Diagnosis: Ectoplana undata Sluys, 1983 can be distinguished from other marine triclads living on Horseshoe Crabs by its small number of pre-pharyngeal testes, and the absence of lateral bursae. Further, the species is characterized by (1) a short and blunt penis, (2) a large seminal vesicle in the penis bulb, (3) the wavy course of the male atrium, and (4) oviducts which open separately into the ventral portion of the bursal canal.
Habitus: Preserved specimens up to about 2.5 mm long and 1.25 mm wide. The body is elongate-oval-shaped; front end and hind are rounded. The small eyes are situated at a considerable distance from the anterior margin and lie close together. The body is white, due to lack of pigmentation.
The pharynx is between one-sixth and one-fifth of the body length. The inner circular muscle layer of the pharynx is much thicker than the outer circular muscle zone. The mouth opening is situated at the hind end of the pharyngeal pocket.
The anterior ramus of the intestine terminates at a short distance behind the brain and gives rise to about 6 or 7 pairs of lateral diverticula. The posterior intestinal trunks are confluent in the hind end of the body, whereas commissures may also be present.
Male Reproductive System
There are about 6-8 testes on either side of the body. The follicles occur more or less in pairs between the diverticula of the anterior intestinal ramus, extending from behind the ovaries to the root of the pharynx. The large follicles occupy most of the space between dorsal and ventral body surface.
At the level of the pharynx, the vasa deferentia enlarge to form false seminal vesicles which narrow again before penetrating the penis bulb. Immediately after having entered the bulb, the vasa deferentia open into a large, rounded seminal vesicle. The latter tapers into a narrow ejaculatory duct which opens at the blunt tip of the penis papilla. The seminal vesicle is lined with cuboidal, nucleate cells and is surrounded by a layer of circular muscle fibres.
The penis consists of a small bulb and a short, blunt papilla that is oriented more or less parallel to the body surface. No conclusive information could be obtained on the nature of the lining epithelium of the penis papilla but neither nuclei nor subepithelial muscles could be discerned. At some places the parenchyma of the penis papilla contains a fine granular secretion. The musculature of the penis bulb is only weakly developed.
The penis papilla projects into the spacious dorsal portion of the male atrium. The ventral part of the latter is much narrower and communicates with the common atrium. This narrow, distal, portion of the male atrium is rather long and has a more or less undulating, wavy course. In some specimens the course of the distal part of the bursal canal may not be so smooth but show a more pronounced bend (cf. Sluys 1983: Fig.3). The cells lining the narrow, ventral portion of the male atrium are infranucleate, in contrast to the lining epithelium of the dorsal part of the atrium; the spacious section of the male atrium is lined with nucleate cells. The most distal part of the male atrium shows a narrow, dorsally directed fold. The male atrium is surrounded by a subepithelial layer of circular muscles, consisting of several rows of fibres, overlain with a zone of longitudinal muscles. The thicknes of both muscle layers decreases on the proximal, spacious portion of the male atrium.
Female Reproductive System
The ovaries are situated directly behind the brain. No information is available on the way in which the oviducts arise from the ovaries. At the level of the gonopore the oviducts curve medially and open separately into the distal portion of the bursal canal. The oviducts receive the openings of shell glands shortly before they meet the bursal canal.
The vitellaria are only moderately developed and extend from behind the ovaries up to the level of the copulatory apparatus, while they occupy the entire space between dorsal and ventral body surface.
The oval or sac-shaped copulatory bursa is lined with cuboidal, nucleate cells. The bursa is surrounded by a thin layer of muscle fibres. The bursal canal arises from the anterior surface of the bursa and close to its point of emergence the canal shows a small, dorsally directed extension. The remaining part of the bursal canal runs obliquely towards the common atrium.
The lining epithelium of the bursal canal is penetrated by the openings of orange-red staining shell glands and it is devoid of nuclei. The major part of the musculature around the canal is composed of more or less circularly running muscle fibres, interspersed with longitudinally or irregularly running fibres. Other fibres run from the bursal canal into the antero-dorsal portion of the parenchyma around the canal.
Apart from the shell glands, the bursal canal is also surrounded by deep blue staining dots. These dots may correspond with insunk nuclei or with unicellular glands which discharge into the canal, or even with nuclei of muscle fibres.
The eyes lack a lens; the number of retinal cells could not be determined with certainty.
Life Cycle: The egg-shaped cocoons measure up to about 1.25 mm in length and 0.75 mm in diameter and are provided with a short pedicel.
E. undata lives on the Horseshoe Crab Tachypleus gigas.
Type locality: Singapore. E. undata lives on the Horseshoe Crab Tachypleus gigas. The triclad has been found on crabs from Singapore and Deli, Sumatra. Cocoons, but no triclads, were found on specimens of T. gigas from Java and Sumatra. It is not unreasonable to assume that the distribution of E. undata coincides with the combined ranges of its host species.
Material Examined and Type Material
Z.M.A.: Holotype: V.Pl.603, sagittal sections on 2 slides of specimen collected from Tachypleus gigas from Singapore; Paratypes: V.Pl.604.1, sagittal sections on 2 slides; V.Pl.604.2, sagittal sections on 1 slide. Both paratypes collected from T.gigas from Deli, Sumatra.