There are usually two or four in this group, and in the Tricladida, but in the latter they are sometimes more numerous, and in a terrestrial form from Brazil extend along the lateral margins. They are generally numerous in Polycladida, two groups lying constantly above the ganglia, others upon the tentacles, or where these are absent on the anterior and lateral margins of the body. The most complex eye consists of a pigmented cup into which project visual rods with retinal cells at their outer ends. These cells are continuous with the optic nerves. Such eyes are found in Tricladida and all Polycladida. In the latter group they increase by division, whereas in some fresh-water Tricladida, e. g. Polycelis nigra, the eyes which are simple in structure have been observed to fuse. Otolithic vesicles are very rare in Dcndrocoelida, and when present in the Rhabdocoelida there is usually only one. It lies anteriorly and consists of a vesicle with fluid contents and suspended calcareous otoliths. A pair of ciliated grooves, one on each side, lie close to the ganglia in many Rhabdo-coela. In fresh-water Tricladida and the Polycladida a ciliated groove extends along the anterior margin of the body, and backwards laterally on the dorsal aspect.
These structures appear to be sensory.
1The fore-part of the body in Derostomum(-a) Benedeni is coloured red with haemoglobin (Francotte).
2But Convoluta Schultzii is stated by Delage (A. Z. Expt. (2), iv. 1886) to have a ganglionic mass surrounding the otolith, with two other and smaller masses connected inter se and to it. Six parallel longitudinal nerve-fibres extend backwards. They lie apparently on the dorsal aspect, are connected by transverse commissures, and unite posteriorly in a plexus. There is a peculiar frontal sensory organ. Lang mentions (p. 672, Polycladen) an Acoelan with well developed nervous system.
The mouth varies much in position: it may be anterior, median, or posterior. It leads, as a rule, into a muscular pharyngeal sac, within which lies a muscular pharynx. The pharynx can be extended from the mouth, and is an organ for the prehension of food. It assumes various forms. In the Acoela the mouth leads into a short tube which ends in the parenchyma, into which the food passes. In other Turbellaria there is an intestine, simple and saccular in Rhabdocoela, lobed or irregular in Alloio-coda, branched in Dendrocoelida. In the last-named group it consists, in Tricladida of three chief branches, one extending forwards above the ganglia, and one backwards on each side, all beset with secondary branches; in Polycladida of a main intestine, which usually sends a branch forwards above the ganglia, and gives off a variable number of paired lateral branches, never, however, less than four. These branches divide in their turn, and the points of division are often constricted. In some instances the ultimate divisions form an anastomosing network. The epithelium of the main intestine of Polycladida is ciliated; that of its branches, like the intestinal epithelium of other Turbellaria, emits pseudopodial processes which absorb nutriment. Digestion is therefore parenchymatous.
The cells often contain vacuoles, concretions, and coloured granules. The main intestine of Polycladida possesses a well developed musculature, while the constrictions of the lateral branches are provided with both constrictor and dilator muscles. The branches of the intestine in three genera of Polycladida open externally through the ectoderm1. A nervous commissure generally surrounds the branch of the intestine extending forwards above the ganglia.
1 Yungia with dorsal, Cycloporus with marginal apertures. OUgocladus has a doubtfully present pore situated dorsally and posteriorly.
A nephridial or excretory system is absent in Acoela. In other Rhabdocoelida it may consist of a single median trunk opening posteriorly, or of two lateral trunks which either unite and open posteriorly, or remain independent, and then open ventrally or posteriorly, or, as is most usual, into the pharyngeal cavity. It is rare for them to open near the centre of the ventral surface, and to be connected anteriorly by a cross branch, from which a canal passes backwards on each side parallel to the main trunk. With these main trunks finer vessels are connected, which form a more or less distinct network. Canalicules terminating in flame cells open into this network, or into the blind terminations of its branches in Mesostoma Ehrenbergi and a number of other Rhabdocoeles (Vortex, Prostomum, Monocoelis, &c). Cilia may occur in the main or in the secondary canals. In the Tricladida, Polycoelis nigra possesses both on the ventral and dorsal aspects a network of canals in which there is an undulating membrane. Straight vessels lead to the surface of the body, and probably open upon it; and short canalicules ending in flame-cells are connected to the network. In other cases there appears to be a pair of dorsal, or as in Gunda, of dorsal and ventral, lateral canals, which anastomose anteriorly.
These canals open by more or fewer dorsal pores, which in Gunda are segmentally arranged. There is a system of finer branching canals which end in flame-cells. The excretory system of Polycladida is scarcely known. In Thy-sanozoon, Lang detected.large undulated canals which anastomose, and are provided with stout lateral branches which approach the ectoderm, and probably open externally. Fine branched canals are connected to the main vessels, and end in flame-cells, and small cells with large cilia occur in the course of the canals. It is certain that in some instances, and it is probable that in all, the excretory canals are really intra-cellular 1.
All Turbellaria are hermaphrodite except the Rhabdocoele genera, Microstoma and Stenostoma. But the male organs tend to ripen before the female (= successive hermaphroditism) in Acoela, in Polycladida, and perhaps in others. The glands are paired with the exception of the ger-marium in some Rhabdocoela. The testes in Rhabdocoela are two large compact glands, in all other Turbellaria scattered and numerous globular follicles. Specialised ducts are not developed in Acoela, Alloiocoela, and some Rhabdocoela. The two vasa deferentia when present receive the sperm from finer vessels, and either open together or separately into the copulatory organ 1. They are often dilated into seminal reservoirs, or in some .instances a vesicula seminalis is formed from or attached to the penis. One or more accessory glands are usually present, and their secretion is mixed with the sperm. The penis is either a protrusible or an evaginable organ, and is often contained within a sheath and armed with chitinous spines, especially in Rhabdocoela. In the Polyclad Anony-mus there are 2-15 copulatory organs on each side the body, and 2-4 occur in three other instances in the same group.