136 



ELEMENTS OF BIOLOGY 



covering of the endoderm. The cavity between these two sheets o£ 

 mesoderm is the ccelom (Fig. 81); it forms the body cavity of the 

 adult. The cavity is not formed in the Ctenophora, and in the 

 members of the phylum Platyhelminthes it is largely, if not entirely, 

 obliterated in the adult, a second cavity, termed the pseudoccele, 

 appearing in its place. A distinction is thus established between the 

 ACCELOMATE and ccELOMATE animals. The coelom presently becomes 

 the most prominent cavity of the animal body. In the most advanced 



NEURAL GROOVE 



ARCHENTERON 

 ENDODERM 

 YOLK CELLS 



SEGMENTATION 

 CAVITY 



SOMATIC 

 MESODERM 



COELOM 



SPLANCHNIC 

 MESODERM 



ECTODERM 



Fig. 81. — Perspective diagram of a portion of a chordate embryo to show the relative 



positions of cell layers and coelom. 



animals it is the cavity that is occupied by the digestive system and 

 associated structures, by the heart, and by other organs that require 

 space within the body proper. 



In the fishes a portion of the anterior region of the ccelom is 

 separated off by thin membranes; it forms a smaller cavity contain- 

 ing the heart and is known as the pericardial cavity (Fig. 82). In 

 the human body, as well as in all other mammals, in addition to the 

 pericardial cavity, which is thus derived from the ccelom, a trans- 

 verse partition separates the ccelom into an anterior pleural and 

 a posterior peritoneal cavity. This partition contains muscles and 

 is known as the diaphragm. The pleural cavity is further divided by 



