ANIMAL AFFINITIES 



285 



niay arise from cells budded off from the junction of endoderm 

 and ectoderm, or it may arise as vesicles budded off from 

 the endoderm. But a third formative layer, the mesoderm, 

 arises from one or other of the two fundamental germ- 

 layers. This third layer we regard as everywhere homologous 

 in the multi-cellular animals other than the Porifera and 

 Coelenterates. 



94^. The Primary Animal Classification. We may utilize 

 the above homologies in order to make a general and primary 

 classification, which is as follows : 



THE , 

 ANIMAL \ 

 KINGDOM 



/ Protozoa 



' Metazoa \ 



,Metazoa 



without 



tissues 



(2) 



fDiplohlastica , 



Metazoa \ 



zoith "{ 



tissues I Tyipioblastica 

 (3) 



Primary 

 Animal Groups. 

 Protozoa 



Porifera 



. Coelenterata 

 Acoelomata Platyhelminthes, 

 etc. 

 Worms 

 Polyzoa, etc. 

 Echinoderms 

 Molluscs 

 Arthropods 

 Chordates. 



Coelomata 

 (4) 



Here we see : 



(i) The primary subdivision of all animals into unicellular 

 and multicellular forms : the separation we base on the embryo- 

 logical result that a unicellular organism — the ovum, becomes 

 multicellular in the process of segmentation. 



(2) The division of multicellular animals into those in w^hich 

 segmentation results in a multicellular body where there is no 

 distinct differentiation of cells into tissues. For a time this is 

 so in all animal embryogenies and in the cases of the Porifera 

 (sponges) the fully developed organism consists essentially of 

 cells (the " collar-cells ") w^hich exercise all the ordinary animal 

 functions and are all the same in structure. In the rest of the 

 sponge body there are obscurely differentiated supporting struc- 

 tures but no true tissues. The phase of the blastula roughly 

 corresponds with this animal division. 



(3) Tissued animals are separated into those with two developed 

 germ-layers, the endoderm and ectoderm {Diplohlastica) and those 



