STRUCTURE AND LIFE-HISTORY OF CRITHIDIA MELOPHAGIA. 193 



from the sheep. This blood from the sheep in the cesophagus, 

 crop, and anterior part of the stomach of Me lop hag us is 

 always fluid, and of an extremely bright red colour. That in 

 the remaining part of the stomach is duller red but fluid, and 

 in the intestine the blood, now semi-digested, is always 

 darker in hue, sometimes brownish or greenish, while in the 

 extreme rectum it is black. The enhanced red colour in the 

 anterior portions of the alimentary canal has been shown 

 experimentally to be associated apparently with the presence 

 of an anti-coagulin in the digestive tract of the sheep-ked 

 (see Appendix III). 



Crithidia can be found throughout the length of the 

 alimentary canal of Melophagus ovinus. In the anterior 

 parts of the canal they are small, rounded, non-flagellated 

 forms, which, when they come in contact with the blood, 

 rapidly develop and divide, the products of division becoming 

 the typical flagellates found throughout the rest of the canal. 

 The parasites, after this rapid development, pass backwards 

 towards the partly digested blood, which would appear to be 

 a medium more suited to their requirements. In the posterior 

 third of the stomach there are large numbers of young 

 flagellates which form great aggregation rosettes (PI. 12, 

 fig. 43) and clumps, while true division rosettes are also 

 present (PI. 12, fig. 56). 



In the intestine the- same holds good. When many 

 Crithidia are present in a ked, they usually swarm in the 

 fore-part of the intestine. Repeated division occurs in the 

 intestine, so that small flagellates are found in the rectum. 

 Most of these attach themselves to the gut-wall or to debris 

 and encyst, the resting (post-flagellate) stage of the parasite 

 then being found on the walls of the rectum and in the 

 faeces. 



The ovaries and ova serve as places in which a kind of 

 post-flagellate development occurs, the ova being penetrated 

 by flagellate forms of Crithidia, Avhich rapidly lose their 

 flagella and ultimately round themselves off, and pass through 

 a resting stage (PI. 13, figs. 57-94). 



