r82 FLEAS, FLUKES AND CUCKOOS 



wriggle up the stems of heather or some other suitable plant and wait. 

 A grouse, partaking of an early breakfast, inadvertently swallows the 

 larvae which on reaching the caeca undergo two further moults and 

 become adult worms. 



One of" the most interesting worms found in the caeca of chickens 

 and also in wild birds such as the coot is Strongyloides avium. In this whole 

 family there is a most peculiar life-cycle which may possibly throw a 

 little light on the evolution of parasitic nematodes. 



The eggs hatch after being voided with the faeces and young worms 

 develop in the soil into both adult male and female free-living 

 individuals. These worms copulate and lay eggs which in turn give rise 

 to larvae which feed, moult and develop into another generation of free- 

 living worms. This process may be repeated several times but sooner or 

 later a different type of larva is produced, which, if ingested by the right 

 host, develops into an outsize parasitic female which reaches maturity 

 inside the bird and lays parthenogenetic eggs. No parasitic male has 

 ever been found. 



The worms of this family seem to form a link between the free-living 

 and parasitic forms of nematodes. It is uncertain what causes the 

 production of the parasitic types of larvae, but experiments on allied 

 species suggest that abundance of food and certain other environmental 

 factors influence the course of development. Some strains of the same 

 worms seem more susceptible to a luxurious environment than others 

 and abandon the free-living life and produce parasitic forms more 

 readily. Caullery has suggested that all the special types of reproduction 

 so characteristic of parasites, such as parthenogenesis, polyembryony, 

 strobilisation, budding and so forth, occurred originally because of the 

 particular type of environment in which the eggs happened to develop 

 — conditions not necessarily linked with parasitism, but characters 

 which subsequently made adaptation to such a precarious life possible. 

 Many of the Strong)data are bright red in colour — due to their habit 

 of sucking blood from their hosts. They bite the intestinal wall or seize 

 it in their mouths, simultaneously pouring out a secretion which 

 prevents coagulation of the blood. They may also perhaps obtain a 

 supply of oxygen, which is lacking in the intestinal tract, by keeping up 

 a constant flow of blood through their bodies. Some species have the 

 power of digesting the tissues of the host without first swallowing them. 

 The secretions of their oesophageal glands are poured out and they then 

 imbibe the pulpy, semi-liquid mass produced in this manner. 



