4o6 THE BIOLOGY OF BIRDS 



not insects at all but belong to the Acarine order of 

 Arachnids. 



Apart from microscopic Protozoa, the endoparasites 

 of birds are chiefly " worms." There are (i) flukes or 

 Trematodes, e.g. Distomum macrostomtim in various perching 

 birds ; (2) tapeworms or Cestodes, e.g. Tcenia anatina in the 

 duck ; and (3) threadworms or Nematodes, e.g. Tricho- 

 strongylus pergracilis of the grouse. The three examples 

 mentioned all live in the food-canal, but other organs of 

 the bird's body may be aflFected. 



Some species of parasites occur in several diff"erent 

 kinds of birds, but in many cases a particular parasite is 

 peculiar to a particular kind of bird. The discovery of 

 a new bird usually means the discovery of new parasites. 

 This may illustrate the establishment of new species in 

 isolated habitats, for each kind of bird is in a way like an 

 island. Its parasites are linked to it and adapted to it, 

 and cannot readily pass to a different kind of bird. But the 

 question rises insistently whether many of these so-called 

 species are more than different modifications of one species. 

 Crucial experiments are much needed. 



Linked by a Common Parasite. — Many parasitic worms 

 require two hosts, one for their juvenile and asexual stages, 

 another for the full-grown and sexually reproductive forms. 

 Thus the young of the liver-fluke of the sheep lives in the 

 little freshwater snail, and the bladderworms of the pig 

 and the ox become tapeworms in man. Similarly, there 

 are certain flukes and tapeworms which occur as adults in 

 birds, while the young stages live in fishes which form a 

 normal part of the birds' diet. It seems that some of the 

 pearls found in bivalves are formed round the larvae of 

 parasitic worms whose adult stages occur in mollusc-eating 

 birds. 



§ 5. Strange Linkages 



As an example of the quaint linkages that sometimes 

 occur, we may take the partnership between the Crocodile 



