Defence among certain Medusa. 349 
podidz have no cystons or similar excretory organs, nor has 
the function of excretion yet been referred in them to any 
special organs. Is it possible that the discharge of coloured 
matter from the pigment-cells of the bract of Agalma is also 
a method of excretion? and is it the same as that of the cystons 
of Horskalia? It seems to me improbable that we have to 
deal with excretions only in this case, although we may have 
an instance of a novel means of protection, which is in part 
accomplished by the discharge of the excretion in Forskalia. 
Upon this theory, however, we need much more light, which 
can best come from more observation. 
It is legitimate to conclude that the discharge of a highly 
coloured fluid by the scales of Agalma is iti part a means of 
protection for the Medusa, and it would seem natural to con- 
nect it with the function of excretion; but we know so little 
about the character of the excretions and the manner in which 
they are produced in Medusee, that at present we can hardly 
definitely ascribe the special function to these glands. Pos- 
sibly similar glands are found in other Physophores, and the 
excretion has not been recognized from the fact that it is not 
so highly coloured as in Agalma Claust and Forskalia. The 
discharge of this fluid from a living animal, if it take place 
without rupture of the wall of the scale, would imply special 
excretory openings somewhere on the bract; and one is 
tempted to search for such openings, if they exist, on the 
distal tip of the scale, when they would be homologous with 
the excretory openings known to exist on the bell-margin of 
certain Hydromeduse, as Metschnikoff and others have 
shown. 
If we accept the theory that the discharge of a coloured 
fluid is a method of defence, the question arises, How is that 
defence accomplished? Does the fluid darken the water in 
the immediate vicinity of the Medusa which possesses this 
power and in that way conceal it from its foes, as in the case 
of the Cephalopoda? or does it serve, as is possibly the case 
with the rattle of the rattlesnake, to warn away its enemies ? 
May it not even bewilder its prey and thus be rather a means 
of capturing its food than of self-protection? Has it possibly 
a poisonous nature fatal to its prey or foes? Our knowledge 
of its nature is all too small to give us an answer to these 
questions. Its bright colour would indicate that even if it is 
poisonous this is not its only property, or its sole function 
that of killing its enemies or prey. The ability to change the 
colour mentioned in Ceratocymba by Dr. Chun might come 
in the same category as a similar power in fishes and Cepha- 
lopoda. In that case we might have a kind of cutaneous 
Ann, & Mag. N. Hist. Ser. 6. Vol. iv. 2a 
