POLYPI. 



123 



resumes its ordinary movements. Siebold states that when a nais, 

 a daphnia, or the larva of a Cheironomus have been wounded by the 

 darts, but not seized, they do not recover, but die.* 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, seem paralysed almost immediately after they have been 

 seized, and so countenance the opinion of Corda that the secretion 

 of a poison enters the wounds. 



The most extraordinary properties of the Hydra are, however, 

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

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

 the greatest astonishment in the physiological world when first 

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

 wonderful. 



If a polype be partially bisected, each portion forms a perfect 

 polype : RoeselJ devotes three plates to figures of monstrous Hydrae, 

 the results of such incisions. If a Hydra 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, notwith- 

 standing the want of a nervous centre thus indicated, and the 

 absence of any hitherto recognised nervous filaments, manifests an 

 obvious predilection for light, and, when confined to a glass, always 

 moves itself to the brightest side. Trembley succeeded in inverting 

 these delicate animalcules, and retaining them inverted until they 

 accommodated themselves to this singular change in their condition. 

 The pigment or hepatic cells of the gastric membrane, now external, 

 are cast off* : the orifices, by which the cavities of the tentacula 

 communicate with the stomach, are exposed a little below the rim 

 of the mouth ; these orifices are soon obliterated, and new ones are 

 formed in communication with the new digestive sac. The ciliated 

 epithelium and hepatic-cells must be developed on what was the 

 outer surface : and the dart-cells in what before was the gastric 

 surface. At least Trembley assures us, and gives corroborative 

 evidence, that digestion was eff*ected as actively by the surface 

 which before was external, as by that which had been the digestive 

 surface ; whilst this as readily assumed the ordinary gemmiparous 

 function of the skin.§ 



* XXIV. p. 30. t CI. t CV. (Tab. Ixxx., Lxxxi., and Ixxxii.) 



§ J'ai retourne un nombre considerable de Polypes de la seconde espece 



