Second SectioB. 



Ccelenterata (Zoophyta). 



General Review. 



§ 71. 



This division is the first of the Metazoa, or organisms which are 

 undoubtedly animals. The embryonic body separates into two 

 cell-layers — ectoderm and cndoderm ; which in many Sponges alone 

 form the permanent body, though in many a mesoderm is developed. 

 In the lower Acalephte the formation of a mesoderm is incomplete ; 

 that is, the mesoderm is not an independent tissue as it is in all the 

 higher Acalephaa. The most essential character of the animals which 

 make ujd this division is the arrangement of the nutritive apparatus, a 

 cavity sunk into the parenchyma of the body, and either divided into 

 canals or extended into wider spaces. This digestive cavity, with its 

 appended spaces, is invested by the endoderm, and in the lower 

 forms is the sole representative of hollow organs in the body. 

 When several individuals are united to form a colony, the canal 

 system^ which arises from the digestive cavity, is common to all of 

 them, and is continued into the common substance of the colony or 

 coenenchyma. The primary axis alone can be made out in the body, 

 and the secondary axes ai^e indifferent, or if present appear to be 

 equivalent. 



I. Spongiae. 



Gastrteades.* 



Haliphysema, Gastrophysema. 



Porifera. 



Myxospongise. 



Halisarca. 



* The Gastrseades represent stages which are not permanent in the rest of the 

 Spongise. 



