112 THE ANATOMY OF INVERTEBRATED ANIMALS. 



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

 ary canal. 1 Between these two layers, a third layer the 



FIG. 13. Diagrams illustrative of the mutual relations of the Hydrozoa : 



1. Hydra. 2."Sertularian. 3. Calycophoridan. 4. Physonhoridan. 5. Lucernarian. 



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



P. Tentacles. N. Nect,ocalyx. T. Ccenosarc. B. Hydrophyllinm. C. Hydrothoca. S. 



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



Digestive and somatic cavity. 

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



gonophore. 



mesoderm which represents the structures which lie between 



1 " 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 



