MOLLUSCOUS ANIMALS GENERALLY. 191 



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 Gasteropod has been already alluded-to ( 582). 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 presented by the ciliary movement of the gills, as well 

 as the active play of the foot (of which the adult can make no such use), 

 to be worthy of more than a cursory glance. 



588. Chromatopliores of Cephalopods. 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/ contain- 

 ing 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 con- 

 tents over a comparatively-large surface, or to limit them within a com- 

 paratively small area. Very commonly there are different layers of these 

 pigment-cells, their contents having different hues in each layer and thus 

 a great variety of coloration may be given, by the alteration in 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 ob- 

 ject; 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 sometimes be found in a state of sufficient advancement in the grape- 

 like eggs of these animals attached to Sea- weeds, Zoophytes, etc. The 

 eggs of the small cuttle-fish termed the Sepiola, which is very common 

 on our southern coasts, are imbedded, like those of the Doris, in gelati- 

 nous masses, which are attached to Sea-weeds, Zoophytes, etc. ; and their 

 embryoes, when near maturity, are extremely beautiful and interesting 

 objects, being sufficiently transparent to allow the action of the heart to 

 be distinguished, as well as to show most advantageously the changes 

 incessantly occurring in the form and hue of the * chromatophores. 7 



