The _Rhabdocoela_ live under varied conditions. One form, _Prorhynchus sphyrocephalus_, has been found among plants far from water in the neighbourhood of Leyden, by De Man.[64] With this exception the group is purely aquatic, and though a few genera and even individuals of the same species occur both in salt and fresh water, whole sub-families and genera are either marine or paludicolous. Among the latter, _Mesostoma_, _Castrada_, _Vortex_, and _Derostoma_ are common in brooks and ponds, especially at certain times, often only for one month (May or June) in the year. Species of _Macrostoma_, _Stenostoma_, and _Microstoma_ are also abundant in similar places. The two latter occur in chains formed by fission; but the sexual individuals (which are of distinct sexes, contrary to the usual hermaphrodite condition of Flat Worms) only appear at stated times and are not well known. A large number of genera are purely marine, and one family, the Proboscidae (distinguished by having the anterior end invaginated by special muscles and converted into a sensory organ), is entirely so. The most cursory examination of littoral weeds reveals species of _Macrorhynchus_, _Acrorhynchus_, _Promesostoma_, _Byrsophlebs_, and _Proxenetes_, the character of which may be gathered from von Graffs great monograph, or from Gamble's paper on the "British Marine Turbellaria."[65] Much, however, still remains to be done before we possess an adequate idea of the occurrence of this group on our coasts. {45}[Illustration: FIG. 19.—Forms of Rhabdocoelida. A, _Mesostoma tetragonum_ O. F. M. (Rhabdocoela), × 10; B, _Convoluta paradoxa_ Oe. (Acoela), × 10; C, _Vorticeros auriculatum_ O. F. M., × 6; D, _Monotus fuscus_ Oe. (Alloeocoela), × 4. _ap_, Adhesive papillae; _d_, intestine; _m_, pharynx; _ot_, otolith; _rh_, rhabdites; _te_, testes; _ut_, uterus with eggs; _yg_, yolk-glands; ♂, male, ♀, female genital pores. (A after Braun.)]