54 ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



because it is more easily obtained for laboratory study than any other 

 Sporozoan, and Plasmodium, chosen on account of its great practical 

 interest as being the cause of one of the most destructive of all diseases 

 Malaria. 



MONOCYSTIS 



Monocystis is a very common parasite of the ordinary earthworm 

 Lumbricus. If the body of a freshly killed earthworm be slit open and 

 the body wall pinned out flat so as to display the internal organs there 

 will be seen towards the head end a clump of irregular yellowish- 

 white organs known as the seminal vesicles (Fig. 67, p. 138). If a piece 

 of one of these be pulled off with a pair of forceps and dabbed up and down 

 in a drop of normal saline solution 1 the latter will be made milky by the 

 whitish contents of the seminal vesicle. Examination with the micro- 

 scope shows these to consist of various stages of the developing micro- 

 gametes or spermatozoa of the worm. Amongst these a very conspicuous 

 stage is that known as the sperm-morula, from its resemblance to a 

 microscopic raspberry or mulberry (tnorula), spherical in shape and 

 having its surface covered by a layer of little rounded bodies destined to 

 lengthen out and become microgametes. 



The young Monocystis is to be found as a small spherical or ellipsoidal 

 cell embedded within the central protoplasm of the sperm-morula 

 (Fig. 21, A). Within this it grows rapidly, absorbing nourishment from 

 the surrounding protoplasm which becomes stretched out by the growing 

 body of the parasite (Fig. 21, B) until eventually it forms merely a thin 

 film (Fig. 21, C). In the meantime the microgametes of the worm have 

 been going on with their development, each little rounded body becoming 

 first pointed at its outer end and finally drawn out into a fine thread. 

 When this stage has been reached the Monocystis is enclosed in a thick 

 furry coat, each hair of which represents a spermatozoon of the worm. 

 Eventually the Monocystis becomes freed from its furry coat and presents 

 the appearance shown in Fig. 21, D. It contains a single round nucleus. 

 Its protoplasm is bounded by contractile ectoplasm the surface layer of 

 which is stiffened to form a distinct pellicle. The endoplasm is laden 

 with stored-up food material in the form of highly refracting granules 

 of paraglycogen, a substance allied to glycogen or animal starch, which 

 give the Monocystis a snowy white appearance when seen against a 

 black background by reflected light. The portion of the life-history so 

 far described is above all characterized by the active absorption of food, 

 which finds its expression first in growth and later in the storing up 

 1 '75% Common Salt (NaCl) in water. 



