MULTICELLULAR ANIMALS 



131 



tInuoLis around the rim of the cup. The open mouth of the cup is 

 termed the blastopore and the process of its formation is known as 

 CASTRULATioN. One speaks of this two-layer stage as a gastrula 

 (Fig. 78). In all Metazoa some modification of this process of gas- 

 trulation occurs and all embryos pass through a gastrula stage dur- 

 ing development. 



Among the Metazoa only the Porifera and the Coelenterata are 

 diploblastic in their adult stages. All other animals consist of the 

 derivatives of three layers, that is, are triploblastic. The third layer, 



BLASTOCOELE 



PRIMITIVE GUT 



BLASTOPORE 



MESODERM 



ECTODERM 



ENDODERM 



Fig. 78. — Diagrammatic long section through the gastrula stage in the development 



of an echinoderm (starfish). 



the MESODERM, originates immediately following gastrulation, be- 

 tween the ectoderm and the endoderm, largely in the region of the 

 lips of the blastopore (Fig. 78). Concurrent with the appearance of 

 the mesoderm the embryo begins to exhibit the basic plan of sym- 

 metry of the future adult. 



Diploblastic Animals. The members of the phylum Porifera 

 and the phylum Coelenterata are described as the most simple Meta- 

 zoa because they consist of the derivatives of but two layers of cells, 

 that is, they are diploblastic. The bodies of sponges^ and of coelen- 



^ The origins of the ectoderm and endoderm in the development of the Porifera are not 

 identical with the processes of development of these layers in other Metazoa; hence these 

 layers in the sponges are not homologous with ectoderm and endoderm of other animals. 

 However, the adult sponge is a diploblastic animal and here we are regarding it as one 

 of the most simple and primitive of the Metazoa. 



