82 



ZOOLOGY 



SECT. 



Reproduction takes place by binary fission, the nucleus dividing indirectly. 

 Spore-formation also occurs, sometimes preceded by conjugation, sometimes 

 not. The spores (b, c), formed by the breaking up of the protoplasm of the 

 parent, escape as flagellulse. 



CLASS IV.-SPOROZOA. 



1. EXAMPLE OF THE CLASS Monocystis agilis. 



One of the most readily procured Sporozoa is the microscopic 

 worm-like Monocystis agilis (Fig. 62, A), which is commonly found 

 leading a parasitic life in the vesiculse seminales of the common 



A B C 



D 



.FiG. 62. Monocystis.^.4, Trophozoites in different stages of contraction. B, encysted 

 gametocytes. C, division cf gametocytes into gametes. D, conjugation of gametes to form 

 zygotes. E, Cyst enclosing ripe spores formed from the zygotes. F, single spore, showing 

 the (8) sporozoites iu its interior. G, group of developing sperm-cells of the earthworm, 

 enclosing a sporozoite in the centre. H, young trophozoites still surrounded with the tails 

 of the degenerated sperms, nu. nuclei. (From Parker's Practical Zoology.) 



Earthworm. It is flattened, greatly elongated, pointed at both 

 ends, and performs slow movements of expansion and contraction, 

 reminding us of those of Euglena. In this, the trophozoite or adult 

 condition, the protoplasmic body is covered with a firm cuticle, 

 and is distinctly divided into a denser superficial portion, the cortex, 

 and a central semi-fluid mass, the medulla. The innermost layer 

 of the cortex consists of contractile elements or myonemet which act 

 like the muscular fibres of higher animals. There is a large clear 

 nucleus (nu.) with a distinct nucleolus and nuclear membrane, but 

 the other organs of the protozoan cell-body are absent : there is 



