158 LECTURE IX. 



wastes of waters most remote from land, have always attacted the 

 attention of the navigator, who sometimes finds the surface of the 

 sea studded with these gelatinous gems, glistening by day with all 

 the brilliant hues of the rainbow, and betraying their course during 

 the night-season by the lambent phosphorescent light which they 

 dilFuse.* The Acalephcs are represented on our own coast by nume- 

 rous discoid and spheroid species, varying in size from an almost 

 invisible speck to a yard in diameter, and which, besides the ver- 

 nacular name above cited, are known as "sea-blubber," "jelly-fish," 

 or by the Linntean generic term " medusa." 



The form of many of the species is most typical of the great group 

 '^ Radiata " as characterised in the " Regne Animal," and they were 

 called by Lamarck f, on account of their tissue, "Radiaires Mollasses," 

 or soft Radiaries, in contradistinction to the hard-skinned " Radiaires 

 Echinodermes." Cuvier retained for them their ancient name of Aca- 

 lephcB, and he characterises the class as free-swimming gelatinous 

 animals, having a vascular system superadded to the digestive one, 

 although he admits that the former may be only a continuation of the 

 intestinal tubes ramified through the parenchyma of the body. J He 

 acknowledges, also, that the vascular system has not been demon- 

 strated in every species of the class ; and that those species in which 

 it cannot be shown to exist, can hardly be distinguished from the 

 Polypi. 



The division of this class has been founded on their mode of loco- 

 motion. There are some singular forms which float by means of 

 air-bladders, and in which the place of a stomach is supplied by 

 many hollow tentacles : the Acalephes of this order are called " Phy- 

 sogrades," and have sometimes been denominated Siphonophora. In 

 a second group locomotion is efiected by longitudinal series of 

 cilia, whence the name " Ciliogrades ; " these have a single central 

 mouth and a digestive cavity ; they have also been called Ctenophora. 

 The third order have also a central, but sometimes ramified, nutritive 

 sac ; and they swim by means of the rhythmical contractions of a 

 musculo-membranous disc, whence they have been called " Pulmo- 

 grades " and " DiscophoraP The species of the last order are those 

 that are most constantly met with in the seas washing our own coasts. 

 But some of the tropical forms of the other orders are occasionally 

 stranded on the south-western shores of England. I have picked up on 

 those of Cornwall the little Velella, which had been wafted thither, un- 



* The Mammaria scintillans has, perhaps, the greatest share in producing the 

 luminosity of the ocean. 

 . t CXXXVI. torn. ii. p. 450. _ t ^H. torn. iii. p. 274. 



