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CHAPTER XIII. 

 SUB-KINGDOM III.—CCELENTERATA. 



GENERAL CHARACTERS AND DIVISIONS OF CCELENTERATES 



HYDROZOA DIVISIONS OF HYDROZOA. 



The sub-kingdom Ccele?iterata (Frey and Leuckart) may be con- 

 sidered as a modern representative of the Radiata of Cuvier. 

 From the Radiata, however, the Echi?iodermata and Rotifera have 

 been removed, the entire sub-kingdom of the Pi-otozoa has been 

 taken away, and the Polyzoa have been relegated to a place near 

 the Mottusca. 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 Cozlenterata may be defined as radially symmetrical animals, 

 in which the mouth opens into a simple or variously divided space 

 (" cozlenteric cavity"), ivhich acts as an alimentary cavity, and which 

 may or may not be divided into two portions, of ivhich one forms a 

 rudimentary digestive tube. The body-wall consists of tivo funda- 

 mental layers ("ectoderm" and " endoderm"), between which an i?iter- 

 mediate layer (" mesoderm ") is usually developed. Peculiar urticating 

 organs, or " thread-cells," are present. The nervous system is some- 

 times specialised, sometimes diffused ; but no vascular organs are de- 

 veloped. Reproductive organs are iiivariably p?-esent at some period 

 or another of life, though asexual reproductioji is also very general. 



The sub -kingdom Ccele?iterata is divided into the two great 

 primary divisions or classes of the Hydrozoa and Actinozoa, and 

 the following table indicates the main subdivisions of these : — 



TABLE OF THE DIVISIONS OF THE CCELENTERATA. 

 Class I. — Hydrozoa. 



Sub-class i. Hydroida (Hydroid Zoophytes). 



a. Hydrida. — Ex. Hydra. 



b. Cory?iida. — Ex. Tubularia, Hydractinia. 



