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(Ijima and Kaburaki, 1916)

Overview
Diagnosis: Paucumara trigonocephala (Ijima and Kaburaki, 1916). Live specimens can be recognized easily by their characteristic external appearance. Anatomically, the species is characterized by (1) a highly muscularized bursal canal, (2) the large, curved common vas deferens inside the penis bulb, and (3) oviducts which open separately into the bursal canal.
Habitus: Fully extended live specimens are about 2 mm long (L. Winsor in litt.), while according to Kaburaki (1922) they may even be as large as 4 mm in length and 1 mm in breadth. Preserved specimens measure from about 1.5 x 0.5 mm up to 2.75 x 1mm. In extended position the body is slender, with pointed hind end. The greatest breadth of the body is about the posterior third of the animal and from there the body margins converge gradually towards the front end. Just in front of the eyes the body shows a narrowing after which the body margins diverge to form a triangularly shaped front end. The eyes are situated at a considerable distance from the anterior margin.
From a short distance behind the eyes, a brown band runs towards the hind end on either side of the midline of the body. A narrow dark line extends forewards from each of the eyes, while a short portion of these lines extends posteriorly to the eyes. The eyes are surrounded by whitish patches which in turn are surrounded by dark pigmented parts. The obtusely pointed anterior end is also provided with a broad band of dark pigment, leaving a large opaque-cream area between it and the dark pigmented parts around the eye-spots. Behind the eyes an opaque-cream band runs across the body, and on the mid-dorsal surface a creamish line or row of patches extends backwards to the root of the pharynx. The latter, as well as the copulatory apparatus appear as more or less clear areas.

Alimentary System
The pharynx is between one-sixth and one-fourth of the body length; the inner circular muscle layer is much thicker than the outer layer. The mouth opening is situated at the hind end of the pharyngeal pocket.
The anterior ramus of the intestine extends anterior to the eyes. The material examined could not provide information on the presence or absence of pre-ocellar diverticula, but according to Kaburaki (1922) the anterior ramus gives off a pair of branched pre-ocellar diverticula. Behind the eyes the anterior intestinal trunk gives off about 8 pairs (6-9, according to Kaburaki (1922)) of irregularly branched diverticula. From each posterior ramus arise 9-12 (10-15 according to Kaburaki (1922)) pairs of lateral diverticula, which are either not branched or at least more regularly branched than the ones of the anterior trunk. In the material examined the two posterior intestinal rami unite behind the copulatory apparatus to a single branch, the latter giving off a small number of lateral diverticula. According to Kaburaki (1922), however, the posterior branches do not communicate.

Male Reproductive System
Testicular follicles extend from immediately behind the ovaries to about half-way along the pharyngeal pocket; according to Kaburaki (1922) the follicles may even extend backwards up to the level of the mouth. With two or three follicles together the testes are situated ventrally between the intestinal diverticula. In sagittal sections the follicles appear as slender vesicles which may extend dorsally beyond the midline of the body. There are about 17 follicles on either side of the body.
At the level of the root of the pharynx, the vasa deferentia enlarge to form false seminal vesicles which gradually decrease in diameter before penetrating the penis bulb. At about the level of the gonopore, the vasa deferentia bend dorso-medially; they enter the postero-dorsal portion of the penis bulb. Immediately after having penetrated the bulb, the vasa deferentia fuse into a common vas deferens which curves antero-ventrally and, subsequently, enlarges to form a broad ejaculatory duct. The latter narrows gradually before opening at the tip of the penis papilla. The ejaculatory duct is lined with a tall, infranucleate epithelium. The epithelial cells contain a fine granular secretion which probably comes from gland cells within the penis bulb, although I could not find any evidence of gland cells either within or outside the bulb. According to Kaburaki (1922: Fig. 5), penis glands are situated outside of the bulb. Common vas deferens and ejaculatory duct are surrounded by a well developed layer of circular muscles. The penis bulb is rather large but only moderately muscularized.
The penis papilla is a conical structure with a blunt tip; it is situated in a narrow atrium, which is surrounded by a layer of circular and a layer of longitudinal muscles, respectively.

Female Reproductive System
The large, oval-shaped ovaries are situated at a short distance behind the brain. In the contracted specimens which were examined, this distance may appear to be somewhat less than it is in reality. Kaburaki's (1922) Fig. 2 shows the ovaries to be at a considerable distance behind the brain. The oviducts open separately into the ventral portion of the bursal canal; their openings may be directed slightly towards the hind wall of the canal. According to Kaburaki (1922) the distal sections of the oviducts, i.e the parts closest to the bursal canal, are infranucleate and non-ciliated. The material examined did not allow me to check Kaburaki's observation.
Vitellaria extend from the level of the ovaries into the hind end of the body and occupy the entire space between dorsal and ventral body surfaces.
The copulatory bursa is a large sac-shaped vesicle, lined with a cuboidal, nucleate epithelium. From the anterior surface of the bursa arises a broad bursal canal which at first shows a more or less pronounced, dorsally directed bend before it runs obliquely towards the atrium. The lining of the canal consists of a ciliated, infranucleate epithelium. The bursal canal is surrounded by a thick, subepithelial layer of circular muscle fibres, bounded by a thin layer of longitudinal fibres. Some space is left between this longitudinal muscle layer and nuclei, probably of the infranucleate epithelium, which surround the bursal canal. The bursal canal receives the secretion of numerous unicellular glands. The openings of the weakly developed shell glands are situated ectally to the openings of the oviducts into the bursal canal. According to Kaburaki (1922) the copulatory bursa is surrounded by a muscle layer consisting of circular fibres immediately beneath the basement membrane and a layer of longitudinal fibres. I have been unable to find any traces of musculature around the bursae in the material examined.

Eyes
The pigment cups house three retinal cells and a large semi-circular lens.

Ecology
Paucumara trigonocephala appears to prefer low-salinity biotopes such as (1) the mouth of rivers where, at least during ebb, the water is fresh, or (2) lakes in the proximity of the sea. On one occasion specimens have been collected from an entirely freshwater biotope, viz. a brook at Komgi, New Britain (Material Examined). Specimens may be found under stones or other material, such as fouling plates in the case of the Australian material examined.

Distribution
Type locality: Oginohama Port, Rikuzen Province, Japan (38 22' N 141 27' E). The species has been reported from Japan (Oginohama, Province Rikuzen (38 22'N 141 27'E) and Itsukushima, Province Aki (34 18'N 132 19'E)) (Kaburaki 1922) and also occurs in Australia and the Bismarck Islands (Material Examined); F. graciliceps, which very likely is a junior synonyms of P. trigonocephala, has been reported from Hongkong. Kawakatsu, Nunomura and Suzuki (1989) reported the following new localities: Asô Bay, the Tsushima Islands, Nagasaki Prefecture, Kyûshû, Japan; Okinawa Island, Okinawa Prefecture, Japan.

Material Examined
Q.M.: GL 4893, Ross River, Townsville, N.E. Queensland, Australia, 11.8.1982, sagittal sections on 1 slide; GL 4894, sagittal sections on 2 slides; GL 4895, horizontal sections on 1 slide; GL 4896, sagittal sections on 2 slides.
Z.M.K.: Noona Dan Expedition 1961-62, st.34-6, Lake Noinala, Mussau Island, Bismarck Islands, 11.06.1962, sagittal sections on 1 slide; st.19-1, Komgi, 30 km S.E. Cape Lambert, New Britain, Bismarck Islands, 14.05.1962, sagittal sections on 2 slides.

Type Material
No longer available.

Paucumara trigonocephala