22 



INTRODUCTION TO CLASSIFICATION. 



cles, as well as other parts of the body, are provided with 

 those peculiar weapons of offence which have been termed 

 " thread-cells." 



Fig. 6. 



Fig. 6. Diagrams illustrative of the mutual relations of the Hydrozoa. 1. Hydra. 

 2. Sertularian. 3. Diphyes. 4. Physophorid. 5. Lucernaria. a. Ectocyst. 6. En- 

 docyst. c. Their enclosed cavity. 



P. Tentacles. N. Natatorial organ. T. Coenosarc. B. Bract. C. Cell. S. 

 Polypite or digestive cavity. G. Reproductive organ. A. Air vesicle. F. Float. 



I., JI., III., IV. represent the successive stages of development of a Medusiform 

 zooid or reproductive organ. 



VII. THE ACTINOZOA. 



This class contains those animals which are familiar to us as 

 Sea-anemones and Coral-polypes, by the latter of which, in many 

 parts of the world, those huge reefs, which are so well known to 

 navigators, are constructed. It embraces the Sea-pens and the Bed 



