Note 257 — ON PARASITES IN GENERAL. (continued 6)

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.