136 AN INTRODUCTION TO ZOOLOGY 



visible to the naked eye. It is to be found inside the sperm sacs 

 attached to the rosette-shaped funnels of which there is a pair in 

 segments 10 and n, and which lead from the sacs to the sperm 

 ducts or vasa deferentia. M. agilis, on the other hand, is a far 

 smaller form, only reaching a length of 2 mm. when fully grown. 

 It floats freely in the fluid contents of the sperm sac, and can be 

 found by smearing these on a slide and examining the smear under a 

 microscope. The two species are very similar save in size, and pass 

 through practically similar life histories, so that one description will 

 apply almost equally well to either. 



The adult organism is of an elongated spindle shape, somewhat 

 flattened, and contains near the middle a fair-sized nucleus. Its 

 protoplasm is divided into a fairly clear firm ectoplasm, outside which 

 is a very thin but distinct cuticle marked with delicate longitudinal 

 striations and a more fluid internal endoplasm. In the deeper layers 

 of the ectoplasm are a number of myoneme fibrillse which branch and 

 anastomose, forming a fine but somewhat feeble contractile network 

 by means of which the movements of the body are brought about. 

 The endoplasm is opaque, owing to the presence in it of a large 

 number of granules of reserve food material composed of a Carbo- 

 hydrate substance allied to starch. The opacity partly hides the 

 nucleus which, however, cannot even be rendered conspicuous by 

 ordinary stains. The nucleus, which is spherical with a definite 

 nuclear membrane, contains a clear nuclear sap wherein float several 

 deeply staining nuclear corpuscles. These bodies are composed of a 

 basis of a substance plastin, impregnated with chromatin, and are 

 termed karyosomes in order to distinguish them from true nucleoli, 

 which contain plastin alone. The animal just described is in its 

 feeding or trophic stage, and is in consequence called a trophozoite. 

 Its movements are very restricted, it may bend slightly arid is only 

 able to move quite slowly. When it progresses a wave of contraction 

 passes from one end of the body to the other, followed by another 

 wave, and so on. This produces a very characteristic form of motion 

 termed euglenoid movement, since it is exhibited in a typical manner 

 by the protozoon Euglena, one of the MASTIGOPHORA. As has 

 already been pointed out, feeding takes place by osmosis, proteid 

 material, built up by its host, the earthworm, being absorbed from 

 its surroundings. In the same way respiration and nitrogenous 

 excretion occur all over the surface, and no special organs are 

 developed. Reproduction is more complicated than in Amceba or 

 Paramcecium, and forms parts of a noteworthy cyclical series of 

 changes. 



