60 INVERTEBRATE ZOOLOGY 



perform diverse limited functions. This subject of histological 

 differentiation, which marks off the Metazoa from all Protozoa, 

 is discussed in the second section of this chapter. A brief 

 discussion of the organ systems of the invertebrates concludes 

 the chapter. 



A. The Mesozoa 



A small group of relatively simple many-celled animals com- 

 posed of a layer of ectoderm cells covering a single or several 

 entoderm cells has frequently been considered as standing inter- 

 mediate between the Protozoa and the Metazoa and has conse- 

 quently been termed the Mesozoa. The phylogenetic significance 

 which has thus been attached to this group is a subject regarding 

 the tenability of which very grave doubts are justifiable, for all 

 of the more important representatives of the group are internal 

 parasites. It seems probable that their simplicity of organiza- 

 tion is an accompaniment of degeneracy directly traceable to 

 adaptation to the parasitic existence. Not even the known facts 

 regarding the development of these forms aid in pointing out 

 relationships with any other group of organisms. 



The Dicyemidae and the Heterocyemidae include species which 

 are parasitic in cephalopods, while Rhopalura, which represents 

 the Orthonectidae, lives as a parasite in various invertebrate 

 hosts. 



B. Histological Differentiation 



Differentiation of Tissues. — As emphasized in the foregoing 

 chapter, all Metazoa differ from the Protozoa in that the cells 

 of the former become specialized for restricted functions, that is, 

 undergo histological differentiation. Certain groups of cells 

 become similarly specialized for carrying out one specific function 

 more effectively than that function may be executed by unmodi- 

 fied protoplasm. Such a group of similarly specialized cells is 

 termed a tissue. A tissue is made up not only of units of cyto- 

 plasm and their contained nuclei, but frequently the cytoplasm 

 forms substances which are essential in the effective functioning 

 of the tissue. These substances, which are termed plasmic 

 products, may be either inconspicuous or so prominent that the 

 cells which produce them are obscured. 



Classification of Tissues. — Since the function of a tissue is one 

 of its most important characteristics, the classification of tissues 



