CCELENTERA TA. 



CHAPTER VII. 



THE SUB-KINGDOM CCELENTERATA. 



I. Characters of the Sub-Kingdom. 2. Divisions. 

 3. General Characters of the Hydrozoa. 4. Ex- 

 planation OF Technical Terms. 



The sub-kingdom Cmlenterata (Frey and Leuckh'art) may be 

 considered as the modern representative of the Radiata of 

 Cuvier. From the Radiata, however, the Echinodermaia and 

 Scokcida have been removed to form the Annuloida, 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 

 Cxlenterata. 



The Cxienterata may be defined as animals whose alimentary 

 canal communicates freely with the general cavity of the body 

 {^'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." There are no 

 distinct neural and hcemal regions, and in the great majority of 

 the members of the sub-kingdom there are no traces of a nervous 

 system. Peculiar urticating organs, or " thread -cells," are usually 

 present; and, generally speaking, a radiate condition of the organs 

 is perceptible, especially in the tentacles with which most are 

 ■provided. In all the Cxlenterata distinct reproductive organs have 

 been shoivn to exist. By Professor AUman the Ccelenterata have 

 been defined as follows: — "Animals composed of numerous 

 merosomes (body-segments), which are disposed radially round 

 a longitudinal (antero-posterior) axis ; frequently with a de- 

 terminable antero - posterior and dorso - ventral plane (bilat- 

 eral^ ; a distinct body-cavity, which always communicates with 

 the outer world through the mouth." 



