The central NERVOUS SYSTEM, which is embedded in the parenchyma of the body, consists of cerebral ganglia, united together in the shape of dumb-bells, and of two or more longitudinal MEDULLARY FASCICLES, often forming transverse anastomoses. Organs of sense usually occur only in the free-living species, more rarely during the free-living stages of a few parasitic species and in a few ectoparasitic forms.
[In Platyhelminthes simple eye-spots frequently occur, and in a few an auditory vesicle.]
Blood-vessels and definite RESPIRATORY ORGANS are lacking [except in Nemertinea]; the EXCRETORY APPARATUS (formerly termed water-vascular system) is typical of the entire class. It commences in the interstices of the parenchyma, with peculiar terminal cells (ciliated funnels), which will be described later (p. [219]), the capillary processes of which go on uniting into larger branches, and finally form two large collecting vessels, which, sometimes separately and sometimes united, open to the exterior through one, two, or numerous pores.
Nearly all the Platyhelminthes are HERMAPHRODITIC, and in nearly all there are, in addition to the ovaries producing ova, other glands attached to the female genital apparatus, namely, the vitellaria or yolk glands, which provide a substance termed yolk, which serves as nourishment for the embryo. The fully formed eggs have shells and are “compound,” i.e., composed of the egg or ovarian cell, which is surrounded by numerous yolk cells or their products of disintegration. The two sexual openings usually lie close together, frequently in the fundus of a genital atrium; they are rarely separated from one another. Shell glands also usually occur (p. [221]).
Reproduction is sexual, often, however, combined with asexual methods of propagation (segmentation, budding). The Platyhelminthes live partly free in fresh or salt water, exceptionally also on land. The greater part, however, live as parasites on or in animals.
Classification of the Platyhelminthes.
Class I.—Turbellaria (or Eddy Worms). Flat worms for the most part, free living, and always covered with a ciliated epithelium.
Order 1.—Rhabdocœlida, gut unbranched.
Order 2.—Tricladida, gut with three main branches.