130 



MANUAL OF ZOOLOGY 



FIG. 63. Distomum hepaticum, 

 natural size. excr. excretory 

 pore ; mo. mouth ; repr. repro- 

 ductive aperture ; sckr. pos- 

 terior sucker. 



which is an imaginary line running through the middle of 

 the mouth and enteric cavity. In the Fluke, on the other 

 hand, the parts are disposed to the right and left of an 



imaginary median vertical plane, 

 along which the entire animal 

 is capable of being divided into 

 two completely symmetrical, 

 right and left, halves. The type 

 of symmetry here exemplified is 

 termed bilateral ; it has already 

 been met with in some of the 

 Protozoa, and is characteristic 

 of nearly all animals higher than 

 the Ccelenterata. 



The broader end of the body 

 is determined as anterior, 

 owing to the mouth and the central part of the nervous 

 system being situated at that extremity. One of the broad 

 flat surfaces is the dorsal, the other the ventral. The 

 mouth (mo.), situated at the anterior extremity of the head- 

 lobe, is surrounded by a muscular oral sucker, and some 

 distance back, on the ventral surface, just behind the head- 

 lobe, is a second much larger posterior sucker (sckr.). 

 Between the two suckers is a median aperture, the genital 

 opening (repr.), through which a curved muscular process, 

 the cirrus or penis, may be protruded. In the middle of 

 the posterior end of the body is a minute opening, the 

 excretory pore (excr.). 



The surface is covered with innumerable minute spinules, 

 but vibratile cilia are absent. 



The mouth (Fig. 63, mo.) leads to a small bulb-like body, 

 \hepharynx (Fig. 64, ph.), with thick muscular walls and a 

 small cavity. From this a short passage, the oesophagus, 



