Branch Coelenterata 29 



Campanularian. In what respects are the two alike? 

 In what respects are they different ? 

 VIII. — Find two or three hydranths with medusa-buds 

 growing upon the body wall. Make drawings of these, 

 showing different stages of the development. 



THE CLASSIFICATION OF THE CCELENTERATA 



The third phylum or branch of the animal kingdom, and 

 the second phylum of the Metazoa, is the Coelenterata. 

 This phylum is characterized by individuals having a dis- 

 tinct body cavity with but one external opening, the mouth, 

 and a body wall composed of two layers of cells, the ectoderm 

 and the endoderm. Sometimes a thin or thick layer called 

 the mesogloea occurs between the other two. The mouth 

 corresponds to the mouth of higher animals, and the 

 body cavity into which the mouth opens corresponds to 

 the alimentary canal of the higher animals. The endoderm 

 of the Coelenterata is homologous in general w\th the lining 

 of the alimentary canal, and the ectoderm is homologous 

 with the skin of the higher animals. 



The Coelenterata are universally aquatic animals, occur- 

 ring most abundantly in salt water. There is generally a 

 row of tentacles around the mouth, the cavities of these 

 tentacles being continuous with the main body cavity ; they 

 aid in the capture of food and in locomotion. Another 

 characteristic feature of the branch is the possession of 

 nettle cells, within each of which is a minute sac containing 

 a fluid and a barbed filament capable of being thrown out 

 for stinging purposes. They reproduce by budding, by 

 fission, and by eggs. 



The Coelenterata are divided into four classes, of which 

 the Hydrozoa and the Actinozoa are the most important. 

 The members of the first class, the Hydrozoa, are the most 



