528 MOLLUSCOUS ANIMALS GENERALLY. 



number of minute calcareous particles (named otolithes or ear- 

 stones), which are kept in a state of continual movement by the 

 action of cilia lining the vesicles. This " wonderful spectacle," 

 as it was truly designated by its discoverer Siebold, may be 

 brought into view without any dissection, by submitting the 

 head of any small and not very thick-skinned Gasteropod, or 

 the young of the larger forms, to gentle compression under the 

 Microscope, and transmitting a strong light through it. The 

 very early appearance of the auditory vesicles in the embryo Gaste- 

 ropod has been already alluded to ( 350). Those who have the 

 opportunity of examining young specimens of the common Pecten, 

 will find it extremely interesting to watch the action of the very 

 delicate tentacles which they have the power of putting forth 

 from the margin of their mantle, -the animal being confined in a 

 shallow cell, or in the zoophyte-trough ; and if the observer 

 should be fortunate enough to obtain a specimen so young that 

 the valves are quite transparent, he will find the spectacle pre- 

 sented by the ciliary movement of the gills, as well as the active 

 play of the foot (of which the adult animal can make no such 

 use), to be worthy of more than a cursory glance. 



^355. Ohromatophores of Cephalopoda. Almost any species of 

 Cuttle-fish (Sepia) or Squid (Loligo) will afford the opportunity of 

 examining the very curious provision which their skin contains 

 for changing its hue. This consists in the presence of numerous 

 large "pigment- cells," containing coloring matter of various 

 tints ; the prevailing color, however, being that of the fluid of 

 the ink-bag. These pigment-cells may present very different 

 forms, being sometimes nearly globular, whilst at other times 

 they are flattened and extended into radiating prolongations ; 

 and, by the peculiar contractility with which they are endowed, 

 they can pass from one to the other of these conditions, so as to 

 spread their colored contents over a comparatively large surface, 

 or to limit them within a comparatively small area. Very com- 

 monly there are different layers of these pigment-cells, their con- 

 tents having different hues in each layer ; and thus a great variety 

 of coloration may be given, by the alteration of the form of the 

 cells of which one or another layer is made up. It is curious 

 that the changes in the hue of the skin appear to be influenced, 

 as in the case of the Chameleon, by the color of the surface with 

 which it may be in proximity. The alternate contractions and 

 extensions of these pigment-cells or " chromatophores" may be 

 easily observed in a piece of skin detached from the living animal, 

 and viewed as a transparent object; since they will continue for 

 some time, if the skin be placed in sea-water. And they may 

 also be well seen in the embryo Cuttle-fish, which will some- 

 times be found in a state of sufficient advancement, in the 

 grape-like eggs of these animals attached to sea-weeds, zoo- 

 phytes, &c. 



