CCELENTERATA. 73 



CHAPTER VIII. 



SUB-KINGDOM IL CCELENTERATA. 

 FOSSIL HYDROZOA. 



SUB-KINGDOM II. CCELENTERATA. Animals whose alimen- 

 tary canal communicates freely with the general cavity of the body 

 (" somatic cavity "), so that the body-cavity communicates with the 

 outer world through the mouth. Body composed of tivo funda- 

 mental layers, an outer layer or " ectoderm" and an inner layer or 

 " endoderm" The parts of the body, and especially the organs 

 round the mouth, arranged in a star-like or radiated form. 



TABLE OF THE DIVISIONS OF THE CCELENTERATA. 



CLASS A. HYDROZOA. The walls of the digestive sac not separated 

 from those of the general body-cavity, the two coinciding with one another. 

 Reproductive organs in the form of external processes of the body-wall. 

 Sub-class I. HYDROIDA (Hydroid Zoophytes). 

 Order I. Hydrida.Ex. Hydra. 

 Order 2. Corynida. Ex. Tubularia. 

 Order 3. Thecaphora. Ex. Sertularia, Campanularia. 

 Sub-class II. SIPHONOPHORA (Oceanic Hydrozoa). 

 Order 4. Calycophoridce. Ex. Diphyes. 

 Order 5. Physophoridce, Ex. Physalia. 

 Sub-class III. DISCOPHORA (Jelly-fishes). 



Order 6. Medusidce. Ex. ^Egina. 

 Sub-class IV. LUCERNARIDA (Sea-blubbers). 

 Order 7. Lucernariadcz. Ex. Lucernaria. 

 Order 8. Pelagidce.Ex. Pelagia. 

 Order 9. Rhizostomida Ex. Rhizostoma. 

 Sub-class V. GRAPTOLITID^ (Graptolites.) 



CLASS B. ACTINOZOA. Animal with a differentiated digestive sac open- 

 ing below into the body-cavity, but separated from it by an intervening 

 " perivisceral space," which is divided into compartments by a series of 

 radiating vertical partitions or "mesenteries," to the faces of which the re- 

 productive organs are attached. 



Order I. Zoantharia. Ex. Sea-anemones, Star-corals, Brain- 

 corals. 



Order 2. Alcyonaria. Ex. Sea-pens, Fan-corals, Sea-shrubs, Red- 

 coral. 



Order 3. Rugosa. Ex. Cyathophyllum. 

 Order 4. Ctenophofa Ex. Venus's Girdle. 



The Coelenterata appear to have commenced their existence 

 in the Cambrian period, at which time both the great classes 

 of the sub-kingdom were differentiated from one another. So 

 far as we can judge, the Ccelenterate animals have attained 

 their greatest development at the present day; but two large 



