68 INVERTEBRATE ZOOLOGY 



arthropods that sections of nerve cells (Fig. 45) have the 

 appearance of being highly vacuolated. 



Interspersed between the neurons and fibers of a ganglion 

 are varying amounts of mesodermal connective tissue and a 

 highly modified type of ectoderm cells (Fig. 46) which are called 

 neuroglia. The neuroglia cells of invertebrates usually have 

 some portion of the cell body on the surface of the ganglion, while 

 a fiber-bearing, supporting portion extends between the nervous 

 elements of the ganglion. 



In function, structure, and origin, nervous tissue and the 

 sensory epithelium discussed earlier are intimately associated. 



Morphological Changes in the Nucleus 



In the foregoing discussion, attention has been focused upon 

 the changes in the form and structure of the cytoplasm and of the 

 plasmic products which are involved in histological differentia- 

 tion. Omission of the nucleus from this discussion does not 

 imply that it is not concerned in differentiation. There are, 

 however, few conspicuous changes in form or appearance of the 

 nucleus in comparison with the radical changes in the extra- 

 nuclear portions of the cells. As the controlling center of 

 most of the activities of the cell, doubtless the nucleus must have 

 varying functions depending upon the line along which the cell 

 is specialized, but there are few reflections of this in the morphol- 

 ogy of the nucleus. The nuclei in some highly active tissues 

 become irregular or even much branched but these morphological 

 changes seem to be correlated with rate of activity rather than 

 with actual differentiation of the nucleus in a particular kind of 

 tissue. 



C. The Organ Systems 



Just as the individual cells in the many-celled animals become 

 dependent one upon another through histological differentiation 

 so also do the individual tissues become interdependent. Few 

 tissues function as utterly isolated units, for they become com- 

 bined in groups which are known as organs. These organs are 

 the readily recognizable morphological units of which the meta- 

 zoan body is composed. All of the tissues which form an organ 

 cooperate in the performance of some function. Obviously, not 

 all tissues in an organ function to the same extent, but usually 

 one kind of tissue becomes the dominant or essential tissue and is 



