Sphalloplana (Sphalloplana) evaginata Kenk 1977

Sphalloplana (Sphalloplana) evaginata

TYPE MATERIAL.—Holotype, series of sagittal sections of anterior region (6 slides) and of posterior region (12 slides), USNM 53427. Paratypes, three sets of sagittal and transverse sections on 41 slides, USNM 53428–53430; one whole mount, USNM 53431.

EXTERNAL FEATURES (Figures 2, 12).—Sphalloplana evaginata is a blind, rather large and plump species, measuring up to 30 mm in length and 6 mm in width when fully extended. The truncate anterior end has a convex frontal margin and bears a pair of rather prominent rounded auricular lobes protruding anterolaterally. Behind the head is an insignificant narrowing or neck, then the lateral body margins diverge, run parallel up to the level of the mouth, then narrow again in the postpharyngeal region. The tail end is bluntly pointed. The root of the pharynx lies at about the middle of the body and its length amounts to one-sixth the body length. The copulatory complex is situated in the anterior half of the postpharyngeal region. The intestinal system ends anteriorly in an arc (Figure 20) without forming the V-shaped extension of the lateral branches which is characteristic of some species of the subgenus Speophila.

The color of the living specimens is a light pink, due to a diffuse nongranular pigment permeating the subepidermal tissues. This pigment is not evenly distributed but is more concentrated in the head, the pharynx, the copulatory organs, and in a pair of longitudinal bands along both sides of the midline in the posterior two-thirds of the prepharyngeal region (corresponding to the site of the testes). A similar pinkish tint of the body is seen in Macrocotyla glandulosa Hyman, which, however, shows the denser organs (adhesive organ, pharynx, and copulatory apparatus) in a lighter hue than the general body coloration.

The movements of the worm are rather sluggish. Upon mechanical stimulation, the adhesive organ may be protruded temporarily as a pointed, conical projection in the center of the frontal margin.

ANATOMY.—The marginal zone with tall epidermal cells and long rhabdites is developed typically. The adhesive organ (Figure 31) appears in the slides as a subterminal shallow depression in the middle of the frontal margin. The central part of the organ may be invaginated, but not as deeply as is seen in representatives of the subgenus Speophila. The epidermis lining the depression is infranucleate, free of rhabdites, and pierced by numerous gland ducts containing a granular eosinophilic secretion. The glands forming this secretion are located in the mesenchyme some distance behind the organ. A network of muscle fibers is attached to the epithelium, functioning as retractors. The anatomy of the pharynx shows no peculiarities.

The two ovaries are situated behind or below the first to third lateral branches of the anterior intestinal ramus. In fully mature animals the zone of yolk glands or vitellaria begins a certain distance anterior to the ovaries. The testes are numerous and small, located dorsally, arranged in a pair of broad longitudinal rows, beginning some distance behind the ovaries and extending to the level of the pharyngeal root or somewhat behind it. In the zone of the testes, the thin sperm ducts or vasa deferentia run on top of the ventral nerve cords, generally medial to the oviducts or, in some places, lateral to them. They connect with the testes by long efferent ductules. At the level of the pharynx and behind it, the vasa deferentia form the usual expanded, tortuous spermiductal vesicles or false seminal vesicles.

The copulatory complex (Figure 47) was studied in series of sagittal sections of nine individuals. The gonopore (gp) leads into a voluminous common atrium (ac), which connects anteriorly with the male atrium (am) and posteriorly with the widely expanded terminal part of the bursal duct or vagina (v). The wall of the ventral section of the common atrium forms 8–19 (average, 13) cylindrical diverticula or evaginations (da), which project into the surrounding mesenchyme in all directions, but predominantly laterally. The lining of the genital atria is generally a cuboidal epithelium underlaid with a thin layer of circular muscle fibers followed by a layer of longitudinal muscles. Only the dorsal part of the common atrium has a muscle coat of interlaced circular and longitudinal fibers.

