8 COMPARATIVE ANATOMY 



parenchyma, the excretory system, and generative organs 

 belong to the mesoderm. 



It is evident from the above description that the structure 

 of Distomum shows a considerable advance on that of a 

 ccelenterate polype, and that the greater complexity of structure 

 is associated with the existence of the middle cellular layer or 

 mesoderm which is not present in the Ccelenterata. 



The adult liver-fluke, as has been said., inhabits the bile 

 ducts of the sheep, and is therefore called an endoparasite. 

 Not all the Trematoda are endoparasites. A considerable 

 number of them cling to the external surfaces of animals, and 

 are therefore called ectoparasites. The eggs of ectoparasitic 

 Trematoda are generally laid on the place inhabited by the 

 parent and develop directly into the adult form, without the 

 embryo undergoing a metamorphosis or becoming parasitic 

 in or upon another host.* Such forms are known as mono- 

 genetic. But in the endoparasitic Trematodes the ovum gives 

 rise to a larval form which enters a new host, undergoes a 

 metamorphosis, produces one or more generations different 

 from the parent form, arid eventually these generations escape 

 from the second host (usually known as the intermediate host), 

 and under favourable circumstances find their way back to a 

 host of the same species that was inhabited by their parent. 

 Such forms are described as digenetic. 



Distomum hepaticum is a very good example of a digenetic 

 Trematode. The eggs, as has been described, acquire chiti- 

 nous shells secreted by the shell-gland, and pass into the 

 uterus, where they undergo the first stages of segmentation. 

 An individual egg is a minute oval body measuring about '13 

 mm. in length by '08 mm. in diameter. One end is rather 

 more rounded than the other, and a serrated line running 

 round it marks off a small lid or operculum, by means of which 

 the embryo eventually escapes. From the. uterus the eggs are 

 discharged into the bile ducts of the infected sheep, where they 

 may be found in large numbers, sometimes forming a stiff 

 brown mass, completely blocking up the smaller ducts. A 

 single fluke produces a very large number of eggs ; it has been 

 computed that each individual may produce half-a-million, 



* The animal upon or in which a parasite lives is usually described as 

 its "host." 



