58 Colouration in Animals and Plants. 



inflowing system of water and an out-flowing system of effete 

 sewerage quite uncontrolled, and, alas, generally quite unheeded by 

 the individuals whose wants are so supplied ; so in the sponges we 

 have a system of inflowing food-bearing water and an out-flowing 

 sewage, or exhausted-water system. This is brought about by a 

 peculiar system of cilia-lined cells which, as it were, by their motion 

 suck the water in, bringing with it the food, and an efferent system 

 by which the exhausted liquid escapes. These cilia-lined cells are the 

 first true organs that are to be found in the animal kingdom, and 

 according to the views we hold, they ought to be emphasized with 

 colour, even though their internal position renders the colouration 

 less likely. This we find actually to be the case, and these flagel- 

 lated cells, as they are called, are often the seat of vividest colour. 



The animal matter, or sarcode, or protoplasm of sponges falls 

 into three layers, just as we find the primitive embryo of the highest 

 animals ; and just as the middle membrane of a mammalian ovum 

 develops into bone, muscle and nerve, so the middle membrane 

 (mesosarc) of the sponges develops the hard skeleton, and in this 

 most important part we find the colour cells prevail. Sollas, one of our 

 best English authorities upon sponges, writes, " The colours of 

 sponges, which are very various, are usually due to the presence of 

 pigment granules, interbedded either in the endosarc of the flagellated 

 cells, or in the mesodermic cells, usually of the skin only, but some- 

 times of the whole body." * 



We can, then, appeal most confidently to the protozoa as illus- 

 trating the morphological character of colouration. 



* Sollas. Spongidse. Cassell's Nat. Hist. Vol. vi., p. 318. 



