28 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



CHAPTER III PLATYHELMIA (Flat-worms). 

 5. DISTOMA (The Liver-fluke). 



THE best-known kind of fluke is Distoma hepaticum, the Liver- 

 fluke. This is an internal parasite (endoparasite) when adult 

 about an inch and a half long, which lives in the bile-ducts of 

 several animals, more especially the sheep, and causes the disease 

 known as liver-rot. 



MORPHOLOGY AND PHYSIOLOGY. 



1. External Characters. The animal (Fig. 6) is lilaterally sym- 

 metrical, right and left halves, anterior and posterior ends, and 

 upper (dorsal) and lower (ventral) surfaces being distinguishable. 

 The animal can be divided into corresponding halves by one 

 plane only, the median vertical. 



The body is flattened and somewhat leaf-shaped, its broad 

 dorsal and ventral surfaces passing into sharp edges at the sides 

 and behind. The gently curved outline is interrupted at the 

 anterior end by the head-papilla, a projection which tapers to a 

 blunt point, and ends in an adhesive cup or sucker which looks 

 somewhat ventralwards. Behind the head-papilla the sides of 

 the body curve outwards and backwards till the maximum 

 breadth is attained at about the junction of the anterior and 

 middle thirds, while the hinder two-thirds narrow gradually to 

 the posterior end of the body. There is a second sucker (v.s) on 

 the ventral surface, a little way behind the head-papilla. 



Four median apertures are present on the surface of the body. 

 These are: (1) The mouth (m), in the centre of the anterior 

 sucker; the minute excretory pore (ex.p) at the posterior end; (3) 

 the genital aperture (g.o) on the ventral surface between the 

 suckers; and (4) the minute opening of the vagina (Laurer's 

 canal) on the dorsal surface. 



With the exception of the smooth suckers the body surface is 

 rough, owing to the presence of an immense number of minute 

 backwardly directed spines, which project from the cuticle, a firm 

 membrane which invests the body and is secreted by an under- 

 lying epidermis, that consists of a single layer of large granular 

 cells. The hard cuticle protects and supports the body, and its 



