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(Darwin, 1844)

Overview
Diagnosis: Procerodella macrostoma (Darwin, 1844) is characterized by a broad, infranucleate bursal canal, receiving the opening of the common oviduct and the abundant secretion of unicellular glands. The presence of three lenses in each eye cup forms another characteristic feature.
Habitus: Preserved specimens up to 7 mm long and 2 mm wide, the greatest breadth being at the level of the copulatory apparatus. From the broad, hemispherical hind end the body margins converge towards the much narrower front end, which is provided with a pair of small auricles. The entire dorsal body surface is darkly pigmented, except for the auricles and areas around the eyes. Concentrations of pigment occur in the form of (1) a broad patch on the head, (2) a mid-dorsal stripe running from the afore-mentioned patch between the eyes to about the root of the pharynx, (3) irregular lines along the ramifications of the intestinal system. The ventral body surface is pale.

Alimentary System
The pharynx measures between one-sixth and one-fifth of the body length, its root being located at about the middle of the body. The muscle layers of the pharynx are well developed; the inner circular muscle layer is about twice as thick as the outer layer of circular muscles. The mouth opening is at the hind end of the pharyngeal cavity.
The anterior ramus of the intestine extends anterior to the eyes, without giving rise to preocellar diverticula. The anterior gut trunk gives off about 7 pairs of lateral diverticula of which the anteriormost pair curves forwards to reach beyond the eyes. Each of the caudally running gut trunks gives rise to 14-20 lateral diverticula, and they meet in the hind end of the body. The lateral diverticula of the intestine are highly ramified, especially those of the anterior gut trunk, which apparently made Darwin (1844) to use the expression "moss-like subdivision" in describing the branching pattern of the intestinal system.

Male Reproductive System
The testes are situated in front of the pharynx, close to the median line of the body. There are 6-10 follicles on either side of the body, lying in two's or three's between the bases of two intestinal diverticula. The testes do not reach up to the ovaries; the most anterior testicular follicles lie at some distance behind the brain, being situated behind the second pair of intestinal diverticula of the anterior gut trunk. The rather large testes occupy the entire space between ventral and dorsal body surface.
The vasa deferentia expand behind the pharyngeal cavity to form only moderately sized false seminal vesicles. The two ducts probably unite at about the point where they enter the dorsal part of the penis bulb. The ejaculatory duct, which opens at the tip of the penis papilla, is lined with an infranucleate epithelium and is surrounded by a relatively thick layer of intermingled, and irregularly arranged longitudinal and circular muscles.
The penis consists of a well defined bulb, delimited by strong muscle fibres, and an elongate, cone-shaped papilla. The penis papilla is provided with a relatively thick, subepithelial layer of circular muscles and a somewhat thinner layer of longitudinal muscle fibres. The major portion of the penis papilla is covered with a flat, infranucleate epithelium.

Female Reproductive System
The vitelline follicles reach from dorsal to ventral body surface and extend from the level of the ovaries into the hind end of the body.
The ovaries are situated directly behind the brain, medially to the ventral nerve cords. The most anterior parts of the oviducts expand to form tubae which are lined with large, cuboidal and nucleate cells. These tubae lie dorsally to the ventral nerve cords and just laterally to the ovaries. Communication between tubae and ovaries goes via a very short and narrow duct, which meets the ventro-lateral wall of the ovary; this short duct is surrounded by some circular muscle fibres. According to Marcus (1954a), the oviducts run laterally to the ventral nerve cords. Behind the gonopore the oviducts join to form a common oviduct which meets the posterior wall of the bursal canal. This common oviduct is surrounded by a layer of circular muscles and is lined with an infranucleate, ciliated epithelium.
The bursal canal runs from the anterior surface of the small copulatory bursa to the atrium; the canal is very broad, excepting the proximal part close to the bursa. In the material examined the spacious lumen of the proximal third of the bursal canal is completely filled with a dense, cyanophilous secretion, being produced by unicellular glands which are arranged around the canal. The common oviduct also receives the secretion of these unicellular glands. The bursal canal is lined with an infranucleate epithelium and is surrounded by a well developed layer of muscles. The precise arrangement of the rows of muscles around the bursal canal was difficult to interpret in the material examined, but it appeared to consist of alternating rows of circular and longitudinal muscles.
Openings of shell glands pierce the bursal canal ectally to the opening of the common oviduct.

Eyes
Each pigment cup contains three retinal cells and three lenses.

Reproduction
Life Cycle:

The oval-shaped cocoons are about 1 x 0.7 mm, and lack a pedicel (Marcus 1954a).

Ecology
Darwin (1844) reported that he had collected his specimens from under stones in brackish water; the animals from the Lund University Chile Expedition were obtained from the intertidal zone, some of them being collected from among algae and one from an Acorn Barnacle.

Distribution
Type locality: Chonos Archipelago, Chile.

P. macrostoma is known from the Chonos Archipelago (Darwin 1844), two localities on the north coast of Chiloé (Playa Brava at 41°51'35"S 73°49'20"W, and Punta Corona at 41°47'S 73°53'07"W), Cayo Blanco at Moraleda Channel (44°48'20"S 73°35'W), and the Bay of Reloncavi on the west coast of the island Guar (41°40'55"S 73°00'W) (Marcus 1954a).

Material Examined
S.M.N.H.: North coast of Chiloé, Punta Corona, Chile, Lund University Chile Expedition, station M56, 28.02.1949, sagittal sections on 2 slides; transverse sections of front end on 1 slide; sagittal sections of front end on 1 slide; two whole mounts on 1 slide.

Procerodella macrostoma