METAZOA. 157 



PHYLUM IV. MOLLUSCA. 

 PHYLUM V. ARTHROPODA. 

 PHYLUM VI. ECHIXODERMA. 

 PHYLUM VII. CHORDATA. 



SUMMARY OF IMPORTANT FACTS. 



1. The METAZOA are many-celled animals. The cells 

 are differentiated and arranged in layers, no one cell or 

 layer performing all the vital functions. 



2. An outer layer (ectoderm) and an inner layer (en- 

 toderm) are always present; a middle layer (mesoderm) 

 between the other two occurs in most Metazoa. 



3. The ectoderm is protective, nervous, and sensory; 

 the entoderm is concerned in digestion; the mesoderm 

 usually gives rise to muscular, skeletal, circulatory, and 

 excretory organs; respiration may be effected by either 

 ectoderm or entoderm. 



4. The lavers are usuallv distributed in tissues, each 



*/ \j 



tissue being composed of essentially similar cells. 



5. Organs may consist of a single tissue or of a com- 

 plex of tissues. An organ is a structure for the perform- 

 ance of a definite function. 



