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Holmquist and Karling, 1972

Overview
Diagnosis: Pacifides psammophilus Holmquist and Karling, 1972 is characterized by unique structures such as the sclerotized musculo-glandular organ, the saddle-shaped copulatory bursa, and the anterior intestinal ramus running dorsally to the pharyngeal cavity. Further characteristics are the anteriorly situated pharynx, the posteriorly located ovaries, and the spinose ejaculatory duct.
Habitus: Live animals are about 2.5 mm long. Dorsal body surface richly provided with pigment granules and, consequently, rather opaque. Body lanceolate but provided with a pair of small, pointed auricles.

Alimentary System
The pharynx lies in the anterior half of the body and measures between one-sixth and one-fifth of the body length; it is inserted at the antero-dorsal wall of the pharyngeal cavity. The inner circular muscle layer of the pharynx is considerably thicker than the outer circular muscle layer. The mouth opening is situated at the front end of the pharyngeal cavity.
The anterior ramus of the intestine extends from anteriorly to the eyes to behind the pharyngeal pouch, giving off several pairs of postocellar diverticula, but without giving rise to preocellar diverticles. The anterior ramus communicates with the pharynx and runs dorsally to the pharyngeal cavity. Between the pharyngeal cavity and the penis bulb the anterior ramus communicates with the two caudal gut trunks, which meet in the hind end of the body.

Male Reproductive System
There are 10-12 testes on either side of the body, extending from about half-way the pharyngeal cavity to the level of the penis bulb. The follicles are principally situated dorsally but each follicle may extend considerably towards the ventral body surface.
The vasa deferentia unite to form an expanded, winding common vas deferens which narrows considerably before penetrating the penis bulb. Within the bulb the vas deferens makes an S-shaped bend before opening into the ejaculatory duct. The proximal third of the last-mentioned duct is lined with a cuboidal, nucleate epithelium bearing well developed cilia. In the distal section of the ejaculatory duct the lining epithelium is infranucleate and carries strong, sclerotized spines in the distal third of the duct. The spines have a broad base and a narrow, extended top section that curves towards the opening of the ejaculatory duct, which is at the tip of the penis papilla. Extensive penis glands discharge a coarse-grained secretion into the ejaculatory duct, which receives the openings of the glands over its entire length.
The penis is a plump structure, consisting of a well developed hemispherical bulb and a short and broad cone-shaped papilla. The latter is lined with a flat, infranucleate epithelium which is underlain by layers of circular and longitudinal muscles, respectively.

Musculo-Glandular Organ:

A pointed musculo-glandular organ lies to the right of the penis, being attached to the wall of the male atrium and the base of the penis papilla. The basis of this organ consists of a strongly muscularized hemispherical bulb. The muscle fibres of the bulb extend into the distal part of the organ, consisting of an elogate cone which projects freely into the atrium. This cone-shaped part is lined with a flat infranucleate epithelium, and its distal part is provided with a sclerotized tip. The musculo-glandular organ is provided with a duct-shaped lumen, opening at the tip of the organ. This lumen is lined with cuboidal, nucleate cells and receives the same secretion of penis glands which is discharged also into the ejaculatory duct.

Atria
The penis papilla projects into a small male atrium which is lined with cuboidal, infranucleate cells bearing a thick coat of long cilia. The same type of epithelium lines the female and common atrium. A small region of the ventro-posterior wall of the common atrium is pierced by the openings of shell glands. The atria are surrounded by a subepithelial layer of circular muscles and a layer of longitudinal muscles.

Female Reproductive System
The ovaries are situated at the level of the most posterior end of the pharyngeal cavity, or just posterior to the latter; the follicles lie medially to the ventral nerve cords. From the ventro-lateral wall of each ovary arises a short and narrow duct which leads to a sac-shaped expansion of the oviducts. This expansion lies dorso-laterally to the ventral nerve cords and is filled with poorly delimited, nucleated cells, interspersed with sperm. From the ovaries one branch of the oviducts extends forwards up to about the level of the brain, whereas another branch runs backwards. The caudally running branches of the oviducts unite in the hind end of the body to form a common oviduct which communicates with the most distal, i.e. ventral part of the bursal canal. The common oviduct is lined with cuboidal, nucleate cells which bear well developed cilia.
The vitellaria extend from directly behind the brain into the hind end of the body, the follicles reaching from dorsal to ventral body surface.
A saddle-shaped copulatory bursa is placed over the male atrium. The bursa is lined with vacuolated, nucleate cells. A well developed bursal canal arises from the postero-dorsal wall of the bursa and curves postero-ventrally to open into the female atrium. The bursal canal is lined with cuboidal, nucleate cells bearing well developed cilia and is surrounded by a layer of strong circular muscle fibres.

Eyes
The eyes are devoid of a lens; the number of retinal cells in each eye cup could not be established.

Ecology
The species is only known from its type locality, where it was collected from interstitial habitats in pure sand in the Otoplana-zone.

Distribution
The species is only known from its type locality: Montery Bay, beach off Elkhorn Slough, California, U.S.A.

Material Examined, Type Material
S.M.N.H.: Holotype: one set of sagittal sections on 2 slides.
S.M.N.H.: Paratypes: Montery Bay, California, 9.04.1969, one set of transverse sections on 3 slides; two sets of sagittal sections, each on 1 slide; one set of sagittal sections on 2 slides.

Pacifides psammophilus