128 



ANTHOZOA HYDROIDA. 



Fig. 29. 



V-^?ipf^^ 





This consists of an obovate transparent sac, immersed in the nodule, 

 with a small aperture even with the surface. At the bottom of the 

 sac, and within it, there is a saucer-like vesicle, on whose upper 

 depressed surface is seated a solid ovate corpuscle, that gives origin 

 to, or terminates in, a calcareous sharp sagitta or arrow, that can be 

 pushed out at pleasure, or withdrawn till its point is brought within 

 the sac. (6.) When the Hydra wishes to seize an animal, the sagittce 

 are protruded, by which means the surface of the tentacula are rough- 

 ened, and the prey more easily retained ; and Corda believes that a 

 poison is at the same time injected — a conjecture offered to explain 

 the remarkable fact of the almost instant death of the prey. 



The nodules of the tentacula are connected together by means of 

 foru muscular fibres or bands, which run up forming lozenge-shaped 

 spaces by their intersections. These are the extensor muscles of the 

 tentaculum. {a.) They are again joined together by transverse fibres, 

 which Corda believes to be adductor muscles, and to have also the 

 power of shortening the tentacula. But it may be doubted whether 

 this muscular apparatus is of itself sufficient to eflfect the wonderful 