The penis has a rather small bulb (bp) and a large, generally plug-shaped, papilla (pp) of variable outline, subject to contraction and distortion when the animal is being preserved. The papilla has a very feeble musculature, confined chiefly to the rather thin layers of circular and longitudinal muscles underlying its cuboidal epithelial cover, and circular muscles surrounding the penial lumen, while its general tissue is parenchymal, with considerable plasticity. The lumen of the penis (pl), also variable in shape, is uniform both morphologically and histologically, not differentiated into a seminal vesicle and an ejaculatory duct. It begins in the ventral part of the penis bulb, proceeds first dorsally, then arches posteriorly toward the papilla. In some of the slides, this lumen appears funnel-shaped, narrow anteriorly and enlarging toward the papilla, with a wide opening into the male atrium. In the specimen depicted in Figure 47, the distal part of the lumen was compressed dorsoventrally, simulating a canal in the sagittal section. The two vasa deferentia (vd), enlarged as spermiductal vesicles, approach the base of the penis bulb, enter it ventrolaterally, diminish in diameter, and acquire a distinct layer of circular muscles. They penetrate the bulb, unite in the midline, and connect with the anterior end of the penial lumen.

The two oviducts unite in the space between the genital atrium and the bursal duct to form a rather short common oviduct (odc) that opens into the atrium from the dorsal side near the transition between the male and common atria. The copulatory bursa (b), variable in size, is a rounded sac in the usual position between the pharyngeal pouch and the penial bulb. Its outlet, the bursal duct (bd) runs first as a narrow straight canal in the midline above the penis to a level behind the gonopore. Then it curves sharply anteroventrally and expands into a voluminous cavity, the vagina (v), which fuses with the common atrium. The anterior part of the duct has a rather thin muscular layer, whereas the covering of the vagina consists of a thicker, but rather loose, coat of intermingled circular and longitudinal muscle fibers. The epithelium of the duct is cuboidal and nucleate. None of the epithelia of the entire copulatory apparatus are infranucleate.

DISTRIBUTION AND ECOLOGY.—Sphalloplana evaginata is known to occur in four caves in Perry County, Missouri:

Klump Cave (type-locality), situated east of Perryville; 3 February 1968, 2 specimens collected in still pools in the stream bed in shallow water, given to me by Dr. Joseph A. Beatty of Southern Illinois University; 8 April and 30 September 1973, 17 specimens collected by Jerry Lewis and Brent Opell, together with the isopod, Asellus antricolus (Creaser), and the amphipod, Gammarus troglophilus Hubricht and Mackin.

Crevice Cave, near Perryville; 23 September 1961 and 9 June 1964, 8 specimens collected by Stewart Peck and others, transmitted to me by Russell M. Norton.

Berome Moore Cave, 4 October 1964, 3 specimens collected by R. A. Brandon and A. Altig, sent to me by Russell M. Norton.

Garbage Hole Cave, part of the Rimstone Cave System, southeast of Perryville; 3 February 1973, 2 specimens collected by Jerry Lewis, together with amphipods and isopods, sent to me alive.

TAXONOMIC POSITION.—Sphalloplana evaginata is placed in Carpenter's subgenus Sphalloplana on account of its feebly developed adhesive organ, although the occasional appearance of a short invagination in the center of the adhesive pit indicates a transition to the subgenus Speophila. It shares other distinctive characters, such as the dorsal location of the testes, the configuration of the anterior border of the intestinal system, the development of a common genital atrium, etc., with other species of both subgenera. The most conspicuous feature of the species is the presence of cylindrical coeca or evaginations on the wall of the common genital atrium, which is expressed also in the specific name evaginata. Similar differentiations have been reported only in S. kansensis Hyman (1945:479), where the evaginations are described and depicted as “irregular epithelial outgrowths.” In other details (shape of the penis, length of the common oviduct, etc.) S. kansensis differs from S. evaginata.