TREMATODES : BIOLOGY 229 



membrane, which may be striated or contain granules. The tail is 

 always cast off when encystment occurs, and organs peculiar to the 

 cercaria stage (boring papilla, eyes) almost entirely disappear. On 

 the other hand, the genitalia appear or become more or less highly 

 developed, in extreme cases to such an extent that they become 

 functional, and after autocopulation the creatures produce ova within 

 the cysts. 



The cycle of development of the digenetic Trematodes has hitherto 

 been generally explained as a typical ALTERNATION OF GENERATIONS, 

 one sexual generation regularly alternating with one or two asexually 

 reproducing generations. Recent authors, however, regard the cells 

 in the sporocysts from which rediae or eventually cercariae arise as 

 parthenogenetically developing ova, and the sporocysts as well as the 

 rediae as generations propagating parthenogenetically. In this case, 

 however, it is an alternation of a sexual not with an asexual but with 

 firstly a parthenogenetic generation (the sporocyst), the central cells 

 of which are regarded as ova which develop parthenogenetically into 

 the redia, and this the second parthenogenetic generation finally 

 produces larvae (cercariae) capable of developing into the sexually 

 mature form. 



Other authors, again, regard the development of the Digenea as 

 only a complicated metamorphosis (p. 283), which is distributed over 

 several generations before it is concluded. 



BIOLOGY. 



Endoparasitic Trematodes, as fully developed organisms, occur in 

 vertebrate animals only, with very few exceptions; they inhabit almost 

 all the organs (with the exception of the nervous and osseous systems 

 and the male genitalia), but by preference the intestine in all its 

 extent from the oral cavity to the anus; and, further/certain species 

 or groups inhabit only quite restricted parts of the intestine. Besides 

 in the intestine other species live in the liver, or in the bile-ducts, 

 or in the gall-bladder; other accessory organs of the intestine, such 

 as the pancreas, bursa Fabricii (of birds), are only infected by a few 

 species. Many inhabit the lungs, or the air sacs in fowls, a few 

 the trachea. Trematodes have also been known to occur in the 

 urinary bladder, the urethra and the kidneys of all classes of 

 vertebrates ; they are also present in the vascular system of a few 

 tortoises, birds and mammals ; in birds they even penetrate from 

 the cloaca into the oviducts, and are occasionally found enclosed in 

 the laid eggs ; one species is known to occur in the cavum 

 tympani and in the Eustachian tube of a mammal (Dugong), 

 another in the frontal sinus of the polecat ; several species infest the 



