112 



THE ANATOMY OF INVERTEBRATED ANIMALS. 



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

 ary canal. ^ 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. Physophoridan. 5. Lucernarian. 



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



P. Tentacles. N. Nectoealyx. T. Ccenosarc. B. Hydropliylliura. C. Hyd roth oca. S. 



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



Digestive and somatic cavity. 

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



gonophore. 



mesoderm — which represents the structures which lie between 



1 " The body of every Ilydrozoon 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 tlie sac, which will 

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



