METAZOA. 155 



In the Coelenterata all of the functions of the animal are 

 performed by either one or the other of these two layers. 

 In all the other divisions a third layer occurs between 

 ectoderm and entoderm the mesoderm (middle skin)- 

 and this mesoderm takes some of the functions which are 

 divided between the ectoderm and entoderm of the Coelen- 

 terata. The study of the development of these three- 

 layered animals shows a very interesting fact. At first 

 there are but two layers in the body, and later the meso- 

 derm develops between these two. In other words, all of 

 the higher Metazoa pass through a stage in which they 

 exhibit a ccelenterate condition. 



These three layers reach their highest condition in the 



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Vertebrates, and it may be interesting to see how the 

 various structures which are found in a shark, a bird, and 

 a rat are related to these layers. 



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To the ECTODERM belong the outer layer of the skin, 

 the outer layer of scales, the hair, feathers, sweat-glands, 

 the enamel of the teeth, the nervous system, the sensory 

 portions of sensory organs, and the lens of the eye. 



The ENTODERM furnishes the lining of the alimentary 

 canal, the notochorcl, gills, tracheal lining, lungs, liver, 

 pancreas, urinary bladder. 



The contributions of the MESODERM to the body are 

 more extensive. They include the deeper layers of the 

 skin, fat, muscles, connective tissue, cartilage, bones, liga- 

 ments, blood-vessels, blood, the lining (pleural, pericardial, 

 and peritoneal membranes) of the body-cavity, the deeper 

 layer of the scales, the dentine of the teeth, the outer 

 layers of the alimentary canal, and the reproductive and 

 excretory organs and their ducts. 



If we study any part of any one of the Metazoa under 

 the higher powers of the microscope having first treated 



