CNIDABIA—BODY EPITHELIUM 



81 



can be compared with the cesophageal cavity of the Ctaiophora. The further homo- 

 logies (or analogies ?) in the gastro-eanal system and in the tentacles then follow of 

 themselves. 



II. The Body Epithelium. 



It is not possible to carry out a sharp histological distinction be- 

 tween the ectodermal epithelium which clothes the whole exterior of 

 the Cnidarian body and the endodermal gastro-canal epithelium. We 

 here find the ectoderm and the endoderm, histologically, still in a 

 rather undifferentiated condition ; this is seen most clearly from the 

 fact that the endoderm can in some groups supply nerve and muscle 

 elements and stinging cells. These are tissue elements, which among 

 the higher animals arise almost exclusively out of and in the ectoderm. 



The close connection of the body epithelium of the Cnidaria with 

 the nervous and muscular systems is characteristic. The cell-elements 

 of these systems often take part with the other ectoderm cells in limit- 

 ing the outer surface of the body ; or they lie wedged in between 

 them somewhat below the surface. Finally, we find them in many 

 Cnidaria close under the body epithelium, but often still outside of the 

 supporting membrane which divides the ectoderm from the endoderm. 



The body epithelium is either naked, or may be covered with cilia 

 or flagella over greater or smaller ex- 

 panses. The swimming or rowing 

 plates, which are arranged in eight 

 meridional rows in the Cienophwa, arise 

 out of cilia cemented together. 



In the body epithelium of all 

 Onidarict, except the Ctenophora, the 

 stinging cells are found as a very 

 characteristic element (Fig. 69, a, h). 

 These stinging cells contain a stinging 

 capsule, with a spirally coiled filament, 

 often bearing barbed hooks ; when the 

 skin is irritated this filament is evagin- 

 ated and shot forward, and has the 

 effect of a sting. 



The stinging capsules or nematocysts are 

 microscopic adhesive organs, and are to the 

 Cnidccria at the same time weapons of defence 

 and of offence. They are particularly numerous 

 in exposed parts of the body, and in organs 

 which are used in seizing prey — round the 

 mouth, on the tentacles, at the margin of the 

 disc in the Medusce. On the tentacles of 

 Hydro- and Scypho-medusce, and especially on the capturing filaments of the 

 Siphonophora, they are assembled in masses, and form "stinging knobs " or " stinging 

 batteries. " 



VOL. I G 



Fig. 69.— a, 6, Stinging cells of a Hy- 

 arold, Corailophora, after F. E. Sohulze. 

 c, Seizing or adhesive cells of a Cteno- 

 phora, after Chun. 



