Page:EB1911 - Volume 21.djvu/740

Rh Triclads, on the other hand, fragment, without undergoing preparatory changes.

The male and female genital ducts (gono-ducts) open to the exterior, either through a common chamber on the ventral surface (most Rhabdocoelida and all Tricladida, figs. 12, 14) or by separate apertures that are also usually ventral In the latter case, the male gonopore is usually in front of the female one (all Polycladida and some Rhabdocoelida). A separate opening is sometimes acquired by one or other of the accessory reproductive organs (as by the spermotheca in some Rhabdocoelida in which it is dorsal).

The generative organs of the Planarians are complex. Male and female germ-cells develop in one and the same individual and reach the exterior by independent ducts. These ducts are provided with accessory glands along their course and terminate in penial or vaginal structures, often of great complexity, which are surrounded by an “atrium” or invagination of the ventral body-wall. From this invagination a special vesicle “uterus” is often deeloped for the reception of the fertilized egg previous to oviposition.

The Acoela present the simplest arrangement. In this group (fig. 11) the male germ-cells arise in follicles each of which is the product of a single sperm-mother-cell. From these follicles, the motile spermatozoa enter the paired sperm-duct, which opens by a single aperture near the hinder end of the animal, and is provided with a simple unarmed glandular penis. The female germ cells or ova arise from a paired ovary, some of the cells of which appear to act as nurse cells, supplying the young eggs with nourishment. When mature the eggs are transferred to the oviduct. At the point where the two oviducts join in order to open to the exterior they receive a conical sac (spermotheca) which contains spermatozoa. At this point the eggs are fertilized, and deposited in a mucilaginous mass which is attached to algae or buried in the sand. It is characteristic of the Acoela that the testes and ovaries should not be continuous with either the sperm-duct or the oviduct respectively.

In one genus of the Acoelous Turbellaria—Polychaerus—this primitive arrangement undergoes a development which foreshadows the complicated ovaria and vitellaria of higher forms. In Polychaerus the eggs mature in a special roomy chamber and are here provided with yolk which is elaborated by a sterile part of the ovary. Thus we have a differentiation of germ-cells into two portions allocated to two chambers: fertile ova which open eventually into the oviduct, and sterile ova that become yolk-cells and open into the brood-pouch.

The remaining Rhabdocoelida possess separate ovaries and yolk-glands. The union between the two sets of ducts takes place in the genital atrium which is provided with a spermotheca for the fertilization of the ova, but in at least one sub-family (Cylindrostominae) the spermotheca opens by a special dorsal pore. These ova, together with the yolk and spermatozoa, are then transferred to another atrial diverticulum—the uterus, in which a shell is formed and from which they are deposited in the form of a cocoon. In addition, a muscular pouch, the so-called “bursa copulatrix,” is usually present. The male organs of Rhabdocoelida are no less complex. The testes are either follicular (Alloeocoela) or compact (Rhabdocoela), and communicate indirectly or directly with the paired seminal ducts. The ducts unite at the base of an evaginable penis. This muscular organ is provided with glandular and chitinoid appendages of considerable complexity, and, in addition to these, a poison gland and duct are sometimes present. In certain genera (Macrorhynchus, Prorhynchus) the penis is used for catching prey. Perhaps exclusively so in the former genus. The opening of the atrium into the oral cavity in Cylindrostominae and of the male organ into the mouth of Prorhynchus is possibly explained by this fact.

From the Alloeocoela we pass readily to the Triclads. In both of these groups the reproductive organs are based on the same plan; but in Triclads the separation of ovarian and vitellarian portions of the gonad is less perfectly effected. The oviduct transmits the eggs from the anteriorly placed ovary, and receives in its course the openings of numerous vitellaria (fig. 14). No distinct spermotheca is developed, but a cocoon is formed in a special