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Holleman and Hand, 1962

Overview
Diagnosis: The structure of the entire copulatory apparatus, the presence of atrial folds, and a gonopore at the hind end of the body, make it easy to distinguish Nexilis from any other marine triclad known to date.
Habitus: Live animals are about 3 mm long and 1.5-2.0 mm wide. From the obtusely pointed or rounded front end, the body margins diverge towards the hind end. The largest diameter is reached at about the hind end of the pharyngeal pocket or even somewhat posteriorly to the latter. From that point the body margins run towards the broadly rounded hind end. The eyes are set close together at some distance from the anterior margin. The species is devoid of pigment (Holleman and Hand 1962, Holleman 1972).

Alimentary System
The short pharynx is situated at about the middle of the body and measures one-sixth to one-fifth of the body length. The inner and outer circular muscle layers of the pharynx are well developed, the last-mentioned layer being slightly thinner than the first-mentioned one. The diameter of these muscle layers decreases strongly towards the tip of the pharynx. The mouth opening lies at the hind end of the pharyngeal pocket.
The anterior ramus of the intestine extends in front of the eyes and gives off one pair of rather large, preocellar diverticula. Behind the eyes the anterior trunk gives rise to about four pairs of branched diverticula. From each of the posterior rami rise about nine lateral diverticula.

Male Reproductive System
The oval-shaped and ventrally located testes are situated between the root of the pharynx and the ovaries. The number of follicles varies from 5 to 8 pairs. The specimens examined showed 5 pairs of testicular follicles, whereas in the type material 4 to 5 pairs of testes are present (Holleman and Hand 1962, Ball 1975a). But in specimens collected from Washington, Holleman (1972) recorded 7 to 8 pairs of testes. The follicles of the pair of testes immediately behind the ovaries are situated close together, whereas the more posteriorly located testes are progressively spaced wider apart.
At the hind end of the pharyngeal pocket the vasa deferentia enlarge to form only moderately sized false seminal vesicles. The latter run caudally to about the male atrium and, subsequently, recurve to penetrate separately the antero-lateral wall of the seminal vesicle.
The penis consists of a muscular bulbar portion, in which the above-mentioned seminal vesicle is situated, and a highly reduced papilla. The seminal vesicle opens via a narrow opening into a small chamber which, in turn, opens into the papilla. The papilla projects into the male atrium which posteriorly narrows to form the common atrium. The musculature of the penis bulb is well-developed and consists of irregularly arranged muscle fibres. The male atrium is surrounded by a layer of circular and longitudinal muscle fibres, respectively. These layers extend onto the tip of the penis papilla. The atrium is lined with nucleate, columnar cells.

Female Reproductive System
The rounded ovaries are situated directly behind the brain; they lie close together and medially to the ventral nerve cords. From the antero-dorsal surface of the ovaries arises a narrow, strongly muscularized, duct which opens into the enlarged proximal portion of the oviducts. The oviducts arise from the antero-lateral surface of these rounded, enlarged portions or tubae. In running backwards, the oviducts stay laterally to the ventral nerve cords. Posterior to the level of the penis bulb the ducts turn dorso-medially and enter separately the proximal portion of the female genital duct.
The vitellaria are well-developed and extend from anterior to the ovaries up to about the level of the posterior part of the atrial folds. The follicles are situated between the intestinal branches and occupy the entire space between the dorsal and ventral body surfaces.
A copulatory bursa is absent; a female genital duct opens into the antero-dorsal part of the male atrium. Consequently, this particular part of the atrium may be considered as the equivalent of the female atrium. The female genital duct and the female atrium are surrounded by a layer of circular muscles and a layer of longitudinal muscle fibres. Shell glands surround the female genital duct and discharge into its lumen.

