PROSTHIOSTOMATIDAE. { Tentacles absent. Body { elongated. Pharynx {_Prosthiostomum_ (Fig. 8, F). long, cylindrical. { Penis with accessory { muscular vesicles. { {20}APPEARANCE AND SIZE OF POLYCLAD TURBELLARIA.—Polyclads are almost unique amongst animals in possessing a broad and thin, delicate body that glides like a living pellicle over stones and weeds, moulding itself on to any inequalities of the surface over which it is travelling, yet so fragile that a touch of the finger will rend its tissues and often cause its speedy dissolution. The dorsal surface in a few forms is raised into fine processes (_Planocera villosa_), or into hollow papillae (_Thysanozoon brocchii_), and in very rare cases may be armed with spines (_Acanthozoon armatum_,[26] _Enantia spinifera_); in others, again, nettle-cells (nematocysts) are found (_Stylochoplana tarda_, _Anonymus virilis_). Some Polyclads, especially the pelagic forms, are almost transparent; in others, the colour may be an intense orange or velvety black, and is then due to peculiar deposits in the epidermal cells. Between these two extremes the colour is dependent upon the blending of two sources, the pigment of the body itself and the tint of the food. Thus a starved _Leptoplana_ is almost or quite white, a specimen fed on vascular tissue reddish. Many forms are coloured in such a way as to make their detection exceedingly difficult, but this is probably not merely due, as Dalyell supposed, to the substratum furnishing them with food and thus colouring them sympathetically, but is probably a result of natural selection. The largest Polyclad, the bulkiest Turbellarian, is _Leptoplana gigas_ (6 inches long and 4 in breadth), taken by Schmarda, free-swimming, off the coast of Ceylon. The largest European form is _Pseudoceros maximus_, 3½ inches in length and stoutly built. A British species, _Prostheceraeus vittatus_, attains a length of from 2 to 3 inches. These large forms, especially the Pseudoceridae (pre-eminently the family of big Polyclads), are brightly coloured, and usually possess good swimming powers, since, being broad and flat, they are certainly not well adapted for creeping rapidly, and this is well shown by the way these Polyclads take to swimming when in pursuit of prey at night. The size of any individual is determined, amongst other factors, by the period at which maturity sets in, after which probably no increase takes place. Polyclads apparently live about twelve months, and mature specimens of the same species vary from ½ inch to 2½ inches in length (_Thysanozoon brocchii_), {21}showing that growth is, under favourable conditions, very rapid.