Phagocata nivea
From Kenk, 1953:
This is a slender, rather delicate species. Mature specimens measure up to 8 mm. in length and about 1.5 mm. in width. In the quietly gliding animal the anterior end is truncated, with a very slightly bulging frontal outline and with rounded lateral corners (auricles). There is no distinct narrowing or neck behind the auricles and the lateral margins of the head are approximately parallel. Behind the head, the body widens and soon reaches its greatest width. From there on, the lateral margins of the body run parallel up to the level of the mouth, to converge again in the postpharyngeal region and to meet in a bluntly pointed posterior end.
The species lacks pigment and usually appears white and somewhat transparent. The intestinal contents may shine through the body wall and give the animal a certain amount of color; the margins of the body, the head region, and the areas occupied by the pharynx and the copulatory apparatus, however, are always white.
There are two rather small eyes, situated close together (about one-fourth the body width apart at the level of the eyes) and far removed from the frontal end. This character, easily recognized in life, distinguishes the species from another white triclad with which it shares its habitat, Dendrocoelopsis alaskensis, described as new on p. 178.
The pharynx is inserted, in sexually mature specimens, somewhat behind the middle of the body and measures about one-sixth of the body length. The copulatory organs occupy the anterior half of the postpharyngeal region.
The animal moves by gliding only; crawling movements, such as are seen in other triclads, particularly in those equipped with anterior adhesive organs, have not been observed in this species.
From the description it may be seen that the species in life shows a close resemblance to other species of the same genus, particularly to the American Phagocata morgani, the European P. albissima, P. vitta, and related forms. A separations of these species can be made only on the basis of anatomical characters.
Only those characters of the digestive system that have a taxonomic significance are discussed here. The pharynx has a structure typical of the family Planariidae; i. e., the fibers of the internal musclezone are arranged in two distinct layers, a thick inner circular layer and a narrower outer longitudinal one. The anterior intestinal trunk bears 10 or 11 branches on each side. Each posterior trunk has 21 to 27 lateral branches and numerous short medial branches in both the pharyngeal and postpharyngeal regions.
The testes are numerous and are arranged, on each side of the midline, in a longitudinal zone extending from a short distance behind the ovary almost to the posterior end of the body. Their position is predominantly ventral, below the intestinal branches. Only a few testes extend into the mesenchymatic "septa" between the branches toward the dorsal side.
The two ovaries are typical, each situated approximately below the second intestinal branch. An undifferentitated mass of cells, the parovarium, is attached to the dorsolateral side of each ovary.
The genital pore (pg), situated about halfway between the mouth and the posterior end of the body, leads into a small cavity (ac) which continues to the left and dorsally into the duct of the copulatory bursa (bd) and to the right and anteriorly into the male atrium (am). This cavity may be considered to represent a common genital atrium. In some specimens, however, there appears to be no differentitation of the atrium into male and common parts, and the bursa duct and undivided atrium meet at or near the genital pore. These variations are obviously due to the different states of muscular contraction in which the animals were killed. The atrium, narrow at the genital aperture, widens as it extends forward, to the right side of the midline. It is lined with a tall, glandular epithelium, the cells of which project, into the cavity in a villuslike fashion. Below the epithelium there are two layers of muscle fibers, one circular and the other longitudinal.
The penis consists of a spherical, muscular bulb embedded in the mesenchyme, and a moderately large papilla projecting into the male atrium. The bulb is pierced by numerous gland ducts which open into the lumen of both the bulb and the papilla. The shape of the papilla is subject to great variation, due apparently to the state of contraction of the organ. It may be twisted to one side and even partly inverted into the lumen of the penis (similar to the pseudoflagellum of various dendrocoelids). The shape shown in figure 21 appears to be that of the organ at rest. The outer wall of the papilla is covered with a tall to cubical epithelium similar to that lining the atrium. Below the epithelium there is a layer of circular muscle fibers followed by another of longitudinal fibers. The shape of the penis lumen (lp) is a changeable as that of the papilla. Typically, it appears to be wider in the bulb than it is in the papilla, though there is no distinct ejaculatory duct differentitatied. The lumen opens ventrally to the tip of the papilla. The two vasa deferentia (vd) penetrate the penis bulb from both sides and empty into the penis lumen separately, but not far apart.
The two oviducts converge at the level of the copulatory apparatus, the left one passing between the bursa duct and the male atrium, and unite at a point dorsally to the atrium. The rather long common oviduct (odc) curves ventrally and opens into the posterior part of the male atrium. The terminal sections of the paired oviducts and the greater part of the common oviduct receive the outlets of numerous eosinophilic glands, the cell bodies of which are scattered in the surrounding mesenchyme, particularly dorsally to the atrium.
The copulatory bursa (b) is of moderate to large size and is irregularly lobed. The bursa duct or stalk (bd) is wide, runs posteriorly to the left of the midline, and curves ventrally to reach the genital apearture. It is lined with a tall, glandular epithelium and surrounded with a strong muscular coat consisting of intermingled circular and longitudinal fibers.