162 COMPARATIVE ANATOMY 



they can easily be found in almost every individual of the 

 common earthworm. The former is a sporozoon of large 

 size, attaining to a length of 5 mm. and therefore easily visible 

 to the naked eye. The mature individuals are found adhering 

 by one end of their elongated bodies to the rosette-shaped 

 sperm-funnels of the sperm-ducts of the earthworm. The 

 mature form of M. agilis is only about "2 mm. in length 

 and is found in the sperm-sacs of the earthworm,, where, as its 

 name implies, it moves actively among the developing sper- 

 matozoa of its host. In either case the parasite is a single 

 elongated cell (fig. 34, D\ consisting of a granular, opaque, 

 central medullary mass or endoplasm, and an external, hyaline 

 denser layer of ectoplasm. External to the ectoplasm is a very 

 thin but distinct membrane or cuticle, which in the two species 

 under consideration is covered with very fine longitudinal 

 striations. Careful examination with the microscope shows 

 that the ectoplasm is more highly differentiated than is the 

 case in Amoeba. Its deeper layer is modified to form a 

 number of very fine contractile fibres generally known as the 

 myocyte-fibrillae, which run transversely round the body, and 

 are therefore at right angles to the longitudinal striations of 

 the cuticle. These myocyte-fibrillae give off branches which 

 anastomose with one another, so that the whole forms a net- 

 work whose meshes are elongated in a direction transverse to 

 the long axis of the body. The presence of a cuticle and the 

 differentiation of a special contractile layer of the ectoplasm 

 together confer a more or less definite shape upon the body. 

 The shape, however, is not constant, for if a lively specimen 

 of Monocystis is kept under observation it will be seen to 

 exhibit regular but sluggish movements, due to the contractility 

 of the myocyte-fibrillae. A constriction appears at one end of 

 the body and travels slowly to the other end, and as it passes 

 the animal is swollen now at one end, now in the middle and 

 now at the other end, these changes of form being accom- 

 panied by more or less pronounced bending movements from 

 side to side. The endoplasm is not contractile and during 

 these changes of form undergoes passive movements such as 

 would be exhibited by the fluid contents of an elastic bag 

 squeezed by passing the hand from one end to the other. 

 As the parasite approaches maturity the endoplasm becomes 

 filled with a number of coarse ovoid granules which give a 



