84 LECTURE VII. 



inclosing another with thicker parietes, and within this there is a 

 small cavity. From the point where the two sacs coalesce above, 

 there projects a long spine, which is non-retractile. The seizing 

 organ consists of an obovate transparent sac, immersed in the 

 nodule with a small aperture. At the bottom of the sac, and within 

 it, there is a solid corpuscule, which gives origin to a calcareous 

 sharp sagitta or spine, that can be pushed out at pleasure, or with- 

 drawn until its point is brought within the sac. When the hydra 

 wishes to seize an animal, the sagittse are protruded, by which means 

 the surface of the tentacula are roughened, and the prey more 

 easily retained : Corda believes that a poison is at the same time 

 ejected. The nodules of the tentacula are connected together by 

 means of four muscular bands, which run up, forming lozenge-shaped 

 spaces by their intersections : these are joined together by transverse 

 bands. There is no communication between the tube of the tenta- 

 culum and the cavity of the body. The lip of the mouth is armed 

 with spines, similar to those of the tentacula ; but the rest of the body 

 is destitute of them. 



That the tentacula have the power of communicating some be- 

 numbing shocks to the living animals which constitute the food of the 

 Hydra, is evident from the effect produced, for example, upon an En- 

 tomostracan, which may have been touched, but not seized, by one 

 of these organs. The little active crustacean is arrested in the midst 

 of its rapid, darting motion, and sinks, apparently lifeless, for some 

 distance ; then slowly recovers itself, and resumes its ordinary move- 

 ments. These and other active inhabitants of fresh waters, whose 

 powers should be equivalent to rend asunder the delicate gelatinous 

 arms of their low-organised captor, do, nevertheless, perish almost 

 immediately after they have been seized, and so countenance the 

 opinion of Corda of the secretion of a poison ; unless, indeed, the 

 little polype may have the power of communicating an electric shock. 



The most extraordinary properties of the Hydra are, however, 

 those which best accord, and might be expected to be associated, 

 Mdth its low and simple grade of organisation ; although they excited 

 the greatest astonishment in the physiological world when first 

 announced by their discoverer, Trembley, and are often still called 

 wonderful. 



If a polype be transversely bisected, both halves survive ; the 

 cephalic one developing a terminal sucker, the caudal one shooting 

 forth a crown of tentacula ; each moiety thus acquiring the characters 

 of the perfect individual. But in a healthy and well-fed Hydra, the 

 same phenomena will take place if it be divided into ten pieces. The 

 Hydra, notwithstanding the want of a nervous centre thus indicated, 



