BRANCHIFEROUS MOLLUSCA. 



271 



Fig. 44. 



of functions. This is one of the most remarkable and most 

 beautiful of the Gasteropods. The body glows with a fine 

 cerulean blue colour, which 

 deepens in hue towards the 

 ends of the fringes of its 

 ptero-branchiae ; the centre 

 of the back is of pearly 

 whiteness, bordered with a 

 line of deep blue ; and the 

 sides are adorned with an 

 interrupted series of fan- 

 like laciniated gills, by aid 

 of which, as I have said, it 

 swims reversed at the sur- 

 face of the Mediterranean 

 Sea, in numerous swarms.* 

 But, generally, the ex- 

 ternal branchiae are distinct 

 and independent organs. 

 Of the Pteropoda, almost Glaucus hexapterygius, copied from 

 each genus presents them uvler ' 



under some new modification in form, or structure, or 

 position, " as Nature in them strove to show variety." 

 Thus, in the Pneumodermon (Fig. 45), they 

 are placed nearly on the posterior extremity 

 of the body, which is naked, and resemble 

 two Cs placed back to back in this manner, 

 oc, united by a little transverse bar across 

 the middle, or at each end, the lines being 

 garnished with a number of regular promi- 

 nent leaflets of minute size. In the Hyales 

 again, the branchiae are pectinated, and lie 

 concealed in a space between the lobes of the Pneumodermon 

 cloak, to which the water gains admission by copied from Cuvie'r. 

 certain fissures on the sides of the shell; 

 while, to make, as it were, the dissimilarity perfect, they 

 appear, in the genus Cuvieria of Rang (Fig. 46), in the form 



*An early, but anonymous, notice of this mollusk is worth quoting : 

 " The mid-line of the back part appeared through a common magnifier like 

 a single leaf, and was in continual undulating motion, either from the 

 muscles or circulation of juices. Two side lines, extending the whole crea- 

 ture's length, and ending in one in the tail, of a deep blue. The fingers, or 

 tentacles, end in a deep blue ; a silvery cast intermingled with the blue over 

 the whole back, or upper parts, where the blue is lighter." Phil. Trans. 

 liii. 58. Mr. George Bennett has given a very interesting account of the 

 habits of the Glaucus in the Proc. of the Zool. Society, pt. iv. 1836, p. 

 113, 119. 



Fig. 4 5. 



