The oral cavity, pharyngeal pouches, pharynx, and œsophagus are lined with a continuation of the cuticle of the body; the gut cæca are lined with tall cylindrical epithelium (fig. 120). The œsophagus and intestinal branches often have also one layer of annular and longitudinal muscles; the pharynx has essentially the structure of a sucker (fig. 122). [Illustration: FIG. 122.--Median section through the anterior part of _Fasciola hepatica_: the oral sucker, pharyngeal pouches, pharynx, œsophagus, cuticle with spines, and the body parenchyma.] The accessory organs of the alimentary canal consist of groups of unicellular SALIVARY GLANDS that discharge into the œsophagus in front of or behind the pharynx, or even into the pharynx itself. The food of the Trematodes consists of mucus, epithelial cells, the intestinal contents of the hosts, and often also of blood, and this not only in those species living in the vascular system, but also in species living as ectoparasites or in the intestine or biliary passages of their hosts. [Illustration: FIG. 123.--_Polystomum integerrimum_, a monogenetic fluke from the urinary bladder of the frog. _i._, intestine; _h._, large hooks of the sucking disc; _h.k._, smaller hooklets; _l.c.v._, longitudinal vitelline ducts; _o._, oral orifice; _Oot._, oötype; _ov._, ovary; _s.p._, suckers of the disc; _tr.c.v._, transverse vitelline ducts; _Ut._, uterus with ova; _v._, entrance to the vagina; _v.d.e._, vas deferens; _v.d.i._, ductus vitello-intestinalis; the vitellaria and testes are not shown. Magnified. (After Zeller.)] [Illustration: FIG. 124.--_Allocreadium isoporum_, Looss. Excretory apparatus. Of the other organs, the oral sucker, pharynx, genital pore, ventral sucker, ovary and testes are shown; the cylindrical excretory bladder is in the posterior end. 38/1. (After Looss.)] The final products of assimilation dissolved in the fluids of the body are distributed throughout the parenchyma and are thence expelled by a definite tubular system (excretory apparatus, proto-nephridia, formerly also termed the water-vascular system). This system, which is distributed throughout the entire body (fig. 124), is symmetrically developed, and, in the ectoparasitic Trematodes, it opens, right and left, at the anterior end on the dorsal surface; in all other flukes, however, it opens singly into the excretory pore (foramen caudale) at the centre of the posterior border; in those cases, however, where a sucker is present at the posterior end, as in the Amphistomata, the excretory pore is situated on the dorsal surface close in front of the sucker.