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Westblad, 1952

Overview
Diagnosis: Meixnerides armatus Westblad, 1952 is characterized by two large testes, an ejaculatory duct with spines, and a musculo-glandular organ behind the penis.
Habitus: Preserved specimens are about 1.3 mm long and 0.6 mm wide and have a broadly rounded front and hind end. The dorsal surface is pigmented.

Alimentary System
The short pharynx is situated in the posterior half of the body and measures about one-seventh of the body length. Both outer and inner circular muscle layers of the pharynx are well developed, the latter being about twice as thick as the first-mentioned layer. The mouth opening is at the hind end of the pharyngeal cavity.
The anterior ramus of the intestine gives rise to about 6 pairs of lateral diverticula, and it terminates between the eyes. The caudal gut branches meet in the hind end of the body, while each branch gives off about 10 lateral diverticula.

Male Reproductive System
A pair of big, rounded testes is situated directly behind the ovaries, only being separated from the latter by a pair of intestinal diverticula. The testicular follicles occupy the entire space between ventral and dorsal body surface.
The penis consists of a broad, hemispherical and strongly muscularized bulb and an also broad, cone-shaped papilla. The musculature of the penis bulb consists of a thick layer of mainly longitudinal muscle fibres, which is continuous with the longitudinal muscle layer around the atrium and with the well developed longitudinal layer on the penis papilla. The papilla also is provided with a subepithelial layer of circular muscles.
The vasa deferentia penetrate the latero-dorsal surface of the penis bulb and, subsequently, unite to form a short common vas deferens. The latter curves in such a way that it opens laterally into the most proximal section of the ejaculatory duct. Three histologically different parts may be recognized in the last-mentioned duct. The most proximal part is a more or less rounded vesicle lined with cuboidal, nucleate and ciliated cells; the vesicle is surrounded by a coat of irregularly arranged muscle fibres. This vesicle receives the opening of the common vas deferens as well as the numerous openings of penis glands, the latter producing a granular, erythrophilous secretion. The vesicle is delimited from the second or middle section of the ejaculatory duct by a more or less pronounced constriction. It is especially at this constriction that numerous openings of penis glands pierce the lining epithelium of the ejaculatory duct.
The middle part of the ejaculatory duct consists of an irregularly shaped, spacious section that is lined with a flat, infranucleate epithelium. This part of the ejaculatory duct is surrounded by numerous muscle fibres, forming septa between which the secretion of penis glands accumulates. The muscle fibres originate from bundles of longitudinal muscles which run from the penis bulb towards the third and most distal part of the ejaculatory duct. The middle part communicates with the distal part through a more or less pronounced narrowing.
The distal section of the ejaculatory duct is provided with numerous sclerotized spines with long extended tips which point towards the opening of the duct at the tip of the penis papilla. This part of the ejaculatory duct is surrounded by a layer of strong, longitudinal muscles which originate from the retractor bundles that run through the penis papilla.

Female Reproductive System
The vitellaria extend from anterior to the ovaries to the level of the musculo-glandular organ and occupy the entire space between ventral and dorsal body surface.
The ovaries lie directly behind the brain, dorsally to the ventral nerve cords. Usually, the ovaries are rather large and extend to almost the dorsal body surface. The oviducts arise from the ventro-lateral wall of the ovaries and in running backwards they stay laterally to the ventral nerve cords.
Posterior to the gonopore the oviducts curve towards the median line of the body and unite to a common oviduct. This common oviduct consists of a small, expanded part where the oviducts meet, and a narrow duct that opens into the musculo-glandular organ through the hind wall of the latter. This narrow duct is surrounded by circular muscles and receives the openings of shell glands.
The musculo-glandular organ is situated directly posterior to the gonopore. It consists of a rounded or pear-shaped vesicle lined with tall, infranucleate cells which are filled with a granular secretion. The musculo-glandular organ is surrounded by a well developed muscle coat of mainly longitudinal fibres. From this layer muscle fibres extend to the lumen of the musculo-glandular organ, thus penetrating the lining epithelium of the vesicle. The musculo-glandular organ opens ventrally to the exterior, its opening coinciding with the opening of the atrium to the exterior.

Eyes
Each eye cup contains three retinal cells, but no lens.

Ecology
Meixnerides armatus has been collected from sandy, gravel, or clay substrates, at depths varying between 12 m and 40 m, on the Falkland Islands (Berkeley Sound, Port William, Port Albemarle) and on South Georgia (Pot Bay) (Westblad, 1952).

Distribution
Type locality: As type locality has been designated the first sampling locality mentioned in Westblad's (1952) paper, viz. station 40 of the Swedish South Polar Expedition: Falkland Islands, Berkeley Sound, 51°33'S 58°0'W (Sluys 1989). This coincides with the provenance of the major part of the type series.
Meixnerides armatus has been collected from the Falkland Islands (Berkeley Sound, Port William, Port Albemarle) and on South Georgia (Pot Bay) (Westblad, 1952).

Material Examined and Type Material
S.M.N.H.: Type series: Swedish South Polar Expedition, station 56, 8.9.1902, 2 whole mounts on 1 slide; station 40, 1 whole mount on 1 slide; st.40, sagittal sections of 13 specimens, each on 1 slide; st.56, sagittal sections on 1 slide; 11.06.1902, sagittal sections on 1 slide; st.40, transverse sections of 6 specimens, each on 1 slide; transverse sections on 2 slides; st.40, horizontal sections of 3 specimens, each on 1 slide; st.33, sagittal sections of the hind end of a specimen on 1 slide and transverse sections of the front end on another slide.

Meixnerides armatus