78 THE TREMATODA 



Heterocotylea and Malacocotylea, exhibit two stages in this 

 parasitism. The former order are nearly exclusively ectoparasitic 

 on marine and fresh-water fish, attaching themselves to the outer 

 surface of the body, or to the wall of the branchial chamber, or 

 to the gills by means of the adhesive apparatus at the posterior 

 end of the body. Less frequently the worms make their way into 

 the canals or tubes formed by the invagination of the epiblast 

 that is, into the nasal sacs, the oral cavity, and even into the 

 cloaca. The genus Polystomum is, as an exception, endoparasitic, 

 living in the urinary bladder of fish, amphibia, or reptiles. 



On the other hand, the members of the Malacocotylea, as well 

 as the Aspidocotylea, are essentially endoparasitic, and must be 

 regarded as having passed through an ectoparasitic stage; and, 

 indeed, a few species of Distomum still retain this habit. These 

 digenetic Trematodes occur in the enteron and its outgrowths of 

 all groups of vertebrates ; the majority live, in the adult con- 

 dition, in warm-blooded members of the group, many in reptiles 

 and amphibia, and but few (Distomum sp.) in fishes. Every system 

 of organ, with the exception of the nervous system and skeleton, 

 is invaded by them, either in a free or encysted condition ; and 

 even the blood-vessels are affected by Bilharzia. 



Although, as in other parasitic animals, it is a general rule 

 that each species attacks only one definite host, as the animal on 

 which the parasite lives is termed, or in hosts nearly related to 

 one another, yet there are instances, such as Distomum liepaticum 

 and D. lanceolatum, of the same species occurring in many widely 

 different hosts, such as man, rabbit, various ungulates, and even 

 the kangaroo. As in other groups, these parasites are only 

 injurious to the host, when they occur in large numbers, or in 

 certain delicate organs. 1 It is well known that D. hepaticum, the 

 liver-fluke, produces " sheep-rot," especially in districts liable to 

 flooding, where the life-history can be readily completed. 



The food of Trematodes consists in some cases of slime secreted 

 by the host; and this secretion is, no doubt, increased by the 

 irritation caused by the insertion of booklets into the host's skin. 

 But more usually nutriment is derived from the blood of the 

 host, which is sucked up by the parasite by means of the power- 

 ful pharynx, the intestine is consequently, in the fresh worm, 

 yellowish or red in colour ; and remains of blood corpuscles, 

 lymph cells, and epithelial cells have been noted in its contents. 



In Polystomum peculiar crystals, reddish in colour, and octahedral 

 in form, suggest a derivative of haemoglobin (Zeller). 



Some authorities (Taschenberg) describe intracellular digestion, 

 yet there is no doubt but that cavitary digestion also occurs. 



1 It lias, however, recently been shown to be extremely probable that Nematodes, 

 Cestodes, and Trematodes excrete an active poison. 



