204 MANUAL OF ELEMENTARY ZOOLOGY 



The liver fluke is hermaphrodite, and has very complex 



terati 

 Organs. 



generative organs 

 Generative ° ° 



The testes are two much-branched tubes lying one 

 behind the other in the middle part of the body. The branches of each 

 are gathered into a vas deferens, and the two vasa deferentia run forwards 

 side by side tojoin, above the posterior sucker, a large, pear-shaped vesicula 

 seminalis. From this a fine, somewhat twisted tube, the ductus ejacula- 

 torius, passes forwards to enter a stout, muscular penis or cirrus, which 

 opens at the generative pore. Normally the penis lies in a cirrus sac, 

 but it can be turned inside out and thus thrust out of the pore. The ovary 

 is a branched tubular structure on the right side in front of the testes. 

 Its branches join to form the oviduct, which passes towards the middle 

 line and there joins the yolk duct. This is formed by the union of two 

 transverse ducts, which lead each from a longitudinal duct at the side 

 of the body. The yolk glands are very numerous, small, round vesicles 

 lying along the sides of the body and communicating by short ducts 

 with the longitudinal ducts. The Laurer-Stieda canal is a short tube of 

 uncertain function leading from the union of oviduct and yolk duct to a 

 pore on the back. Possibly it is used for the reception of spermatozoa 

 from another individual. The oviduct and yolk duct are surrounded 

 where they join by a rounded mass, the shell gland, composed of 

 numerous, minute, unicellular glands. From this point the joined ducts 

 proceed forwards as a wide, twisted tube, the uterus, to the generative 

 opening. The uterus contains eggs, each enclosed in a shell, within 

 which lie, besides the ovum, a number of yolk cells derived from the yolk 

 glands, and spermatozoa. The animal is probably as a rule self- fertilised. 



The life-history of the liver fluke is a very remark- 

 able and interesting process. The eggs, which 



* are very numerous, are laid into the bile ducts 



of the sheep. So long as they remain within the body 

 of the latter they do not develop, but when they have 

 been carried by the bile to the intestine, and thence 

 passed to the exterior with the droppings, they will develop 

 in damp spots if the weather be warm. In a few weeks 

 a larva known as the miracidium emerges. It is conical, 

 covered with a layer formed by five rings of big, ciliated 

 cells, and provided with two eye-spots and two flame cells. 

 Inside it is filled by a mass of cells. It swims by means of 

 the cilia, with the broad end forwards. At this end there 

 is a knob which can be thrust out as a conical spike. If it 

 can find a member of a particular species of water snail 

 known as Limnceus truncatulus 1 it works its way into the 



1 Other species of water snail are sometimes used in foreign 

 countries. 



