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Hyman, 1954

Overview
Diagnosis: Procerodes pacificus Hyman, 1954 may be distinguished from other members of the genus Procerodes by the following combination of characters: (1) oviducts opening separately into distal end of diverticulum, (2) vasa deferentia uniting in the basal portion of the penis papilla, (3) broad ejaculatory duct lined with a tall epithelium which is penetrated by the openings of penis glands.
Habitus: Preserved specimens up to about 2.5 mm long and 0.75 mm wide; one specimen was 3.75 mm long. The hind end of the body is broadly rounded, while the front end shows two distinct auricles (Holleman 1972). In preserved animals the auricles may still be visible as small protuberances of the body margin. The animals are devoid of pigment. The eyes are set rather wide apart.

Alimentary System
The pharynx is between one-fourth and one-half of the body length. Both its circular muscle layers are well developed, but the inner circular muscle layer is between two and three times as thick as the outer circular muscle layer. One specimen from New Zealand had a short, second pharynx, the root of which was inserted next to the root of the normally developed pharynx. It cannot be ruled out that the smaller size of the second pharynx was due to contraction instead of to a lesser degree of development. The mouth opening is situated at the hind end of the pharyngeal pocket.
The anterior ramus of the intestine extends anterior to the eyes and gives off a preocellar pair of forked lateral diverticula. Behind the eyes the anterior trunk gives rise to 4-5 pairs of lateral diverticula. Each posterior ramus gives rise to about 12 lateral branches; the posterior intestinal trunks do not communicate in the hind end of the body.

Male Reproductive System
There are about 20 testes situated on either side of the body. The number of testicular follicles may range from 17 to 21 or, exceptionally, 29 pairs of follicles (Hyman 1954, Holleman 1972). The testes extend from the level of the ovaries into the hind of the body. The follicles may either be small and rounded or somewhat larger and pear-shaped; they are situated dorsally between the tips of the intestinal diverticula.
At about the hind end of the pharyngeal pocket the vasa deferentia enlarge to form false seminal vesicles. The vasa deferentia, subsequently, curve dorso-medially and after narrowing, penetrate the dorsal surface of the shallow penis bulb. In the penis both ducts run side by side and unite in the basal portion of the penis papilla. After having entered the penis bulb, the vasa deferentia may increase slightly in diameter or keep the same width. These parts of the vasa deferentia, i.e before the point of fusion, are lined with a low, nucleate epithelium. However, the ejaculatory duct is lined with a much taller epithelium, which is also nucleate. Especially the distal portion of the ejaculatory duct shows very tall cells. Although the outer diameter of the ejaculatory duct is larger than that of the vasa deferentia, its lumen is of about the same width as the two last-mentioned ducts: the increased outer diameter is compensated by the cells being taller. The sections of the vasa deferentia within the penis are surrounded by a well developed circular muscle layer, consisting of about two rows of fibres. This layer becomes somewhat thinner on the distal portion of the ejaculatory duct. This last-mentioned part of the ejaculatory duct is penetrated by the secretion of penis glands. This secretion is present throughout the penis but the location of the glandular elements remains uncertain.
The penis bulb is shallow and only weakly or moderately muscularized. The penis papilla is a conical structure with an obtusely pointed tip and a vertical orientation. In several specimens from New Zealand the penis has a more or less rectangular appearance, which is most likely due to contraction. The penis papilla is covered with a flat, nucleate epithelium which is underlain with a well developed circular muscle layer. Entally to this layer lies a somewhat thinner layer of longitudinal muscle fibres. Both muscle layers are continuous with those surrounding the male atrium. The latter communicates with the common atrium via a pronounced constriction.

Female Reproductive System
The vitellaria extend from anterior to the ovaries into the hind end of the body and occur between the intestinal diverticula. Vitellarian follicles occupy the entire space between dorsal and ventral body surface.
The two small, oval-shaped ovaries lie directly behind the brain and slightly medially to the ventral nerve cords. The oviducts arise from the ventro-lateral surface of the ovaries and, in running backwards, stay just dorsally to the ventral nerve cords. Behind the gonopore, the oviducts curve medially and open separately into the distal end of the diverticulum of the bursal canal.
The rounded copulatory bursa is lined with large, vacuolated cells with nuclei at their base. The bursa is surrounded by a muscle layer, which in the New Zealand specimens was much more conspicuous than in those from America. From the antero-ventral surface of the bursa arises the obliquely running bursal canal. The canal is lined with cuboidal, nucleate cells bearing well developed cilia. A diverticulum arises from the rear wall of the proximal section of the bursal canal. This diverticulum is lined with cuboidal, nucleate cells which bear cilia. Shell glands discharge into the diverticulum. The glands are situated behind the copulatory bursa, around the diverticulum, and below the male atrium. The bursal canal is surrounded by a subepithelial layer of circular muscles and a layer of longitudinal muscles, respectively; the diverticulum is surrounded by a layer of circular muscle fibres.

Eyes
The eyes consist of three retinal cells being enveloped by an unicellular pigment cup which does not house a lens.

Ecology
P. pacificus was found for the first time on Kelp on the shore near San Diego, California (Hyman 1954). The species has been collected also from coarse gravel at Deadman's Bay on San Juan Island, Washington (Holleman 1972, Material Examined) and from a depth of 45 cm in a beach near Kaikoura, New Zealand (Material Examined).

Distribution
Type locality: Sunset Cliffs near San Diego, California. P. pacificus was found for the first time on Kelp on the shore near San Diego, California (Hyman 1954). The species has been collected also from coarse gravel at Deadman's Bay on San Juan Island, Washington (Holleman 1972, Material Examined) and from a depth of 45 cm in a beach near Kaikoura, New Zealand (Material Examined).

Material Examined
Private collection Prof. Dr. P. Ax (Göttingen): no. 1, Deadman's Bay, San Juan Island, Washington, 2.08.1965, sagittal sections on 7 slides; no. 2, horizontal sections on 4 slides; no. 3, transverse sections on 10 slides; no. 4, horizontal sections on 4 slides; no. 5, sagittal sections on 5 slides; no. 6, sagittal sec tions on 6 slides; no. 7, horizontal sections on 3 slides (These specimens form part of the sample, 70 animals in total, from which also Holleman (1972) studied a number of specimens).
A.I.M.: 6741, South Island Bay, Kaikoura, New Zealand, Jan./Febr. 1983, sagittal sections on 1 slide; 6742, sagittal sections on 1 slide; 6743, sagittal sections on 1 slide; 6744, sagittal sections on 1 slide; 6745, sagittal sections on 1 slide; 6746, horizontal sections on 1 slide; 6747, transverse sections on 2 slides.

Type Material
A.M.N.H.: Holotype: no. 443, serial sections on 3 slides. Paratypes: no. 444, 37 whole mounts on 1 slide; no. 591, one set of serial sections on 1 slide; no. 592, 32 whole mounts on 1 slide.

Procerodes pacificus