ON WANDERING CELLS IN ECHINODERMS. 101 
organ itself the strands of similar tissue in continuity with it 
must also be included (hzmal system). It is obvious that if 
pigment is brought up to the skin faster than it is extruded 
through that structure the surface of the animal will become 
coloured ; but before entering into this more fully it might be 
as well to point out the various means by which pigment is 
eliminated in animals generally, for of course the method by 
the interference of amceboid cells is not the only one which 
occurs in the animal kingdom. 
Pigment (i) may be got rid of in soluble form, e.g. bilirubin, 
biliverdin, &c.; (11) it may be extruded in solid form through 
the special excretory organs; thus in a large proportion of 
animals the excretory organs contain considerable quantities of 
pigment—cf. the nephridia of molluscs, of Capitellide (Hisig, 
27), &c. Here it is interesting to note that in man there is 
some close association between certain pigmentary bodies and 
uric acid (“‘ cayenne pepper coloured crystals”). Or, again (111), 
it may accumulate in organs which have no connection with 
the exterior, where by being deposited it is removed from the 
circulation, e.g. the uratic tracts in Tanais (Claus, No. 24), 
the coxal glands (red brick glands) of Limulus and Scorpio ; 
moreover (iv) it may accumulate as a result of the degeneration 
of an organ, ontogenetically, and remain in the remnants 
thereof, at any rate for a time (Gaskell, No. 32); or lastly (v), 
and this is the method that concerns us here, it gets excreted 
through the free surface by the intervention of amceboid cells. 
It has already been pointed out that pigmentation will ensue 
if pigment-granules are brought to the epidermis more rapidly 
than they can be got rid of, and many instances (Vertebrates, 
Spatangids, &c.) have been cited where this does take place. 
That pigmentation may and does arise through the use of effete 
matters is an excellent illustration of the economical way in 
which the material at the disposal of an animal is used. After 
pigment has done its work in respiration, some part of it be- 
comes used again to colour the individual, and thus help it in 
the struggle for existence ; but cutaneous pigmentation is by 
no means always due to effete respiratory pigment. Thug 
