CHAPTER IV 



THE SUBKINGDOM METAZOA 



The fundamental difference in histology which distinguishes the 

 Metazoa from the Protozoa has already been described in Chapter ii. 

 Something must here be said concerning the main features of the 

 organization of the Metazoa. 



The simplest type of bodily architecture in this subkingdom is that 

 with which the student is familiar in Hydra, where the body consists 

 of a sac with one opening, and with the wall composed of two 

 cellular layers and a layer of secreted jelly between them. The inner 

 layer is the endoderm. It consists of cells specialized for the processes 

 of digestion, and the cavity which it lines is for the reception of food. 

 The outer layer is the ectoderm: by its cells relations with the en- 

 vironment are regulated. Some of these cells form a protective and 

 retaining sheet ; among them stand others which are sensitive ; others 

 — nerve-cells — lying below the sheet, are branched so as to serve 

 for the transmission in various directions of the stimuli received by 

 the sense cells : together they form a nerve-net. At the base of both 

 ectoderm and endoderm there lie muscle fibres — which in Hydra are 

 elongate contractile processes of the retaining cells but in other 

 animals of this type are often whole cells that have left the surface. 

 Lastly, from certain undifferentiated cells at the base of the ectoderm 

 there are formed the generative cells. 



When we compare this organization with that of a protozoon we 

 observe that the cellular structure of the metazoon, primarily, perhaps, 

 necessitated by its size (p. 8), has the following result : by isolating the 

 units specialized for the performance of particular functions it (a) re- 

 moves most of them from the direct action of the outer world, (6) makes 

 it possible that groups of them should constitute independent organs, 

 and {c) enables the relations of such organs, both with the environ- 

 ment and with one another to be regulated by intervening cells and 

 internal media. Already in the simple case we have examined these 

 facts are turned to advantage. Under the protection of the layer 

 which remains in contact with the outer world there are established 

 a special organ of digestion and a system for distributing stimuli 

 which are received by distinct units on the surfaces. Other ele- 

 ments (muscular, genital) are beginning to separate. In the following 

 pages we shall see this process of separation and differentiation carried 

 much further. Its result is that the activities of the organism are less 

 and less liable to interference from or suppression by the environ- 



