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(Kato, 1943)

Overview
Diagnosis: Procerodella asahinai (Kato, 1943) is characterized by a strongly muscularized bursal canal which communicates with a vestibulum, the latter opening into the common atrium. Characteristic are also the numerous unicellular glands around the bursal canal, discharging their secretion into the lumen of the latter. P. asahinai may be distinguished from the few other marine triclads which also show the above-mentioned features by (1) a very short common oviduct which opens into the vestibulum, (2) the lack of a diverticulum of the bursal canal, (3) an unarmed penis papilla, (4) a rather long and winding common intra-bulbar vas deferens.
Habitus: Preserved specimens are about 2.8 mm long and 1.2 mm wide. In such specimens the front end is broadly rounded, and the body margins taper towards a pointed hind end. In preserved animals the entire body is usually white, but in some specimens a small number of black pigment granules may be present along the anterior body margin.

Alimentary System
The pharynx is about one-fifth of the body length and lies in the middle of the body. The mouth opening is situated at the hind end of the pharyngeal pocket.
From the anterior ramus of the intestine arise six pairs of lateral diverticula. The anterior gut trunk extends, unbranched, anterior to the eyes. The posterior intestinal trunks meet in the hind end of the body, each of them being provided with about 15 lateral diverticula. Behind the pharynx the posterior rami give rise to a number of short median diverticula.

Male Reproductive System
There are about 10 testicular follicles on either side of the body. The follicles are situated ventrally, and in pairs, between the branches of the anterior gut trunk. The testes extend from directly behind the ovaries up to the root of the pharynx.
Behind the pharynx the vasa deferentia enlarge to form false seminal vesicles, which decrease again in diameter while ascending to the penis bulb. At the medio-dorsal base of the bulb, the vasa deferentia unite to a common duct, which immediately penetrates the muscle layers of the penis bulb. In the bulb this duct follows a winding course before it opens into a large seminal vesicle at the base of the penis papilla. This vesicle diminishes only gradually in diameter and opens at the tip of the penis papilla through a narrow pore. Common vas deferens and seminal vesicle are lined with large cuboidal cells. Cyanophilous glands which lie in the penis bulb, discharge a fine, granular secretion into the distal portion of the common vas deferens. The seminal vesicle is penetrated by numerous openings of unicellular, eosinophilous glands, which lie in the parenchyma dorsally to the penis bulb.
The penis bulb is large and well-muscularized. The penis papilla has the shape of a cone with a broad base and has an oblique, ventro-caudal orientation. The papilla is covered with a flat epithelium.

Female Reproductive System
The paired ovaries are situated between the first and second pair of lateral diverticula of the anterior gut trunk. In running backwards, the oviducts stay laterally to the ventral nerve cords. Behind the gonopore, the oviducts unite to a very short and slender duct which opens into the vestibulum. Shell glands discharge into this vestibulum, which opens into the common atrium and communicates, via a constriction, with the broad bursal canal. Directly underneath the epithelium of the bursal canal lies a very thick layer of circular muscles, overlain with a thin layer of longitudinal muscle fibres. Immediately outside of these two muscle layers a large number of unicellular glands lies distributed in the parenchyma. These glands discharge a fine, granular and faintly eosinophilous secretion into the bursal canal, opening along the entire length of the latter.
The copulatory bursa is a rather large, sac-shaped vesicle, lined with nucleate, cuboidal cells.
The vitellaria extend from behind the ovaries throughout the body length, the bulk of the follicles being situated along the dorsal body surface.

Eyes
The histology of the eyes was not described by Kato (1943). In his Fig.1, Kato depicted two cresentic eyes without indicating the presence of eye lenses, as he did in the drawings of Procerodella japonica (Kato 1955) and Miroplana trifasciata (Kato 1931).

Distribution
Type locality: Lake Yûdonuma on the coast of Tokachi Province, Hokkaido, Japan (approx. 42°56'N 143°10'E).

Material Examined
The present account is based on the original species description.

Procerodella asahinai