PARAZOA AND ENTEROZOA 



innermost has its units in the shape >of goblets from the interior 

 of which rises a vibratile flagellum (choanocytes). They bound 

 a cavity excavated in the mass of cells, and communicating by 

 apertures, with the exterior. By the movements of their flagella 

 they induce the flow of currents of water within the cavity or 

 chamber which they line. 1 The Enterozoa, on the other hand, are 

 in their simplest expression, two-cell-layered sacs, the outer layer 

 of cells the ectoderm being protective, respiratory, and excre- 

 tory, and often provided with vibratile processes, whilst the inner 

 or endoderm cells are essentially concerned in digestion, assimila- 

 tion, and reproduction and bound a cavity. This cavity is the 

 primitive gut or " archenteron," and opens to the exterior by a 

 single aperture, the mouth-anus. 



The most primitive Enterozoa retain the general features thus 

 indicated, whilst it is possible to trace the development of the in- 

 dividual in the case of representatives of the higher groups of 

 Enterozoa from the same simple structure (the embryonic form 

 known as the Diblastula or Gastrula). 



PARAZOA ENTEROZO 



Branch A 



Grade B. METAZOA. 



t 

 Grade A. PROTOZOA. 



TREE showing primary grades and branches of the Animal Pedigree. 



3. STERILITY OF THE BRANCH PARAZOA. The Parazoa have 

 apparently not given rise to any very great advance or complication of 

 structure. They are represented by the Sponges or Porifera alone. 



4. THE DIVISION OF THE BRANCH ENTEROZOA INTO Two 

 GRADES. The Enterozoa proceeding from the condition of simple 

 two -cell -layered sacs (Fig. 1) have given rise to an immensely 

 increased complexity of structure, and to a vast diversity of form 

 and internal organisation. The most important step in their pro- 

 gressive development of complexity of structure is the production 

 of a second internal cavity distinct from the gut or archenteron 

 (Figs. 2 and 3). To this second cavity the name "coelom" is 

 given. Its nature and origin are discussed below. 



The presence of the coelom is of the highest physiological import- 

 ance. Once developed it became the starting-point for a variety 



1 See further the conclusion of the chapter Porifera, by Prof. Minchin. 



