176 SYSTEMATIC ZOOLOGY. 



generations. The ctenophores are all marine, are per- 

 fectly transparent and so delicate in structure that it is 

 almost impossible to preserve them, some being torn by 

 strong currents of water. They are among the most 

 voracious of animals. As a rule few measure more than 

 three or four inches in diameter. 



SUMMARY OF IMPORTANT FACTS. 



1. The CCELENTERATA and the Echinoderma were for- 

 merly united as a group Radiata on account of their 

 radiate structure. 



2. The Coelenterata have but a single opening to the 

 digestive tract; there is no body-cavity; the body-wall 

 is composed of ectoderm and entoderm with a supporting 

 layer between them. 



3. The nettle-cells are especially characteristic. 



4. Reproduction is by eggs and by fission, or budding. 



5. Some buds may become free and form medusae and 

 there is frequently an alternation of generations. 



6. The Coelenterata are divided into Hydrozoa and 

 Scyphozoa. 



7. The HYDROZOA lack an inturned oesophagus. They 

 have frequently an alternation of fixed or polyp and free 

 or medusa generations. 



8. The Hydrozoa are divided into Hydridse, Hydro- 

 medusse, and Siphonophora. 



9. The Scr;pHozoA have an inturned oesophagus; the 

 digestive tract is subdivided by radial septa bearing mes- 

 enterial filaments. 



10. The Scyphozoa are divided into Actinozoa and 

 Scyphomedusse. 



