The peripheral nerves, which spring from the posterior strands as well as from the commissures, either pass directly to the muscular fibres or to the sensory cells that are situated at the level of the subcuticular cells, or they reach these after the formation of a plexus situated immediately beneath the dermo-muscular layer; the processes directed outwards terminate in small vesicles in the cuticle. As to other ORGANS OF SENSE, simple eyes, two or four in number, are known in several ectoparasitic species as well as in a few free-living larval stages (Cercariæ) of endoparasitic forms. In the adult stage, however, they usually undergo complete atrophy. The ALIMENTARY CANAL commences with an oral aperture, generally terminal or sub-terminal (ventral) at the anterior extremity, which leads into an oral cavity usually surrounded by a sucker; the œsophagus, of various lengths, is directed backwards and is generally surrounded by a muscular pharynx (fig. 122). In some cases there exists between the sucker and pharynx, pharyngeal pouches (præpharynx). Sooner or later the intestine divides into two lateral branches directed backwards, both of which end blindly (cæca) at the same level.[257] In many ectoparasites (_Monogenea_ [p. 222]) a connection exists between the genital glands and one of the intestinal branches (ductus vitello-intestinalis [fig. 123]). [257] The following conditions represent deviations from this type: (1) In _Gasterostomum_ the oral aperture is situated in the middle of the ventral surface, and occasionally is even nearer to the posterior than to the anterior end. There is no proper oral sucker, but the pharynx is thus termed. (2) A few genera, such as _Gasterostomum_, _Aspidogaster_, _Diplozoon_, etc., have only _one_ intestinal diverticulum, which is undoubtedly to be taken as representing the primitive condition, as it is also often found in the young stages of the _Trematoda_. (3) The branches of the intestines are curved and united behind (several _Tristomidæ_ and _Monostomidæ_), while in _Polystomum integerrimum_ (in the bladder of frogs) there are several commissures between the intestinal branches, and in the _Schistosomidæ_ the united intestinal branches proceed as one channel towards the posterior end. (4) The termination of the two intestinal branches is not always on a level; they are therefore of different lengths. (5) When the œsophagus is very long the intestinal branches extend both forward and backward, so that the gut exhibits the form of an *H*. (6) In the broad and flat species the gut-forks form diverticula mostly externally but also internally; these again may branch (fig. 139). (7) In a few cases (_Nematobothrium_, _Didymozoon_) the intestine completely disappears up to the pharynx.