112 



THE ANATOMY OF INVERTEBRATED ANIMALS. 



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

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



Fig. 13.— Diasramfi illustrative of the mutual relations of the Hyrlrozoa : 



1. Hydra. 2.~^Sertularian. 3. Calycophoridau. 4. Physaohoridan. 5. Lucernarian. 



«. Ectoderm, b. Endoderm. c. The dige;<tive and somatic cavity. 



P. Tenfacles. N. Nectocalyx. T. Ccenosarc. B. Hydroi)hyllium. C. Hydrothfca. S. 



Hydranth. G Gonophore, A. Air- Vesicle contained" in P. Pneninatophore. c, 



Disrestivc 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 the sac, which will 

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



