98 PEACTICAL PABASITOLOGY 



be peeled out, together with the cystic membrane, which is derived 

 from the tissues of the host ; or the membrane may be carefully cut 

 through and the parasite taken out. Mature Nematodes and Cestodes 

 are found in the liver of very few host-species. 



The other internal organs are examined in a similar manner. 

 Hollow organs are slit up and solid organs carefully cut into slices. 

 Ample material is forthcoming from indigenous vertebrates, and it 

 may be supplemented by material obtained from slaughter-houses. 

 Fresh carcases of extra-European animals, as also of the rarer 

 European land vertebrates, can sometimes be obtained from mena- 

 geries and zoological collections, while foreign fish may be got from 

 live-stock dealers and hatcheries. It should be borne in mind, how- 

 ever, that animals which have been long in captivity, or have died 

 after lingering illness, do not as a rule yield very abundant material. 

 As a consequence of changes in diet, or of illness, Helminthes are fre- 

 quently absent from the intestine, though they are sometimes found 

 in the other organs, generally in the encysted stages. The chances 

 of finding parasites in the intestine are greater in the case of animals 

 which have died soon after importation. The attendants, moreover, 

 are aware that Helminthes are passed in the dung of freshly imported 

 animals, and use should be made of this knowledge. 



Even in fishing towns, fish are usually emptied of their viscera 

 before coming upon the market, and for this reason the encysted 

 forms of Helminthes are the ones most frequently found in them. 

 Occasionally, they are found free upon the skin and gills. Interesting 

 material is sometimes found in the gut of certain species ; such are 

 the shark and the skate. 



Invertebrates, both of the land and of fresh and salt water, may 

 also harbour Helminthes, though in certain developmental stages 

 only. These occur free or encysted in the substance of the internal 

 organs and, with the exception of the gut, they are rarely found in 

 the hollow organs. In- addition to the Protozoa, the parasites most 

 frequently present in the gut are a small species of Nematode. 



CHAPTER II. 

 METHODS OF PRESERVING HELMINTHES. 



Helminthes should be examined in the first instance in the fresh 

 state. As a general rule, they are sufficiently transparent for the 

 purpose, and this transparency may be increased by gently pressing 

 them between the glass slide and the cover-glass (or two glass slides 

 may be used). It is very necessary, however, to preserve them, both 

 for museum purposes and for section cutting. The method varies 



