Mouth In the middle, or behind the In the middle, or in front of middle, of the ventral the middle, of the ventral surface. surface. Pharynx More or less intricately Rarely folded. Usually folded. cylindrical or trumpet- shaped. Tentacles A pair of dorsal tentacles A pair of marginal tentacles usually present. (except in _Anonymus_). Development Usually direct. Larva when Müller's larva present. present, not a typical Metamorphosis, however, Müller's larva. extremely slight. {17}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. [Illustration: 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.] {18}[Illustration: 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.)] {19}CLASSIFICATION OF POLYCLADIDA. ACOTYLEA. Family. Genus. British Representatives.