20 Colouration in Animals and Plants. 



indeed, they have even been considered to be the lowest type of 

 Vertebrates. This is owing to the possession of a hard axial organ, 

 occupying much the position of the backbone, and is the well- 

 known cuttle-bone. Now, these animals are peculiar amongst their 

 class, from possessing, very frequently, an axial stripe. We thus 

 see clearly that the dorsal stripe is directly related to the internal 

 axial skeleton. 



Turning now to the invertebrata, we are at once struck with the 

 entire absence of the peculiar vertebrate plan of decoration ; and find 

 ourselves face to face with several distinct plans. 



From a colouration point of view, we might readily divide the 

 animal kingdom into two classes, marked by the presence or absence 

 of distinct organs. The first of these includes all the animals except 

 the Protozoa the lowest members of the animal kingdom which 

 are simply masses of jelly-like protoplasm, without any distinct 

 organs. 



Now, on our view, that colouration follows structure, we ought 

 to find an absence of decoration in this structureless group. This is 

 what we actually do find. The lowest Protozoa are entirely with- 

 out any system of colouring ; being merely of uniform tint, generally 

 of brown colour. As if to place this fact beyond doubt, we find in 

 the higher members a tendency to organization in a pulsating 

 vesicle, which constantly retains the same position, and may, hence, 

 be deemed an incipient organ. Now, this vesicle is invariably tinged 

 with a different hue from the rest of the being. We seem, indeed, 

 here to be brought into contact with the first trace of colouration, 

 and we find it to arise with the commencement of organization, and 

 to be actually applied to the incipient organ itself. 



Ascending still higher in the scale, we come to distinctly organ- 

 ized animals, known as the Ccelenterata ; of which familiar examples 

 are found in the jelly-fishes and sea anemonies. These animals are 

 characterized by the possession of distinct organs, are transparent, 

 or translucent, and the organs are arranged radially. 



No one can have failed to notice on our coasts, as the filmy jelly- 

 fishes float by, that the looped canals of the disc are delicately 

 tinted with violet ; and closer examination will show the radiating 

 muscular bands as pellucid white lines ; and the sense organs fring- 

 ing the umbrella are vividly black the first trace of opaque colour- 

 ation in the animal kingdom. 



These animals were of yore united with the star-fishes and sea- 



