Colouration in the Invertebrata. 



51 



SYSTEMS OF COLOURATION. 



Protozoa. The protozoa are generally very minute, and always 

 composed of structureless protoplasm. Their peculiarities are rather 

 negative than positive, there being neither body segments, muscular, 

 circulatory, nor nervous systems. Even the denser exterior portion 

 (ectosarc) possessed by some of them seems to be rather a temporary 

 coagulation of the protoplasm than a real differentiation of that 

 material. 



Here, then, we have to deal with the simplest forms of life, and if 

 colouration depends upon structure, these structureless transparent 

 creatures should lack all colour-pattern, and such is really the case. 

 Possessing no organs, they have no colouration, and are generally 

 either colourless or a faint uniform brown colour, and through their 

 colourless bodies the food particles show, often giving a fictitious 

 appearance of colouring. 



To this general statement there is a curious and most telling ex- 

 ception. In a great many protozoa there exists a curious pulsating 

 cell-like body, called the contractile vesicle, which seems to be a 

 rudimentary organ, whose function is unknown. Here, then, if 

 anywhere, traces of colouring should be found, and here it is 

 accordingly found, for, though generally clear and colourless, it 

 sometimes assumes a pale roseate hue. This may be deemed the 

 first attempt at decoration in the animal kingdom, and it is directly 

 applied to the only part which can be said to possess structure. 

 Beautiful examples are plentiful in Leidy's magnificient volume on 

 Freshwater Rhizopods. 



Ccelenterata. These animals fall into two groups, the Hydrozoa, 

 of which the hydra and jelly-fishes are types, and the Actinozoa, of 

 which the sea-anemonies and corals are types. Most of the coelen- 



