942 MOLLUSCA AND BRACHIOPODA 



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. 1 



Chromatophores of Cephalopods. Almost any species of cuttle- 

 fish (8epia) 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 colouring matter of various tints, the prevailing colour, 

 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 coloured contents over a comparatively large 

 surface, or to limit them within a comparatively small area. Very 

 commonly 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 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 colour 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 sometimes be found in a state of sufficient advance- 

 ment in the grape-like eggs of these animals attached to sea-\veeds, 

 zoophytes, &c. 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 gelatinous masses which are attached to seaweeds, 

 zoophytes, &c. ; and their embryos, when near maturity, are ex- 

 tremely beautiful and interesting objects, being sufficiently trans- 

 parent 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.' 2 



1 Much valuable information concerning the sensory organs of molluscs will be 

 found in Dr. H. Simroth's memoir, ' Ueber die Sinneswerkzeuge unserer einheimi- 

 schen Weichthiere,' Zeitschr. fur iviss. Zb'ol. xxvi. p. 227. 



- For further information regarding the chromatophores see an essay by Dr. 

 Klemensiewicz in the Sitzungsberichte of the Vienna Academy, vol. Ixxviii. p. 7, 

 and Krukenberg, Vergl. physiol. Studien, 1880. 



The following works and memoirs on the Mollusca generally may be consulted by 

 the student : S. P. Woodward, A Manual of the Mollusca, 3rd ed. London, 1875 ; 

 Keferstein, in Bronn's Klassen und Ordnungen des Thierreichs ', the article ' Mollusca,' 

 by Professor Bay Lankester, in the 9th edition of the Encyclopedia Britannica ; 

 M. P. Fischer's Manuel de Conchyliologie, Paris, 1881-87 ; and the Rev. A. H. 

 Cooke's volume in the Cambridge Natural History ; as well as the numerous reports 

 on the Mollusca collected by H.M.S. Challenger. 



