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 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]).
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).
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.
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.)