SOME OF THE WAYS IN WHICH ANIMALS BREATHE. 3138 
have soon much further information. For us the great point is 
that they are all respiratory in function. 
The red coloration of the body of many marine animals 
appear to be associated with yet another colouring matter of 
respiratory significance ; to this Wurm, who first observed it in 
those red parts on the beaks of Pheasants and other birds, 
which ornithologists call the ‘“‘rosette,”’ gave the name of 
tetronerythrin; Merejkowsky has found it in 104 species of 
Invertebrates and fishes, and he tells us that it is often present 
even when the animals appear to be yellow, green, or brown in 
colour ; it is chiefly found in the tissues that come in contact 
with the water, and it is important to observe that it is absent 
from those forms which have symbiotic alge providing them 
with oxygen. Dr. Halliburton has found it in the blood of 
Crustacea. The function of this colouring matter is quite 
possibly respiratory, but, as it does not lose and take up oxygen, 
it cannot be compared with hemoglobin or hemocyanin. 
We have now to consider the ways in which the oxygen in the 
air or in the water makes its way to the tissues, or to special 
organs set apart for respiratory purposes; these ways may be 
ad erouped under three heads :— 
. The simplest and the earliest arrangement is that which 
has en called vague. Here there are no special respiratory 
organs, and in the simplest cases even respiratory colouring 
matters are wanting. The simple Ameba, and the Protozoa 
generally, the lower worms, such as Planarians and others, 
afford us examples of forms in which the whole body, being 
bathed in water (or rarely in air), the carbonic acid of the cell 
or cells makes exchange for oxygen on the general surface of the 
body; in the Starfish, where respiratory pigments appear to be 
present, the thin membrane which lines the body cavity sends 
out a number of delicate processes between the interstices of the 
body-wall; these multiply the opportunities for gaseous exchange 
between the fluid within and the sea-water without the mem- 
brane, and so carbonic acid constantly escapes and oxygen as 
constantly enters. 
In the earthworm, again, respiration takes place over the 
whole surface of the body, but the blood-vessels come very near 
to the surface ; in the leech the general arrangements are much 
the same, save that the blood-vessels are still more superficial. 
