226 



ZOOLOGY 



SECT. 



par 



ect 



test . 



circ.mu<s 



od. rdef 



Fin. 174. Transverse section of a Planarian. c//v:. mu*. circular muscular fibres ; cxc. in 

 testinal coeca ; dors. vent. mus. dorso-ventral mascular fibres; ect. ectoderm; cxt. long, fiiux. 

 external layer of longitudinal muscular fibres ; int. central lumen of the intestine ; int. lonrt. 

 mus. internal layer of longitudinal muscular fibres; ne. nerve-cords; o. <L oviducts; JMIT. 

 parenchyma ; test, testes ; r. dtf. vas deferens ; rit. vitelline glands. (From Hatschek's 

 Lehrbuclt.) 



ii. The Liver-Fluke (Distomum hepaticum). - 



General Features. The Liver-Fluke of the Sheep, which is to 

 be found in the interior of the larger bile-ducts of the infested 

 animal, is a soft-bodied worm of flattened leaf-like shape (Fig. 

 175), with a triangular process, the head-lobe, projecting from the 

 broader end. The symmetry of the parts is distinctly bilateral as 

 in the Planarian. Externally the body is quite equilateral, the 

 right and left portions exactly balancing one another, but, as will 

 appear subsequently, this complete symmetry 

 does not extend to all the internal organs. 



The surface is devoid of vibratile cilia, but 

 is covered with innumerable minute spinules 

 or papillce, which are prolongations of the 

 homogeneous .external layer or cuticle in- 

 vesting the whole animal. At the extreme 

 anterior end of the triangular head-lobe is 

 the small opening of the mouth (mo.) sur- 

 rounded by a muscular oral sucker. Some 

 distance back on the ventral surface, just 

 behind the head-lobe, is a second much larger 

 posterior sucker (sckr.). Between the two 

 suckers, but rather nearer the posterior one, 

 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.). 



Body- wall. The body-wall (Fig. 176) is found on section to 

 comprise three layers: (1) A homogeneous cuticle (cut.) of which 

 the spinules (sp.) are special developments; (2) a layer of circularly 



FIG. 175. Distomum 

 hepaticum, natural 

 size. excr. excretory 



pore ; mo. mouth ; repr. 

 reproductive aperture ; 



.sv/,v. posterior sucker. 



