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(Kato, 1955)

Overview
Diagnosis: Procerodella japonica (Kato, 1955) can be distinguished by the strong layer of circular muscles around the bursal canal and by the unicellular glands discharging into the latter. It differs from the few species which also show these two features in the presence of (1) an unarmed , conical penis papilla, (2) a diverticulum of the bursal canal, (3) a short common vas deferens opening into a large seminal vesicle.
Habitus: Preserved animals up to 3 mm long and 1 mm wide. The body is lanceolate, with the hind end rounded and the front end having a triangular shape. In preserved specimens the dorsal surface shows irregular black spots. Pigmentation is more dense over the pharyngeal region and also on various parts of the head, where it delineates an unpigmented, branched pattern. The ventral surface is white.

Alimentary System
The pharynx measures about one-fifth of the body length, its root being located at about the middle of the body. The mouth opening lies at the hind end of the pharyngeal pocket.
The anterior ramus of the intestine extends as an undivided branch anterior to the eyes. Behind the eyes the anterior trunk gives off 7 pairs of lateral diverticula. The two posterior rami meet in the hind end of the body, while each of them gives rise to about 13-15 pairs of lateral diverticula and an equal number of small median branches.

Male Reproductive System
There are about 14 testicular follicles on either side of the body. The vental testes lie on both sides of the anterior intestinal ramus.
The vasa deferentia unite at the base of the penis bulb to a short common duct which opens into a spacious seminal vesicle. From the latter arises the ejaculatory duct which opens at the tip of the penis papilla. Seminal vesicle and ejaculatory duct are lined with tall, columnar cells with insunk nuclei. Well developed penis glands, which lie in the parenchyma dorsally to the penis bulb, discharge their secretion into the distal section of the ejaculatory duct. The musculature of the penis bulb is well developed. The penis papilla is an elongate cone with an oblique, ventro-caudal orientation.

Female Reproductive System
The two ovaries lie at a short distance behind the brain. The vitellaria extend from behind the ovaries into the hind end of the body. The main portion of the vitellaria is situated along the dorsal body surface.
The copulatory bursa is a rather large and irregularly shaped sac, lined with ciliated cells with a distinct nucleus. From the anterior surface of the bursa arises the obliquely running bursal canal. At about the middle of the canal a narrow diverticulum arises from its ventral surface. The oviducts open separately into the distal end of this diverticulum. Bursal canal and diverticulum communicate with a small expansion or vestibulum. From the latter runs a straight and narrow duct, which meets the male atrium.
The bursal canal is lined with a tall, nucleate epithelium and is surrounded by a very thick, subepithelial layer of circular muscles which is bounded by a much thinner layer of longitudinal muscles. The same muscle coat, albeit much thinner, surrounds the diverticulum. Numerous unicellular gland cells lie just outside of these two muscle layers and discharge their granular and eosinophilous secretion into the bursal canal. Shell glands open into the vestibulum and into the canal connecting the former with the male atrium.

Eyes
No information has been provided on the histology of the eyes. The way in which the eyes are drawn, however, suggests the presence of an eye lens (Fig. 167).

Ecology
P. japonica has been collected from under stones at the muddy beach of the brackish water lake Hôjôzu-gata. The lake is separated from the sea by a sand dune and a swamp, and due to seasonal changes its chlorine content varies from 0.2-2.0 g/L. The species is known only from its type locality.

Distribution
Type locality: Lake Hôjôzu-gata in Toyama Prefecture (36°42' N 137°14'E). This lake is a lagoon connected with Toyama Bay and is now used as the New Harbour at Toyama (or the Toyama New Port) (Kawakatsu 1989). The species is known only from the type locality.

Material Examined
Kato's original description formed the only source of information.

Type Material
The type material has been lost.

Procerodella japonica