80 



ZOOLOGY 



The spores (b, c), formed by the breaking up of the protoplasm of the parent, 

 escape as flagelhila;. 



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. 61, A), which is commonly found 

 leading a parasitic life in the vesiculse seminales of the common 

 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 



A B C ^ 



Fig. til. — Monocystis. A, Trophozoites in different stages of contr.iction. B, encysted 

 gametocytes. jQ^division of gametocytes into gametes. D, conjugation of gametes to form 

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

 the (8) sporozoitcs in its interior. G, group of developing sperm-eells of the e.irthworm, 

 enclosing a sporozoite in tlie centre. H, young trophozites still suirounded with the tails of 

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



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. 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 

 no trace of contractile vacuole, of flagella or pseudopods, of mouth 

 or gullet. Nutrition is effected entirely by absorption. 



JReproduction takes place by a peculiar and characteristic 

 process of spore-formation. Two individuals come together, 

 and become rounded off and enclosed in a common cyst (B). The 

 nucleus of each divides repeatedly, until a large number of nuclei 

 are formed (C). Each of the nuclei becomes surrounded by a thin 

 layer of protoplasm. The minute cells thus formed, after 



