16 



Subkingdom II. CCELENTEEATA. 



EADIATA. ZOOPHYTA. CENTROXIJE. 



Aquatic animals with a distinct body-cavity, and a mouth open- 

 ing into it ; no intestinal canal. Eeproduction normally by sper- 

 matozoa and ova, which are discharged through the mouth. 



The majority of the Coelenterata are composite animals that 

 is, organisms made up of colonies or communities of individuals 

 organically united. The substance of the body, which is mostly 

 of a radiate character, is composed of two membranes ectoderm 

 and entoderm. There are no traces of a nervous system, except 

 in certain Medusae, and there is no proper blood-vascular appa- 

 ratus, although there is a fluid in the body-cavity which " repre- 

 sents " the blood. 



Peculiar stinging-organs, supposed to be poisonous, are inva- 

 riably present, except in Sponges ; they are called " cnidag,'' " ne- 

 matocysts," or " thread-cells ;" they are usually colourless, elastic, 

 double-walled sacs, round or oval, with a fluid in their interior. 

 The inner wall of the sac is produced into a sheath terminating 

 in a long thread [ecthoreum] ; this is usually twisted in many 

 irregular coils round its sheath, and fills up the open end of the 

 sac. " Under pressure or irritation, the cnida suddenly breaks, 

 its fluid escapes, and the delicate thread [cnidocil] is projected, 

 still remaining attached to its sheath." The cnidae are said to be 

 analogous to the tactile organs of the Arthropoda. 



The body-cavity of the Coelenterata does not, according to 

 Hackel, represent the true intestinal cavity [coeloma] of the higher 

 animals, but, in many, a system of cavities [enterocoele] takes its 

 place. 



There are four classes : 



Fixed, or, if free, not moving by means of cilia. 



No urticating organs SPONGIA. 



With urticating organs. 



Digestive cavity continuous with the body- 

 cavity HYDROZOA. 



Digestive cavity separated from the body-cavity ACTINOZOA. 

 Free, swimming by means of cilia CTENOPHORA. 



