LOCOMOTION.—Locomotion is generally performed by Polyclads at night when in search of food, and two methods, creeping {23}and swimming, are usually employed—creeping by the cilia, aided possibly, as in the case of some Gasteropod Mollusca, by the longitudinal muscles of the ventral surface; and swimming, by undulations of the expanded margins of the body. In the former case the cilia work in a glandular secretion which bathes the body, and enables them to effect their purpose equally well on different substrata. The anterior region is generally lifted up, exploring the surroundings by the aid of the tentacles, which are here usually present. The rest of the body is closely appressed to the ground. Swimming is particularly well performed by the Pseudoceridae, certain species of _Prostheceraeus_, the large Planoceridae, some _Stylochoplana_, _Discocelis_, and _Leptoplana_, and in the same manner as in _Leptoplana tremellaris_ (p. 9). In _Cryptocelis_, _Leptoplana alcinoi_, and _L. pallida_, however, the whole body executes serpentine movements like an active leech (e.g. _Nephelis_); a cross section of the body would thus present the same appearance during the whole movement. Many Polyclads, notably _Anonymus_ (Lang), if irritated, spread out in all directions, becoming exceeding thin and transparent. [Illustration: FIG. 9.—_Discocelis lichenoides_ Mert. (after Mertens), creeping on the inner side of a glass vessel by means of the lobes of the extended and exceedingly mobile pharynx (_ph_). These lobes also serve to enclose _Crustacea_ (_a_), and one lobe may then be withdrawn independently of the rest, back into the body (_b_). The brain (_br_) and shell-gland (_sg_) are shown by transparency.] _Discocelis lichenoides_, _Planocera graffii_, and _Anonymus virilis_ have peculiar modes of progression. The first, according to Mertens, will climb up the sides of a vessel by means of the expanded lobes of the pharynx (Fig. 9, _ph_), a habit of considerable interest, since we know that certain Ctenophores—_Lampetia_, for instance—progress when not swimming on the expanded lobes of their "stomach."[33] _Planocera_ and _Anonymus_ {24}creep by extending parts of the anterior margin and dragging the rest of the body behind. In consequence, the brain and dorsal tentacles may come to lie actually behind the middle of the body, and thus no definite anterior end or "head" advances first. Along with this curious habit it may be noticed (Lang) that the radial symmetry of the body is well marked; but even without accepting this author's suggestion of the concurrent development of a "head" with locomotion in a definite direction, the facts, whether these two forms are primitive or not, are highly interesting.