20 



PROTOZOA 



[CH. 



resulted from the oxidation of the living matter, and so we can 

 understand how the chemical action which is essential to life can 

 go on in every part of the living .substance, remembering the 

 fundamental chemical maxim " corpora non agunt nisi soluta." 

 The granules are temporary deposits in a solid form, either of 

 matter resulting, from katabolism, or of nutritious matter not yet 

 assimilated. 



We must now glance at some animals allied to Amoeba, in order 

 to gain some idea of the group Protozoa as a whole. 



Difflugla and Arcella are both found in the mud of pools and 



ponds ; they resemble Amoeba in general structure 



but differ from it in being provided with shells. In 



consequence of having these they are only able to put out 



pseudopodia at one spot, the mouth of the shell. The shell of 



Difflugia is composed simply of grains of sand stuck together with a 



secretion ; it has the shape of a pointed egg with the thick end cut 



off (1, Fig. 2). Arcella, on the other 

 / hand, makes its shell entirely out of 



its own secretion ; this is colourless 

 when thin, but as the animal grows 

 older the shell becomes thicker and 

 acquires a characteristic brown colour, 

 and we are enabled to recognise that 

 it consists of chit in. This is really a 

 name for a class of substances which 

 are constantly met with in the animal 

 kingdom and which according to some 

 investigators are allied in composition 

 to uric acid. Out of chitin, for instance, 

 all insects construct their hard cases. 

 It seems probable, that the self-de- 

 struction of protoplasm, which results 

 from the ordinary vital functions, may 

 in many cases give rise to chitin, so 

 that perhaps in Arcella, the shell is 

 at once a protection and the ordinary excretion. The shape of this 

 shell is like a watch-glass with a flat lid resting on it, and in the 

 middle of the lid there is a round hole through which the pseudo- 

 podia come out. Sometimes gas bubbles (7, Fig. 3) can be seen in 

 the body of the animal, which tend no doubt to balance the weight 

 of the shell. Arcella possesses two so-called primary nuclei and iu 



Fm. 2. Difflugla urceolata x 100. 

 After Leidy. 



1. Shell composed of particles 

 of sand containing body of the 

 animal. 2. Pseudopodia. 



