Plate I. 

 Diagrams illustrating structure of Hydroida. 



1. Diagram of Hydra. 



2. Diagram of Hydroid colony. 



3. 4, 5. Thread-cells, 



6. Medusa of Hydroid (after Hincks). 



7. Diagram of planula. 



For explanations of these diagrams, see chapter on 

 structure of the Hydroida, pp. 22 — 34. 



