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Sluys, 1989a nom. emend.

Overview
Diagnosis: Pacifides gladiator Sluys, 1989a may easily be distinguished from its congener P. psammophilus by the presence of (1) a large musculo-glandular organ with a long sclerotic tip, (2) oviducts which open separately into the bursal canal, (3) a muscular penial fold.
Habitus: Preserved specimens are 1.25-1.75 mm long and 0.25-0.50 mm wide. Anterior and posterior end rounded; head with auricular protuberances. The entire dorsal surface is brownish, with the eyes set in pigment-free patches. The ventral surface is also pigmented, but not as densely as the dorsal body surface.

Alimentary System
The pharynx lies in the anterior half of the body and is between one-seventh and one-fifth of the body length. The inner circular muscle layer of the pharynx is much thicker than the outer one. The mouth opening is situated at the anterior end of the pharyngeal cavity.
The anterior gut trunk extends anterior to the eyes and gives off a pair of small preocellar diverticula. Posterior to the eyes the anterior intestinal ramus gives rise to 4-5 pairs of lateral diverticula.
The two posterior rami show an aberrant course, as compared with other species of triclads. As usual, the posterior gut trunks emerge just anterior to the pharynx and run on either side of the pharyngeal pocket. Behind the pharynx the posterior gut trunks unite to a relatively long common section which bifurcates just anterior to the copulatory apparatus. The two resulting branches run on either side of the copulatory complex and unite again in the hind end of the body.

Male Reproductive System
The testicular follicles lie between the pharynx and the copulatory apparatus and occupy the entire space between dorsal and ventral body surface; there are 8-10 follicles on either side of the body. The testes discharge their sperm into a well developed, winding common vas deferens which narrows before penetrating the shallow penis bulb. Within the penis the common vas deferens opens into the rather broad ejaculatory duct, which is lined with cuboidal, nucleate cells bearing well developed cilia.
The penis papilla is a plump structure with an irregularly shaped tip, which probably is the result of the eversible nature of the papilla. The tip of the penis is provided with small, sclerotized spines. If the penis is indeed eversible, then these spines will be situated in the ejaculatory duct in a more retracted penis papilla. The papilla is covered with a flat, infranucleate epithelium. The antero-ventral surface of the penis papilla carries a large, bean-shaped lump of tissue-or penial fold-which is traversed by strong circular muscle fibres.

Musculo-glandular Organ
The species is provided with a strong musculo-glandular organ, which is attached to the base of the penis papilla and projects into the male atrium. The organ consists of an elongate papilla with a long and curved, sclerotized tip or stylet. The musculo-glandular organ is provided with a lumen which receives the secretion of glands and opens at the tip of the stylet. The papilla is provided with a flat, infranucleate epithelium which is underlain with a thick zone of longitudinal muscles.

Female Reproductive System
The vitelline follicles occupy the entire space between dorsal and ventral body surface and extend throughout the body length.
The ovaries are situated on either side of the pharyngeal pocket, medially to the ventral nerve cords. The oviducts run dorso-lateral to the ventral nerve cords and extend anterior to the ovaries.
Posterior to the gonopore the oviducts curve medially and open separately into the ventral section of the bursal canal. The latter is a dorso-anteriorly curved duct, connecting the small copulatory bursa with the male atrium. The bursal canal is lined with cuboidal or tall, nucleate cells which bear long cilia. Over its entire length the canal receives the numerous openings of shell glands.
The copulatory bursa lies dorsally to the male atrium and is lined with tall, vacuolated cells.

Eyes
The eyes lack a lens; the number of retinal cells present in each eye cup could not be determined.

Ecology
The type specimens were collected from a marine aquarium in the Horniman Museum, London. The flatworms had become very common in the tank and had caused the death of a young sea-horse by entering the gill-chamber of the latter. These sea-horses were of Indo-Pacific (probably Philippine) origin. Several young sea-horses were hatched in a tank and, subsequently, transferred to another tank; the heavy population of P. gladiator developed in this second tank. As a consequence, it remains uncertain whether the flatworms came from the same region of the world as the sea-horses.

Distribution
Type locality: Marine aquarium, Horniman Museum, London.

Material Examined, Type Material
B.M.N.H.: Holotype, 1988.2.23.3, sagittal sections on 1 slide; Paratypes: 1988.2.23.4-5, whole mount on 1 slide; 1988.2.23.4-5, horizontal sections on 1 slide.

Pacifides gladiator