40 Animal Life and Intelligence, 



two is not the same. One remains attached to the stalk, 

 and expands into a complete vorticella. The other remains 

 pear-shaped, and develops round the posterior region of 

 the body a gii'dle of powerful vibratile cilia, by the lashing 

 of which the animalcule tears itself away from the parent 

 stem, and swims off through the water. After a short 

 active existence, it settles down in a convenient spot, 

 adhering by its posterior extremity. The hinder girdle of 

 cilia is lost or absorbed, a stalk is rapidly developed, and 

 the organism expands into a perfect vorticella. 



In some cases, however, the fission is of a different 

 •character, with different results. It may be very unequal, 

 so that a minute, free-swimming animalcule is disengaged ; 

 or minute animalcules may result by repetition of division. 

 In either case the minute form conjugates with an ordinary 

 vorticella, its smaller mass being completely merged in the 

 larger volume of its mate. 



There are, of course, many variations in detail in the 

 modes of protozoan reproduction ; but we may say that, 

 omitting such details, reproduction is either by simple 

 fission or by spore-formation; and that these processes 

 are in some cases associated with, and perhaps dependent 

 on, the temporary or permanent union of two individuals 

 in conjugation. 



It is essential to notice that the results of fission or of 

 spore-formation separate, each going on its own way. Hence 

 such development as we find in the protozoa results from 

 differentiations within the limits of the single cell. Thus 

 the bell-animalcule has a well-defined and constant form ; 

 a definite arrangement of cilia round the rim and in the 

 vestibule by which food finds entrance to the body. The 

 outer layer of the body forms a transparent cuticle, beneath 

 which is a so-called " myophan " layer, continuous with a 

 contractile thread in the stalk. Within the substance of 

 the body is a pulsating cavity, or contractile vesicle, and a 

 nucleus. Such is the nature of the differentiation which 

 may go on within the protozoan cell. 



When we pass to the metazoa, we find that the method 



