Sphalloplana (Sphalloplana) subtilis Kenk 1977

Sphalloplana (Sphalloplana) subtilis

TYPE MATERIAL.—Holotype, set of sagittal sections on 5 slides, USNM 53444. Paratypes, two sets of sagittal and transverse sections on 9 slides, USNM 53445–53446.

EXTERNAL FEATURES (Figures 6, 13).—This is a very slender, unpigmented species, up to 16 mm long and 1 mm wide when extended. The head end is truncated, with almost straight or slightly convex frontal margin and rounded lateral edges, which may extend very little anteriorly and laterally. There is no conspicuous adhesive organ discernible in life. In the quietly gliding animal, no necklike construction is seen behind the head, at most a very insignificant narrowing. The lateral margins of the body run almost parallel for the greater part of the body length, tapering behind the level of the copulatory complex to meet at the posterior end. The short pharynx, measuring about one-ninth of the length of the body, is inserted far back at about the beginning of the third fifth of the body length. The copulatory apparatus is located in the anterior half of the postpharyngeal region.

ANATOMY.—The marginal zone with thickened epithelium and large rhabdites, characteristic of the genus, is developed typically (Figure 29). The adhesive organ (Figure 35) appears in the sections as a small subterminal pit in the center of the frontal margin, lined with an infranucleate epithelium pierced by numerous eosinophilic gland ducts. It has a very feeble musculature, principally fibers that may act as retractors. No protrusion of the organ has been observed in living specimens upon stimulation, such as is seen in many other representatives of the genus.

The anterior ramus of the intestine bears over 20 pairs of lateral branches, each posterior ramus about 25 short branches. The anterior border of the intestinal area is rounded. Behind the copulatory apparatus, the two posterior rami unite to a single trunk.

The two ovaries are located behind the third to fifth lateral branches of the anterior intestinal ramus. The testes, in moderate number, form a pair of longitudinal rows extending through the posterior three-fifths of the prepharyngeal region. They are ventral, lying below the intestinal branches (Figure 42). above and medial to the ventral nerve cords. Many testicular follicles are connected directly to the thin anterior vasa deferentia that run along the upper border of the nerve cords, medial to the oviducts.

The copulatory apparatus (Figure 52) was analyzed in two series of sagittal sections. It occupies the anterior half of the postpharyngeal region. The genital aperture (gp) leads into two cavities, anteriorly into the male atrium (am) and posteriorly into the expanded outlet of the copulatory bursa or vagina (v). The penis consists of a rather small bulb (bp) embedded in the mesenchyme and a larger, plug-shaped papilla (pp). Its lumen (pl) forms an arched canal, histologically rather uniform, without a differentiation into a seminal vesicle and an ejaculatory duct. This canal, lined with a cuboidal epithelium, opens into the atrium on the ventral side of the penis papilla. The two sperm ducts or vasa deferentia (vd). which at the level of the pharynx expand to form the spermiductal vesicles, approach the penis bulb and enter it anterolaterally, diminish in diameter, acquire each a muscle coat, and open into the anterior part of the penial lumen independently from the sides. The musculature of the penis is very weakly developed. That of the penis bulb consists of a loose network of muscle fibers, while the papilla has a thin muscle layer developed below the external epithelium. The penial lumen is enclosed in a coat of circular muscle fibers. Many faintly eosinophilic gland ducts enter the bulb from the surrounding mesenchyme and empty into the penis lumen as well as into the atrium in a field on the dorsal side of the penis papilla.

The two oviducts or ovovitelline ducts unite in the space between the male atrium and the bursal duct, forming a common oviduct (odc), which runs ventrally and opens into the posterior part of the male atrium. As is usual, the terminal parts of the paired oviducts and the common oviduct receive the outlets of strongly eosinophilic so-called shell glands. The copulatory bursa (b) is a voluminous sac lying anterior to the penis bulb. Its outlet, the bursal duct (bd), begins as a rather narrow, straight canal passing posteriorly above the penis and male atrium to a level posterior to the gonopore. There it bends abruptly anteroventrally and widens into a large expansion, the vagina (v). Close to the gonopore, the vagina narrows again and joins the atrial outlet. The epithelial lining of the vaginal sac is infranucleate, while the epithelia of the narrow proximal bursal canal and the narrow terminal parts of the vagina contain nuclei. The vagina has a coating of muscles consisting of intermingled circular and longitudinal fibers, the circular ones predominating. The musculature of the thin anterior part of the bursal canal is very feeble and could not be analyzed in my preparations.

DISTRIBUTION AND ECOLOGY.—Sphalloplana subtilis is known only from a spring on the property of J. W. Biggers at 6278 East Edsall Road, Fairfax County, Virginia, close to the Alexandria city line. The spring (Figures 27, 28) is enclosed in a brick structure with a removable concrete cover. The water is 6 feet (1.8 m) deep, its level reaching to 2 feet (0.6 m) below the rim of the casing. The species was obtained by J. R. Holsinger, W. Biggers, and the writer by baiting with fresh marine shrimp on several occasions between 18 March and 12 August 1973. The water temperature on 25 March was 8.6°C, on 12 August 17.7°C (while the temperature of the surface layer was 22.1°C), the pH 5.4 on both dates. A total of 10 specimens were collected, three of them sexually mature. Besides S. subtilis, the spring contained another species of Sphalloplana, S. holsingeri, which was much more abundant (over 80 specimens were collected).

TAXONOMIC POSITION.—The weakly developed adhesive organ of S. subtilis places the species in the subgenus Sphalloplana. The testes are ventral as they are in some other species of the genus. The most important specific characters are in the configuration of the copulatory organs. The short penis with very weak bulb and plug-shaped papilla and the ventral opening of the penial lumen show some similarity with S. consimilis (which has dorsal testes) and S. virginiana (which has a more highly developed adhesive organ). Another outstanding character, the development of a voluminous vagina with infranucleate epithelium, is shared with S. holsingeri, from which it differs by the anatomy of the penis and of the adhesive organ. The specific name, subtilis (Latin, slender) alludes to the elongated shape of the species.