THE MOST PRIMITIVE METAZOA 55 



the mechanism of this interesting rearrangement is not yet under- 

 stood. This type of cellular organisation is found in the higher 

 Metazoa where it has superimposed on it the co-ordinating 

 influence of a nervous and hormonic integration, both of which 

 are apparently absent in the sponges. The high degree of skeletal 

 material relative to the small amount of living protoplasm makes 

 the sponges very poor food and so a relatively successful group of 

 animals. 



Are the sponges a primitive or an advanced group of animals? 

 To answer this question we should have to know the time of 

 origin of the sponges and there is no certain information on this 

 point. Instead we can examine the apparently simple characters 

 and the apparently complex characters and attempt to derive some 

 satisfaction from this. The reader should always be on his guard 

 when consulting such lists ; it is not possible to come to a conclusion 

 merely by seeing which of the two lists is the longer ! 



Simple characteristics 



(1) The layers of the body are loosely organised. 



(2) The layers of the body do not correspond to the ectoderm, 

 mesoderm and endoderm of the higher forms. 



(3) There is no definite body form. 



(4) They have choanoflagellate cells like those present in the 

 choanoflagellate protozoans. 



(5) There is no nervous system. 



(6) There is no excretory system. 



(7) There is no mouth. 



(8) The gut (gastral cavity) shows little differentiation. 



(9) They have a high regenerative capacity. 



(10) They have a well-developed system of asexual reproduction. 



(11) The larvae have well-developed flagella. 



Complex characteristics 



(1) They have three layers of cells (some coelenterates have only 

 two layers of cells). 



(2) They have a well- developed middle layer, the " mesen- 

 chyme," with an elaborate skeletal system. 



(3) They have some differentiation within the layers, e.g. the 

 pore cells. 



