112 THE ANATOMY OF INVERTEBnATED ANIMALS. 



animals, and the latter that of the epithelium of the aliment- 

 ary canal/ Between these two layers, a third layer — the 



Fis. 13. — Diasrramp illustrative of the mutnal relations! of the HijrTrozon : 



1. Hydra. 2. Sertularian. 3. Calvcophoridati. 4. Physuphoriclan. 5. Lucernariati. 



a. Ectoderm, b. Endoderm. c. The digestive and somatic cavity. 



P. Tentacles. N. Nectoialyx. T. Coeno?arc. B. Hydrophyllium. C. Hydrothoca. S. 



Hydranth. Q Gonophore, A. Air- Vesicle contained in F. Pneumatophore. c, 



Digective and somatic cavity. 

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



gonophore. 



mesoderm — which represents the structures which lie between 



* " The body of every Hydrozoon is essentially a sac composed of two mem- 

 branes, an external and an internal, which have been conveniently denomi- 

 nated by the terms ectoderm and endoderm. The cavity of the sac, which will 

 be called the somatic cavity ^ contains a fluid, charged with nutritive matter in 



