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Holmquist and Karling, 1972

Overview
Diagnosis: Oregoniplana opisthopora Holmquist and Karling, 1972 may be distinguished easily from other triclads by (1) the sacs which communicate with the ovaries through a curved duct that meets the dorsal wall of the ovaries, (2) the presence of one pair of big testes, (3) the broad commissure between the two false seminal vesicles, and (4) the caudal pore of the copulatory apparatus.
Habitus: Living animals are about 2 mm long. The dorsal surface of the lanceolate body is dark. The eyes are close together and situated at a considerable distance from the anterior body margin. The pharynx lies almost in the middle of the body.

Alimentary System
The pharynx measures between one-seventh and one-fifth of the body length. The inner circular muscle layer of the pharynx is thin, consisting of only one or two rows of fibres. The mouth opening is at the hind end of the pharyngeal cavity.
The anterior ramus of the intestine gives rise to two pairs of preocellar diverticula, whereas behind the brain it also gives rise to two pairs of lateral diverticula. The two posterior gut trunks meet in the hind end of the body.

Male Reproductive System
There is only one pair of very large testes situated between the ovaries and the root of the pharynx; the follicles occupy the entire space between dorsal and ventral body surfaces. The vasa deferentia arise from the ventral wall of the testes and expand to form well-developed seminal vesicles, which have a broad commissure at the level of the root of the pharynx. The vasa deferentia penetrate separately, but closely together, the weakly muscularized penis bulb. Within the bulb the ducts unite to form a short common vas deferens, which opens into the ejaculatory duct via a small intrapenial papilla. In one specimen examined, the common vas deferens gave rise to an atypical dorsal expansion (Fig. 39).
The ejaculatory duct, which is lined with an infranucleate epithelium, gives rise to a dorsal expansion at about half-way its length. At the level of this expansion, which may be developed to greater or lesser extent, the ejaculatory duct receives a coarse-grained secretion of penis glands. Another type of penis gland discharges a more fine grained and weakly staining secretion into the proximal part of the ejaculatory duct. The latter is surrounded by a row of strong circular muscle fibres and opens at the tip of the penis papilla.
The penis is positioned more or less parallel to the body surface and is lined with a flat, nucleate epithelium. The penis papilla is provided with a thick, subepithelial layer of strong circular muscles, with entally to this layer a weakly developed layer of longitudinal muscles.

Female Reproductive System
The vitellaria extend from anterior to the ovaries to about the level of the penis, occyping the entire space between dorsal and ventral body surface.
The ovaries are situated at some distance behind the brain, viz. at a position between one-fourth and one-third of the distance between the brain and the root of the pharynx; they lie medially to the ventral nerve cords. The germ centre is located in the ventro-lateral part of the ovaries. From the dorsal surface of each ovary arises a well-developed duct which curves dorso-laterally and, subsequently, communicates with a sac-shaped structure ("stromatic sac). The latter lies more or less laterally, but very close, to each of the ovaries. These sacs are filled with poorly defined, nucleate cells, interspersed with sperm. The connecting duct between sac and ovary is lined with cuboidal, nucleate cells. The oviducts arise from the ventro-lateral walls of the sacs and run laterally to the ventral nerve cords.
In the hind end of the body the oviducts open separately into a ciliated duct which runs between the female atrium and the caudal end of the body, where it opens to the exterior. This duct is lined with cuboidal, nucleate cells bearing well developed cilia.
A small duct opens dorsally into the most posterior part of the duct into which also the oviducts open. This small and narrow duct may be developed to greater or lesser extent and it is lined with nucleate, ciliated cells. In some specimens this small duct appears to end blindly, but in others it opens into a small and ill-defined sac-shaped vesicle. The last-mentioned situation was observed in one of the paratypes examined and appears to occur also in the holotype (cf. Holmquist and Karling 1972: Fig.8).

Nervous System and Eyes
There is a thick commissure between the ventral nerve cords, just anteriorly to the gonopore.
The eyes are devoid of a lens; the number of retinal could not be determined with certainty. The eye cups lie antero-dorsally to the brain.

Ecology
The type specimens were collected from "sandy substrate of Zostera meadows on rocks in the surf zone at low tide" (Holmquist and Karling 1972).

Distribution
Type locality: Yaquina Head, Oregon, U.S.A. The species is only known from its type locality.

Material Examined and Type Material
S.M.N.H.: Paratypes: Oregon, Yaquina Head, 4.05.1969, 6 specimens sectioned sagitally, each on 1 slide; 2 specimens sectioned transversally, each on 2 slides.
TYPE S.M.N.H.: Holotype: sagittal sections of one specimen on 2 slides. Paratypes: see above.

Oregoniplana opisthopora