66 MANUAL OF ZOOLOGY, 



CCELENTERATA. 



CHAPTER VII. 

 THE SUB-KINGDOM C(ELENTEEATA. 



1. CHARACTERS OF THE SUB-KINGDOM. 2. DIVISIONS. 

 3. GENERAL CHARACTERS OF THE HYDROZOA. 4. EX- 

 PLANATION OF TECHNICAL TERMS. 



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

 considered as the modern representative of the Eadiata of 

 Cuvier. From the Radiata, however, the Echinodermata and 

 Scolecida 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 

 Coelenterata. 



The Coelenterata maybe defined as animals whose alimentary 

 canal communicates freely ivith 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 l ectu- 

 derm,' and an inner layer, or ' endoderm. 1 There are no 

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

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

 si/stem. 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 Coelenterata distinct reproductive organs have 

 been shown to exist. By Professor Allman the Coelenterata have 

 been denned as follows : ' Animals composed of numerous 

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

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

 determinable antero-posterior and dorso-ventral plane (bi- 

 lateral) ; a distinct body-cavity, which always communicates 

 with the outer world through the mouth.' 



With regard to the fundamental tissues of the Coelenterata, 

 there exist two primary membranes, of which one forms the 

 outer surface of the body, and is called the * ectoderm ; ' 

 .whilst the other lines the alimentary canal, the general cavity 



