HABITS AND STRUCTURE OF TRICLADS.—The common _Planaria_ (_Dendrocoelum_) _lactea_, which usually progresses by ciliary action, aided, it is said, by muscular contractions of the ventral surface, performs, if alarmed, a series of rapid "looping" movements, by affixing a sucker (Fig. 14, A, _sc_), placed on the under side of the head, to the substratum, and pulling the posterior end close to this. The sucker, discovered by Leydig, is even better developed in _P. punctata_ (Fig. 16, A), _P. mrazekii_, and _P. cavatica_, and is an efficient adhering-organ which has probably been developed from a similar but simpler structure found in a considerable number of both fresh-water and marine Triclads (_P. alpina_, Fig. 16, E). Probably the sucker of the Land Planarian _Cotyloplana_ (D) is the same structure, but the two suckers of _Dicotylus_ (B) are at present unique. _Planaria dioica_, found by Claparède on the coast of Normandy,[51] is covered with minute adhesive papillae, {36}similar to those of certain Rhabdocoelida (e.g. _Monotus_, Fig. 19, D), enabling it to cling tightly to the _Zostera_, and so to resist the loosening action of the waves. [Illustration: FIG. 16.—Suckers of Triclads. A, _Planaria punctata_ Pall.; _a_, dorsal surface of head; _b_, ventral surface (freely moving) showing the sucker; _c_, sucker contracted (after Hallez): B, ventral surface of head of _Dicotylus pulvinar_ Gr., from Lake Baikal (after Grube): C, dorsal surface of _Procotylea fluviatilis_ Gir. (after Girard): D, sucker of _Cotyloplana whiteleggei_ Sp. (after Spencer): E, ventral view of head of _Planaria alpina_ Dana (preserved specimen); _hg_, adhering groove; _m_, thickened musculature forming the margin of the sucker; _sc_, sucker; _t_, tentacles.] The movements of Land Planarians are somewhat peculiar. The ventral surface of _Bipalium_ has a median groove, into which the ducts of numerous mucus-glands open. This is bordered by two ridges clothed with long and powerful cilia, which perform the chief part in propelling the animal, aided, according to Lehnert,[52] by muscular waves which pass from the head, backwards, _i.e._ opposite in direction to those by which a snail slides along. This observation, however, needs confirmation. The whole body executes sinuous movements, during which the crescentic head, lifted slightly above the ground (Fig. 15, A), is constantly altering and regaining its normal shape, somewhat as a _Planaria lactea_ uses the lobes of its head. Further examination shows that the margin of the head of _Bipalium_ is not only provided with eyes, but in addition, with ciliated, (probably) olfactory pits. Such depressions, innervated directly from the cerebral ganglia, have been found in sixteen species of _Geoplana_, {37}and in one or two species of _Rhynchodemus_.[53] Some Land Planarians (a species of _Rhynchodemus_ from Ceylon, and a _Dolichoplana_ from the Philippines) wriggle out of a box or the hand with great speed (Moseley).