{243}The animal consists of a broad prostomium, with a pair of eyes; and of a body, distinctly constricted in immature specimens into five or six segments, followed by a short conical tail. There are neither chaetae nor tentacles; locomotion is chiefly effected by means of the bands of cilia which encircle the body in a regular fashion, two bands round the head, and two round each segment in _D. taeniatus_; in some species there is only a single band on each segment. The whole of the ventral surface is covered with cilia, by the aid of which the animal probably "creeps" along the weeds. The alimentary canal is straight, and divisible into the regions shown in Fig. 121; a muscular protrusible organ, which is a ventral outgrowth of the foregut, is employed as a "sucker." The coelom is more or less obliterated (or ill developed). The excretory system in the genus is varied: in some species, as in _D. gigas_, it is stated to be constructed on the Planarian plan; in others, as in _D. taeniatus_, the organs are definite nephridia. Of these tubes there are five pairs, the last pair in the male serving as a seminal vesicle. Each nephridium is a ciliated tube, the internal end of which lies in the body-cavity and appears to be blocked by a ciliated tongue-shaped appendage. The first pair corresponds to the "larval nephridia" of Trochosphere larvae. The nervous system, which is in contact with the epidermis, consists of a brain in the prostomium, and, on each side of the body, a ventral cord with five ganglia, connected by transverse commissures in as many segments. The sexes are separate, and are usually similar; the male of _D. gyrociliatus_ is, however, much smaller than the female. The generative organs occupy the greater part of the body-cavity; in the male the testes communicate, by means of the pair of seminal vesicles, with a median eversible apparatus. In the female the paired ovaries communicate with a median sac which serves as a spermatheca. The development is simple:[296] the worm itself is more like a larval Polychaete than a full-grown worm. _Dinophilus_ is an extremely interesting form, and it has been suggested that, while still possessing certain Planarian characteristics, it may be looked upon as closely resembling the ancestor from which the Chaetopoda have arisen.