12 MONOCYSTIS 



laden with reserve food substance ; and within it there is a rounded, clear- 

 looking fluid nucleus. This is the vegetative or trophic stage of the parasite, 

 the trophozoite. Notice the characteristic " euglenoid " movement of Mono- 

 cystis, which causes some variation in its long oval shape. 



Structure and Life-history Stages of Monocystis agilis. 



Monocystis magna may also be found free in the cavity of the seminal 

 vesicles. In that situation, in well-infested worms, most of the hfe-history 

 stages of a small species, Monocystis agilis, are generally present. 



With a pipette remove some of the fluid contents of a seminal vesicle and 

 put a drop of it, diluted with salt solution, on a clean slide. Cover with a cover- 

 slip, and examine the slide under the microscope. The fluid chiefly consists of 

 countless hair-like spermatozoa and numerous sperm-morulai — i.e. mulberry- 

 like clusters of developing spermatozoa — and these form the normal contents of 

 the seminal vesicles. Amidst the sperms and morulae the following Monocystis 

 stages may be seen : — 



(a) Free adult trophozoites (Fig. 3, A), in form and structure similar to those 



of M. magna, as noted above. 



(b) Trophozoites which, having attained the limit of growth and being ripe 



for reproduction, have rounded themselves off and become gameto- 

 cytes. These may be seen in closely associated pairs, or at the more 

 advanced stage of 



(c) Paired gametocytes within a cyst {Fig. 3, B) which they have secreted. 

 {d) Cyst containing two gametocytes about to form gametes. Note the 



numerous small nuclei which have been formed and moved towards 

 the surface of the body {Fig. 3, C). 



{e) Cyst showing the gametocjrtes broken up into small portions, each con- 

 taining a nucleus ; these are the gametes {Fig. 3, D), some of which 

 are seen in conjugation, and some pairs are already fused and therefore 

 zygotes. Notice that there is in the cyst a certain amount of residual 

 protoplasm, which serves for the nourishment of the gametes, 



(/) Clear bluish cysts containing munerous small boat-shaped spores or 

 pseudonavicellse {Fig. 3, E). The zygotes become ovoid and secrete 

 tough chitinous coats, and by division each segments into a cluster 

 of eight sausage-shaped sporozoites surrounding a smaU amount of 

 residual protoplasm. In that way the zygote is transformed into a 

 spore {Fig. 3, E and F). These spores must be transmitted to another 

 host before they /can develop further. Probably the earthworm is 

 eaten by a bird, and the spores passed out undigested along with the 

 excreta. The spore may then be swallowed by another worm ; and 

 < the spore-coat being dissolved by the digestive juices, the sporozoites 

 will therefore be set free. From the food-canal they find their way 

 into the seminal vesicles, and they enter the sperm-morulae. 



{g) A sporozoite inside a sperm-morula {Fig. 3, G). It derives nourish- 

 ment from the surrounding protoplasm and develops into a young 

 trophozoite. 



{h) Young trophozoites coated with sperms {Fig, 3, H). 



