TREMATODA 



2I 7 - 



few free-living larval stages (Cercariae) 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 oesophagus, 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 

 (praepharynx). Sooner or later the 

 intestine divides into two lateral 

 branches directed backwards, both of 

 which end blindly (caeca) at the same 

 level. 1 In many ectoparasites (Mouo- 

 genea [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 cesophagus are lined with 

 a continuation of the cuticle of the 

 body ; the gut caeca are lined with tall 

 cylindrical epithelium (fig. 120). The 

 cesophagus 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 ali- 

 mentary canal consist of groups of unicellular SALIVARY GLANDS that 

 discharge into the cesophagus in front of or behind the pharynx, or 

 even into the pharynx itself. 



1 The following conditions represent deviations from this type: (i) In Gasterostomum 

 the oral apetture 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 G aster ostomum> Aspidogaster Y 

 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 Tristomidce and Monostomid<z) , while in Polystonntm integerrimum (in the bladder 

 of frogs) there are several commissures between the intestinal branches, and in the Schisto- 

 somidce 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 oesophagus 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. 



FIG. 122. Median section through 

 the anterior part of Fasdola hcpatica : 

 the oral sucker, pharyngeal pouches, 

 pharynx, oesophagus, cuticle with 

 spines, and the body parenchyma. 



