I PHYLUM PROTOZOA 55 



the single nucleus extends as a narrow branching cord 

 throughout the axis of the entire colony. 



Transverse fission is the universal method of reproduc- 

 tion ; and budding also occurs. Spore-formation has been 

 observed in a few forms. 



Conjugation, in the form of a temporary union of two 

 individuals, with interchange of the substance of the micro- 

 nuclei, occurs in many of the ciliate Infusoria. In some 

 forms the conjugating individuals become completely fused. 

 The effect of the process of conjugation seems to be in- 

 creased activity in multiplication by fission. 



In mode of life the Infusoria are as varied as the Mastigo- 

 phora. Some are holozoic, some saprophytic, some parasitic. 

 Of the parasitic forms some give rise to definite diseases 

 in the bodies of their hosts. The skin affection known as 

 eczema, for example, seems to be caused by the presence 

 of parasitic Vorticellae. 



4. THE SP0R0Z0A 



In the interior of certain organs, termed the seminal 

 vesicles, of the earthworm will often be found a parasitic 

 Protozoan — Monocystis agilis (Fig. 23) — which exempli- 

 fies another of the classes of the phylum, the class Sporozoa. 

 It is flattened, elongated, pointed at both ends, and performs 

 slow movements of expansion and contraction (A, B), 

 reminding us of those of Euglena. There are neither 

 pseudopodia, nor flagella, nor cilia. There is a firm 

 cuticle, and the protoplasm is divided into a denser 

 superficial portion, the cortex, and a central semi-fluid mass, 

 the medulla. There is a large clear nucleus, but no trace of 

 contractile vacuole, or of mouth or gullet. Reproduction 

 takes place by a peculiar and characteristic process of spore- 

 formation. Either a single individual, or two individuals 



