Royal Physical Society. 199 



about a diameter and a half. In shape they resemble some- 

 what a grain of wheat, being respectively flattened and slightly 

 rounded on opposite sides. They are contained within small 

 cells in the ectoderm, and possess dense coats, which, under 

 pressure, burst at one extremity, and protrude a four-barbed 

 dart, from the point of which projects a long stiff thread or 

 hair (fig. 5). These thread-cells, found extensively in all 

 classes of polyps and acalephs, are, I have no doubt, true or- 

 gans of offence, as I have detected numbers of them plunged 

 beyond the barbs in the soft bodies of worms, rescued from 

 the grasp of Hydra vulgaris. But it is to be remarked, that 

 they frequently exist in situations where their mischievous 

 properties are rendered innocuous by the dense coverings un- 

 der which they lie hid. Thus we find them congregated in 

 masses on the polypary, beneath the thick corallum of Coryne 

 glandulosa (Dalyell) ; and in Halecium Beanii, while the 

 naked polyps are armed with unbarbed thread-cells of the most 

 minute and simple kind, these bodies attain a giant size and 

 formidably spinous development of the thread beneath the hard 

 coverings of the reproductive capsules and the corallum. 



The Polyps. 



17. The polypoid appendages of Hydractinia are of five 

 kinds (fig. 1) : — 



1. The Alimentary Polyps, possessing mouth and tenta- 



cles (c). 



2. The Reproductive Polyps, destitute of mouth, and having 



only rudimentary tentacles (b). 



3. The Sessile Ovisacs and Spermsacs of the Polypary (d). 



4. The Ophidian, or Spiral Polyps (a). 

 5. The Tentacular Polyps, or great Tentacles of the Poly- 

 pary (d). 



18. The Alimentary Polyps. — These organs, which repre- 

 sent the prehensile and digestive systems of the zoophyte, first 

 make their appearance as a thickening of the endoderm in one 

 of the canals of the polypary, attended with a copious deposit 

 of red granular matter. This thickening in a few hours rises 

 from the surface in the form of a fleshy stem, on the summit 

 of which an oral aperture presently appears surrounded by 