Common Atrium
The male atrium narrows posteriorly to the common atrium. This narrowing is surrounded by bundles of muscle fibres. Posterior to the narrowing the common atrium widens to an irregularly shaped chamber. Posterior to this chamber, the wall of the common atrium shows a number of folds. These folds are more or less symmetrically arranged on either side of the animal and divide the common atrium in a number of narrowings and chambers. The lumen of the common atrium narrows posterior to its initial enlargement, the atrial folds here being close together. Subsequently, the lumen enlarges considerably, after which it narrows again. The last, most posteriorly located atrial fold consists of a pair of elongated or slightly club-shaped structures pointing towards the posterior end. The common atrium communicates with the gonopore, the latter being located at the hind end of the body.
The narrowing in the common atrium as well as the posteriorly located enlargement of the atrial lumen, are surrounded by a broad layer of unicellular glands which discharge into the atrium.
The appearance of the atrial folds as described above, holds true for CT 02469 specimens of the material examined. In the holotype Ball (1975a) found the atrial folds not to be arranged symmetrically but recorded a large, posteriorly situated, papilla-like structure projecting into the atrium. The specimen depicted by Holleman and Hand (1962) shows two large, equally sized papilla-like folds which project backwards. A similar situation seems to be present in the ill-preserved specimen N.M.N.S. Acc.69-298.
The main portion of the atrial lumen is lined with a cuboidal, nucleate epithelium which is underlain with a layer of circular muscle fibres. I could not discern a layer of longitudinal muscles entally to this circular muscle layer, as was reported by Ball (1975a). Singular muscle fibres which may run just entally to the circular muscle layer, are part of the muscle fibres which are dispersed irregularly in the parenchyma of the atrial folds.
The ectal part of the atrial folds is bounded by strong layers of muscle fibres which may run close to the dorsal and ventral body surfaces. These muscles are arranged in alternating rows of circular and longitudinal fibres and run from the posterior narrowing in the male atrium towards the proximal portion of the posterior pair of atrial folds.
The distal part of the common atrium, i.e. the section from about the club-shaped folds to the gonopore, is pierced by numerous openings of erythrophilous glands, which produce a fine granular secretion. These glands are distributed in the parenchyma, up to the level of the male atrium.
Cement glands discharge their secretion into the gonopore.

Eyes
The eye cups contain three retinal cells and an oval-shaped lens.

Reproduction
Life Cycle: Although Holleman and Hand (1962) observed discharged cocoons as well as cocoons still present in the male atrium, these workers provided no information on the shape or size of the capsules.

Ecology
Nexilis epichitonius has been collected from a variety of animals on which it apparently occurs as a facultative ectosymbiote: Chiton (Mopalia hindsii), Emarginate Dogwinkle (Thais emarginata, Mollusca) (Holleman and Hand 1962), and Mytilus californianus (Holleman 1972). Further, the species has been collected from a dead Crab (Cancer productus) (Holleman and Hand 1962). Nexilis has been collected also from intertidal areas, not being associated with any other living animal (Holleman and Hand 1962, Material Examined).

Distribution
Type locality: Stinson Beach, California (122°37'W 37°53'N). The species is known from the following localities: Sausolito, California; Stinson Beach, California; Coos Bay, Oregon; Tillamook, Oregon; San Juan Island, Washington; Vancouver Island, British Columbia; Amchitka Island, Aleutians, Alaska (Holleman and Hand 1962, Holleman 1972, Material Examined).

Material Examined
N.M.N.S.: Acc.69-298, Glacier Point (= Point No Point), Vancouver Island, British Columbia, 11.03.1969, sagittal sections on 1 slide.
Private collection I. R. Ball: CT 02469.1, Square Bay, Amchitka Island, Aleutians, Alaska, 23.09.1969, sagittal sections on 3 slides; CT 02469.2, sagittal sections on 4 slides; CT 02469.3, horizontal sections on 1 slide; CT 02469, 3 preserved specimens.
B.M.N.H.: 1976.5.13.1-25, Amchitka Island, Aleutians, sagittal sections on 2 slides.

Type Material
A.M.N.H.: Holotype, no. 503, California, sagittal sections on 5 slides; paratypes: initially 10 preserved specimens (AMNH 504) of which a number have been sectioned and one specimen has been preserved as a whole mount (Ball 1975).

Nexilis epichitonius