THE GASTRAEA-THEORY^ ETC. 223 



mucous membranes in general. The word " squamous" 

 sufficiently clearly indicates the general character of an 

 epithelium made up of flattened cells which overlap, as 

 " tesselated" equally clearly signifies aa epithelium of flat- 

 tened cells fitting into each other at their edges. These 

 are general distinctions. Such special forms as the sinuous 

 cells of the commencing lymphatics or the jagged cells 

 of the epidermis do not need any distinctive general ap- 

 pellation. 



We perhaps do want easy terms which shall denote 

 whether the epithelium in any spot consists of several layers, 

 or of one pronounced layer only. The latter might be called 

 monoderic (Sepog = oepfxa), the former polyderic. 



Epithelium itself would simply mean cells lining a cavity 

 or coating a free surface. 



TJie Gastraea-Theory, the Phylogenetic Classification 

 of the Animal Kingdom and the Homology of the 

 Germ-Lamella. By Ernst Haeckel. (Translated by 

 E. Perceval Wright, M.D., F.L.S., Sec. R.I.A.", 

 Professor of Botany, Trin. Coll., DubHn. With PI. VII.) 



{Continued from p. 165.) 



5. — The^Systematic Signification of the Gastraea 



Theory. 



The following conclusions relating to the natural system 

 of the animal kingdom, or, what is the same thing, to its 

 genealogical tree, result from the foregoing discussions, 

 which 1 have already explained, partly in the ' Biology of 

 the Calcareous Sponges ' and partly in the fourth edition of 

 the ' Naturliche Schopfungsgeschichte ' (in the eighteenth 

 lecture). The Avhole animal kingdom divides into two 

 large principal groups, the gastrula forming the separating 

 boundary line between them ; on the one side the stem- 

 group of the primary animals {Protozoa) ; on the other, 

 the six higher stem-groups which we oppose to the others 

 as animals with germ-lam ellse {Metazoa or Blastozoa). 

 In the primary animals (the Protozoa) the entire body 

 consists either (1) of a simple cytode (Monera, Monotha- 

 lamia), or (2) of an aggregate of cytodes (Polythalamia), or 

 (3) of a simple cell (Amcebte, unicellular Gregarinse, Infu- 



