CHAPTER II 



the enteroccela and the ccelomoccela ^ 



1. The Distinction between the Grades Protozoa and 

 Metazoa. — Some discussion of this subject will be found in the first 

 part of the present woi'k. Here we start with the simplest con- 

 ception of a Metazoon, namely, a multicellular organism {i.e. an 

 organism which can be actually as well as optically resolved into 

 a number of constituent " cells " or " cytes ") in which the cell-units 

 are difterentiated into at least two groups, having contrasted pro- 

 perties and functions instead of being equiformal and interchange- 

 able in function as in the multicellular Protozoa. The production 

 of micro- and macro-gametes or male and female reproductive con- 

 jugating cells does not in itself serve to distinguish the Metazoa 

 from the Protozoa, as this occurs not only in multicellular, but 

 also in unicellular Protozoa (Coccidia, Ha^mamoebse). The group- 

 ing of at least two different kinds of cell-units to form at least 

 two distinct permanent layers or masses in the adult organism 

 is the essential character of the Metazoa, and it does not constitute 

 a very great chasm between them and some of the aggregated or 

 multicellular Protozoa. 



2. Division of the Metazoa into Two Branches. — The 

 Metazoa - are divisible into two divergent branches, which possibly 

 may be really two independent stems arising separately from 

 widely different ancestral Protozoa. These are, on the one hand, the 

 Parazoa ^ or Sponges, and, on the other hand, the Enterozoa,'* which 

 comprise the rest of the animal kingdom. The Parazoa are charac- 

 terised by being composed of aggregates of cells, of which the outer 

 layer is protective, trophic, and reproductive in function, whilst the 



^ By E. Ray Lankester, M.A., F.R.S. 



- The term Metazoa was introduced by Haeckel in his Stndien ziir Gastraea 

 Theorie, Jena, 1877. p. 12 and p. 54. Protozoa is a translation of the German word 

 " Urthiere," and was lirst used by von Siebold in 1841. 



' This term is due to Sollas ; see Quart. Journ. Microsc. Sci. N.S. vol. xxiv. ]». 

 614 (1884). 



■* The name Enterozoa was introduced by me in 1876 (preface to the English 

 translation of Gegenbaur's Comparative Anatomy) as a substitute for Haeckel's term 

 Metazoa. It now linds convenient application as the title for one of the two branches 

 into which Metazoa are divisible. 



