CRITHIDIA GEEIRDIS 357 



the flagellum, passes along an undulating membrane to the drawn-out 

 tapering anterior end of the body. These long forms become shorter 

 and finally converted into round leishmania forms, which appear to encyst 

 and escape in the faeces of the invertebrate. As in the case of leptomonas, 

 the cysts lead to infection of a new host. 



Crithidia gerridis Patton, 1908. — This flagellate is an intestinal parasite 

 of the water bug, Gerris fossarum, where it was first seen by Patton. It 

 is also found in a species of Microvelia, and another water bug related 

 to Perittopus (Fig. 166). It was chiefly studied by Patton in Microvelia. 

 The alimentary canal of the Microvelia consists of a narrow oesophagus 

 opening into a sacculated crop. The latter opens into the short, dilated 

 mid-gut, which nearly always contains a greenish-yellow fluid. The mid- 

 gut is followed by the small intestine, at the anterior end of which open 

 four long, narrow Malpighian tubes. The small intestine is followed by 

 the dilated colon continuous with the short, straight rectum. The eggs 

 hatch into nymphs, which by five moults attain the adult condition. In 

 the crop of the nymphs are found the encysted forms which have been 

 ingested with water (Fig. 166, i). Very shortly after their ingestion, 

 these round forms produce flagella and begin to multiply. The smallest 

 round forms are 4 to 6 microns in length by 3 to 4 microns in breadth. 

 At first, these round forms possess only nucleus and kinetoplast. Very 

 soon, from the latter the axoneme is formed, but when it reaches the 

 surface of the body, instead of immediately entering the flagellum, it 

 passes along the edge of a narrow undulating membrane, the rudiment 

 of the structure which is seen fully developed in the adult crithidia forms 

 (Fig. 166, 2-5). These forms, having increased in size, now measure 

 6 to 10 microns by 4 to 8 microns. Multiplication takes place at this stage 

 by binary fission. Patton states that the flagellum actually divides 

 longitudinally, but this is certainly incorrect. By active division rosettes 

 of rounded flagellates are produced, with the flagella directed outwards 

 (Fig. 166, 8). These rosettes are attached in masses to the lining epi- 

 thelium. They gradually break up, and the individual flagellates swim 

 away. The pole opposite that to which the flagellum is attached elon- 

 gates, while the flagellated pole becomes drawn out with the flagellum. 

 In this manner the typical crithidia forms arise. They vary from 15 to 

 45 microns in length and 2 to 4 microns in breadth (Fig. 166, 10-12). 

 The anterior end of the body is drawn out to a fine point where the axoneme 

 enters the flagellum. There is an undulating membrane on the part of 

 the body anterior to the nucleus, and the axoneme passes along its margin. 

 The posterior end of the body is rounded. The nucleus is spherical and 

 situated at the centre of the parasite, while the kinetoplast is 1 to 1-5 

 microns in front of it. These long forms are often agglomerated together 



