140 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS 



(5) In Opalina, from the intestine of the frog, and a few other forms, 

 there are very numerous nuclei, arranged in a symmetrical manner in 

 the cell substance. In some cases these isolated nuclei have been 

 observed to unite to form one large nucleus just before binary fission 

 takes place. Of these various cases the diffuse condition is apparently 

 very primitive. 



The nucleus, when stained and examined under high powers, is 

 observed to be complex in structure. It consists of a nuclear network, 

 or a coil of chromatin threads. Karyokinesis has been observed in 

 some cases. 



While we cannot at present define the physiological import of the 

 nucleus, we must recognise its importance. Thus Bruno Hofer has 

 shown that when an Amceha is cut in two, the part with the nucleus 

 lives and grows normally, while the part without any nucleus sooner or 

 later dies ; and Balbiani has observed that in the case of Infusorians cut 

 into pieces, those parts which have nuclei survive, while if no nucleus is 

 present in the fragment, the wound may remain unhealed, and death 

 ensues. 



The outer part of the cell substance (*' ectoplasm ") is 

 often clearer and less granular than the inner part ('' endo- 

 plasm "). In corticate Protozoa there is a more definite 

 rind or thickened margin of cell substance. Outside this 

 there may be a " cuticle " distinct from the living matter , 

 sometimes consisting of chitin, or gelatin, or rarely of 

 cellulose. The cuticle may form a cyst, which is either a 

 protection during drought, or a sheath within which the 

 unit proceeds to divide into numerous spores. Moreover, 

 the cuticle may become the basis of a shell formed from 

 foreign particles, or made by the animal itself of lime, flint, 

 or organic material. 



In the cell substance there may be bubbles of water taken 

 in with food particles (food vacuoles), contractile vacuoles, 

 fibres which seem to be specially contractile (in Gregarines), 

 spicules of flint or threads of horn-like material, which may 

 build up a connected framework, and the pigments already 

 mentioned. Some marine and fresh-water Protozoa secrete 

 bubbles of oxygen which buoy them up in the water. 



Reproduction of Protozoa 



Growth and reproduction are on a different plane from 

 the other functions. Growth occurs when income exceeds 

 expenditure, and when constructive or anabolic processes 



