134 



THE ANIMAL PARASITES OF MAN 



as well as from the commissures, either pass directly to the mus- 

 cular fibres or to the sensory cells that are situated at about the 

 same level as the sub-cuticular cells, or they reach these after the 

 formation of a plexus situated immediately beneath the fibres of 

 the cuticular muscles ; 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 of endo- 

 parasitic forms (Cercaria). In the 

 adult stage, however, they usually 

 undergo complete retrogression. 



The ALIMENTARY CANAL COm- 



mences with* an oral aperture situated 

 on the ventral surface of the anterior 

 extremity and usually connected with 

 an oral cavity surrounded by a sucker ; 

 the oesophagus of varied length is di- 

 rected backwards and is generally 

 provided with an elastic muscular 

 pharynx (fig. 69). Sooner or later 

 the intestine divides into two lateral 

 branches directed backwards, both of 

 which end blindly at the same level. 1 

 In many ectoparasites (Monogenea) 

 a connection is established between 

 the genital glands and one of the intes- 

 tinal branches [ductus vitello-intestinalis (fig. 66).] [This duct has 



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

 stomum the oral aperture is situated in the middle of the ventral surface, and occasion- 

 ally 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, &c., have only one intestinal pouch, which is undoubtedly 

 to be understood as representing the primitive condition, as it is also often found in 

 the young stages of the Trematodes. (3) The branches of the intestines are curved and 

 united, behind (several Tristomidea and Monostomidea), while in Polystomum integerri- 

 mum (in the bladder of frogs) there are several commissures between the intestinal 

 branches, and in Schistosominea 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 intestinal diverticules 

 mostly develop laterally, or may also run along the median line, and may again branch 

 off. (7) In a few cases (Nematobothrium, Didymozoon) the intestine completely dis- 

 appears up to the pharynx. 



FIG. 69. Median section through 

 the anterior part of Distomum 

 hepaticum ; the oral sucker, 

 pharynx with bursa and the oeso- 

 phagus have been cut through. 



