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CHAPTER IX. 



SUB-KINGDOM 1I.CCELENTERATA, 



FOSSIL HYDROZOA. 



THE sub-kingdom Ccelenterata (Frey and Leuckhart) may be 

 considered as a modern representative of the Radiata of 

 Cuvier. From the Radiata, however, the Echinodermata and 

 Scolecida have been removed to form the Anmdoida, the entire 

 sub-kingdom of the Protozoa has been taken away, and the 

 Polyzoa have been relegated to their proper place amongst 

 the Mollusca. Deducting these groups from the old Radiata, 

 the residue, comprising most of the animals commonly known 

 as Polypes or Zoophytes, remains to constitute the modern 

 Ccelenterata. 



The Ccelenterata may be defined as animals whose alimentary 

 canal communicates freely with the general cavity of the Itody 

 (" somatic cavity "). The substance of the body is made up of 

 two fundamental membranes an outer layer, called the " ecto- 

 derm," and an inner layer, or " endoderm." Peculiar urticating 

 organs, or " thread-cells',' are usually present ; and, generally 

 speaking, a radiate condition of the organs is perceptible, espe- 

 cially in the tentacles with which most are provided. In all 

 the Ccelenterata distinct reproductive organs have been shown 

 to exist. 



The sub-kingdom Ccelenterata is divided into the two great 

 classes of the Hydrozoa and Actinozoa, and the following- 

 table indicates the characters and principal subdivisions of 

 these : 



