of E. limuli coincides with the combined ranges of its host species, then the area of distribution of the triclad covers a large part of southeast Asia.

Material Examined
Kaburaki (1922) has provided a detailed account of the species and his publication formed the basis of the following description. Further I have profited from reading the manuscript of Kawakatsu and Sekiguchi (1988) and from examining a few specimens which I received through the courtesy of Professor Kawakatsu.

Type material
Kawakatsu and Sekiguchi (1988) designated a neotype, which is deposited in the N.S.M.T.

Ectoplana limuli