THE HYDROZOA. 



21 



an outer and an inner — which have been termed, respectively, 

 ectoderm and endoderm. The endoderm is that layer which 

 lines the inner cavities of the body, from the mouth inwards; 

 the ectoderm is that which forms its external covering. 



These two layers are shown in the accompanying diagram- 



's. 5. 



Fig. 5. — Diagrams illustrative of the mutual relations of the Hydrozoa. — 1. Hydra. 

 2. Sertularian. 3. Diphyes. 4. Physophorid. 5. Lucemaria. a. Ectocyst. b. Endo- 

 cyst. c. Their enclosed cavity. 



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

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



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

 zooid or reproductive organ. 



matic sections of the leading forms of Hydrozoa, the ectoderm 

 being represented by the thin line with the adjacent clear 

 space, the endoderm by the thick dark line. 



