Unisegmental Vermes1 with a ciliated ectoderm in which are found rhabdites, pseudo-rhqbdites, or nematocysts; with two cerebral ganglia connected transversely, each of which is continued backwards into a long nerve, with a mouth and muscular pharynx, but no anus; with an excretory system of tubes ending in flame-cells, and a coelome formed by spaces between the parenchyma and viscera when it is present at all. Respiratory arid circulatory organs wanting. Hermaphrodites with rare exceptions and the ovary commonly divided into a germarium and vitellarium.

1 The marine Triclad, Gunda segmentata, is divided (but not externally) into 25 segments by septa formed chiefly by the dorso-ventral muscles. There are 25 simple and paired lateral branches to the intestine; a pair of ovaries with 24 pairs of vitellaria, and 25 pairs of testes. The genital organs correspond to the septa. The excretory canals open dorsally and probably by segmentally arranged pores. A similar segmentation is indicated, but much less markedly, in other Tridadida.

The small Turbellaria are mostly cylindrical, or the ventral surface is flat, the dorsal convex: the larger Turbellaria are usually broad and thin, with the exception of those inhabiting damp earth which are long, narrow, and thick bodied. The anterior extremity is usually wider than the posterior, but it may be narrow and pointed, and partially or wholly retractile, forming the proboscis of the Rhabdocoele Proboscida and some few genera. It is occasionally provided with tentacles as in the Rhabdocoele genus Vorticeros and in many Polycladida. These tentacles are either nuchal, and then solid, contractile, sometimes retractile into pits and placed one on either side the nerve ganglia (Planoceridae), or marginal and anterior, and then either lobes (Pseudoceridae) or pointed processes (Eury-leptidae) into which branches of the intestine pass. The ectoderm consists of a single layer of ciliated cells sometimes covered by a delicate cuticle. Pigment rarely occurs in them, but the majority contain rhabdites - clear homogeneous smooth rods pointed at each end and formed probably as a secretion either in the superficial cells themselves (Polycladida), or also in cells of the ectoderm imbedded in the parenchyma (most Tricladida, all Rhabdocoelida). Pseudorhabdites occur in Alloiocoela: they are rod-like, but granular and with an uneven surface.

True nematocysts are rarely found, e.g. in Microstoma lineare1. Some of the ectoderm cells are glandular, but unicellular glands occur also in the parenchyma with ducts leading up to the surface. Adhesive cells with processes and a sticky secretion are commonly found on the ventral aspect at the posterior end of the body, but not in the adult Polycladida (?), though sometimes present in the larva. Tactile cells with bundles of immobile but bendable hairs are met with in all Polycladida and in some other Turbellaria. Beneath the ectoderm is a basement membrane, the thickness of which varies much. It is stout and contains richly branched cells in Polycladida. The musculature of the body forms a continuous investment. In the Rhabdocoelida it consists of an outer circular, and inner longitudinal layer of fibres which are reversed in position in Microstoma lineare, and in many instances have interposed between them a diagonal layer. The layers are more complicated in Polycladida, but consist generally speaking of an outer and inner longitudinal layer, the latter absent on the dorsal surface, inclosing between them a layer of circular, and two layers of diagonal, fibres. The layers vary in Tricladida, but the outer layer is circular.

Many Polycladida possess a muscular sucker situated near the centre of the body: others (JLeptoplanidae) have a sucker between the male and female generative openings. A system of dorso-ventral fibres traverses the parenchyma. Its fibres branch at each end, and are attached to the basement membrane. The parenchyma consists in Acoela of a soft granular protoplasmic mass into which the food passes, a digestive tract not being differentiated. In Rhabdocoela and Alloiocoela it consists of delicate connective tissue-fibres with nuclei which form a reticulum, and connective tissue cells. The coelome is formed by the spaces between these elements and the viscera. The degree to which it is developed varies much, and the fluid which fills it is usually colourless. On the contrary the parenchyma of Polycladida is composed apparently of nucleated cells, containing vacuoles, and filling up the whole space between the viscera. A coelome is consequently wanting. Pigment is scattered in the parenchyma, in Rhabdocoelida in special cells1. Convoluta paradoxa possess peculiar yellow cells, containing plates of diatomin (?); Vortex viridis, Convoluta Schultzii, etc, cells containing chlorophyl and starch granules.

These coloured cells usually lie near the integument and are perhaps symbiotic algae (see pp. 242-4).

1Still more rare are the structures known as sagittocysts, i. e. capsules similar to those of nematocysts, but inclosing a needle-like rod, which is expelled on irritation of the animal. It is very rare to find a Rhabdocoele without any of the structures named, e. g. the parasitic Graffilla, or Prorhynchus stagnalis. There is some doubt as to whether the rhabdites are expelled, as is usually supposed. They may serve as a skeleton. See Iijima, Z. W. Z. xl. pp. 372-74.

A nervous system does not exist in Acoela2. In other Turbellaria there are a couple of ganglionic swellings usually placed anteriorly, sometimes, as in Polycladida, more posteriorly, in front of or above the pharynx. The two ganglia are usually more or less closely united, and each of them gives origin, except in Polycladida, to a nerve which runs backwards ventral to the digestive system. These cords are rarely united by a transverse commissure in Rhabdocoela, but in the Tricladida they possess many such commissures. The Polycladida have several stout nerves radiating from the ganglia, as well as others from the longitudinal cords. Numerous commissures unite them one with another. The branches given off by the lateral nerves ultimately form a sub-muscular plexus. In addition to the tactile ectodermic cells which are chiefly grouped about the anterior region of the body, or in Polycladida on the dorsal papillae and tentacles, both eyes and auditory organs are found. As to the former, pigment spots without a lens lie in the ectoderm of Acoela and the Micro-stomidae (Rhabdocoela). But in all other Turbellaria the eyes when present lie in the parenchyma. In the Rhabdocoelida they sometimes lie in the ganglia themselves.