| Character. | Acotylea. | Cotylea. |
| Sucker | A sucker absent.[[25]] | A sucker always present (Figs. 8, D, s; 7, A, sc). |
| Mouth | In the middle, or behind the middle, of the ventral surface. | In the middle, or in front of the middle, of the ventral surface. |
| Pharynx | More or less intricately folded. | Rarely folded. Usually cylindrical or trumpet-shaped. |
| Tentacles | A pair of dorsal tentacles usually present. | A pair of marginal tentacles (except in Anonymus). |
| Development | Usually direct. Larva when present, not a typical Müller's larva. | Müller's larva present. Metamorphosis, however, extremely slight. |
Fig. 8 shows that, starting with a member (A, D) of each division, in which the mouth is almost in the middle of the ventral surface, and the brain and sense organs somewhat remote from the anterior end, we find in the Acotylea a series leading to an elongated form (Cestoplanidae), in which the mouth, pharynx, and genital pores are far back near the hinder end of the body; while in the Cotylea the series leads similarly to the elongated Prosthiostomatidae, in which, however, the pharynx and external apertures are in the front part of the body. This view of the morphology of the Polyclads is due to Lang, and is based on the assumption that the more radially-constructed forms (Fig. 8, A, D) are the primitive ones.
Fig. 7.—Diagrammatic vertical longitudinal sections: A, Of Prosthiostomum (type of Cotylea); B, of Leptoplana; C, of Cestoplana (types of Acotylea). (After Lang.) These figures illustrate the changes which follow the shifting of the mouth from a central position (B) to either end of the body. br, Brain; dphm, "diaphragm"; gm, true mouth; lg, openings of lateral gut-branches; m, mouth; mg, main-gut or stomach; mgbr, median gut-branch; ph, pharynx; ph.m, aperture in pharyngeal fold; phs, peripharyngeal sheath; sc, sucker; ♂, male, and ♀, female, genital aperture.
Fig. 8.—Chief forms of Polycladida: A-C, Acotylea; D-F, Cotylea. A, Planocera graffii Lang, nat. size; B, Stylochoplana maculata Stimps, × 7; C, Cestoplana rubrocincta Lang, × 4⁄3; D, Anonymus virilis Lang, × 3, ventral surface; E, Thysanozoon brocchii Grube, nat. size; the head is thrown back and the pharynx (ph) is protruded. F, Prosthiostomum siphunculus Lang, × 3. Br, Brain; CG, cerebral eye group; DM, true mouth; Ey, marginal eyes; m, mouth; MG, main-gut or stomach; P, dorsal papillae; Ph, pharynx; s, sucker (ventral); T, tentacles; UP, dorsal median gut-branch. ♂, male, and ♀, female, genital aperture, except in D, where ♂ refers to the multiple penes. (After Lang and Schmidt.)
Classification of Polycladida.
| ACOTYLEA. | ||
| Family. | Genus. | British Representatives. |
| Planoceridae. With dorsal tentacles. Mouth sub-central. | Planocera (Fig. 8, A). Imogine. Conoceros. Stylochus. Stylochoplana (Fig. 8, B). Diplonchus. Planctoplana. | Planocera folium Grube. Berwick-on-Tweed. Stylochoplana maculata Quatref. Among brown weeds in Laminarian zone. |
| Leptoplanidae. Without dorsal tentacles. Penis directed backwards. | Discocelis. Cryptocelis. Leptoplana. Trigonoporus. ?Polypostia (see p. [27]). | Leptoplana tremellaris O. F. Müll. L. fallax Quatref. Plymouth. L. droebachensis Oe. Plymouth Sound. L. atomata O. F. Müll. Doubtful species. |
| Cestoplanidae. No tentacles. Body elongated. Penis directed forwards. | Cestoplana (Fig. 8, C). In Mediterranean and on French side of the Channel. | |
| Enantiidae. No sucker. No tentacles. Main-gut very short. External apertures as in Euryleptidae. | Enantia. Adriatic Sea. | |
| COTYLEA. | ||
| Anonymidae. Mouth central. No tentacles. With two rows of penes. | Anonymus (Fig. 8, D). Naples (two specimens). | |
| Pseudoceridae. Marginal tentacles folded. Mouth in anterior half. | Thysanozoon (Fig. 8, E). Pseudoceros. Yungia. | |
| Euryleptidae. Tentacles usually present and pointed, or represented by two groups of eyes. Mouth close to anterior end. Pharynx cylindrical. | Prostheceraeus. Cycloporus. Eurylepta. Oligocladus. Stylostomum. Aceros. | Prostheceraeus vittatus Mont. On west coast. P. argus Quatref. Guernsey. Cycloporus papillosus Lang. On Ascidians in 2-30 fms. Eurylepta cornuta O.F. Müll. On sponges and shells, 2-10 fms. Oligocladus sanguinolentus Quatref. O. auritus Clap. Doubtful. Stylostomum variabile Lang. |
| Prosthiostomatidae. Tentacles absent. Body elongated. Pharynx long, cylindrical. Penis with accessory muscular vesicles. | Prosthiostomum (Fig. 8, F). | |
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), showing that growth is, under favourable conditions, very rapid.