566 



TREMATODA 



There are two suckers — an oral, which is situated at the anterior end of the 

 animal and surrounds the mouth opening, and a ventral, the acetabulum, 

 which is a muscular cap situated in the median line near the junction of the 

 anterior projection with the rest of the body. The mouth leads into a mus- 

 cular pharynx, which passes via a short oesophagus into the intestine, which 

 divides just in front of the genital aperture into two tubes running to the 

 posterior end of the body, giving off numerous branched caeca. 



The excretory system starts with dilated tubules, into which project the 

 cilia of the so-called flame cells. The ducts, v/hich freely anastomose, open 

 into a main duct, which runs directly backwards to open at a median pore 

 at the posterior end of the body. 



The two testes are much-branched tubes, lying in the middle of the body, 

 of equal size, one lying in front of the other; from each a vas deferens runs 

 as far as the ventral sucker, where it opens into an elongated sac, the vesicula 

 seminalis, from which the ductus ejaculatorius runs to the end of a large 

 muscular organ, called the cirrus, which lies in a sac situate just in front of 

 the ventral sucker. The genital opening through which the cirrus can be 

 protruded is situated between the two suckers on the ventral surface. 



Fig. 217. — The Redia of Fig. 218. — The Cercaria of 



Fasciola hepatica. Fasciola hepatica. 



(After Thomas, from the Quarterly Journal of Microscopical Science.) 



The branched and tubular ovary lies on the right, in front of the anterior 

 testes. The ovarian ducts run backwards to join the middle line with the 

 median vitellarian duct from the yolk reservoir, into which open the vitel- 

 larian ducts from the large branched yolk glands lying on either side of the 

 body, from the level of the ventral sucker to the posterior extremity. 



The junction of these two ducts is surrounded by the shell gland, and forms 

 a much-convoluted tube, the uterus, which is joined close to its commence- 

 ment by a little tube, Laurer's canal, opening on to the dorsal surface. The 

 uterus, which is generally full of eggs, opens on the left side of the base of 

 the cirrus. 



The nervous system consists of a nerve collar around the pharynx, from 

 which two nerve cords run backwards along the sides of the body. 



Life-History. — The ovum starts from the ovary and travels along the 

 ovarian duct, where it is fertilized, after which it is surrounded by a large 

 number of yolk cells, and then by a shell formed from the secretion of the 



