CHAPTER VII 

 MONOCYSTIS AGILIS AND MONOCYSTIS MAGNA 



WHILST most of the Protozoa are free-living, and obtain solid 

 food by various devices, the Gregarines or Sporozoa, to 

 which order Monocystis magna and Monocystis agilis belong, 

 are endoparasites, spending their existence within the body 

 of another animal known as the host, and deriving their 

 nutriment from its tissues or secretions. Both Monocystis 

 magna and agilis are found in the sperm-sacs of the 

 earthworm, and may nearly always be found if the contents 

 of the sperm-sacs are teased up, with the addition of some 

 salt solution, on a glass slide. Monocystis agilis is the 

 smaller but more common species, and the following descrip- 

 tion will apply chiefly to it : 



The free mature individual is a single elongate ovoid cell, 

 consisting of a granular, opaque central or medullary protoplasm, 

 containing a single spherical nucleus and an external hyaline 

 denser layer, usually called the cortex. This cortex, which 

 must not be confounded with the so-called ectoplasm of 

 Amceba or Badhamia, is a specially differentiated contractile 

 external layer of the protoplasm, and its consistence, admit- 

 ting of no pseudopodial movement, confers a more or less 

 definite shape on the body ; a shape, however, which alters 

 somewhat as a result of the contractility of the cortex. This 

 contractility is manifested as a constriction appearing at one 

 end of the body, and travelling slowly to the other, giving 

 rise to frequent changes of form. These sluggish move- 

 ments are termed euglenoid, because they are highly char- 

 acteristic of the green animalcule Euglena, which will be 

 described in the next chapter. The Gregarines are probably 

 degenerate in consequence of their parasitic habit, and 

 accordingly we find that Monocystis has no pseudopodia, 

 no flagella or cilia, and no contractile vacuole. After moving 



