SOME OF THE WAYS IN WHICH ANIMALS BREATHE. 309 
This useful oxygen-carrying colouring matter is not, however, 
confined to the blood; like some others which we shall have to 
notice soon, it is to be found in some of the tissues; it was 
detected by Lankester in the muscles of the vertebrate heart, in 
the voluntary muscles of mammals, in the muscles which move 
the ‘‘tongue’’ (odontophor) of Gastropod Molluscs, and in the 
central nervous system of the Sea Mouse (Aphrodite aculeata). 
An account of its distribution in the nervous tissue of some 
of the Nemertine Worms has been given by Prof. Hubrecht, and 
its relations to the respiratory processes are here so obvious and 
so instructive that we shall do well to consider the question in 
detail. When I had the honour of addressing you last year I 
directed your attention to certain peculiarities in the structure 
of the Nemertine Worms, which exhibited a wonderful resemblance 
to what was seen in the Vertebrates; I told you then of the sacs 
or grooves which lay on either side of the head, and had the 
function of respiratory passages; these passages appear to be 
specially useful in carrying oxygen to the central brain-mass, 
or, in other words, they appear to act as parts of a cerebral 
respiratory apparatus. Now, in the division of the Schizo- 
nemertini, in which there is a deep lateral fissure on either side 
of the head, the nervous tissue is impregnated with hemoglobin, 
but the blood is free of it; in the Hoplonemertini, on the other 
hand, the blood becomes the chief oxygen-carrier, as is shown 
by its impregnation with hemoglobin ; the lateral fissures close 
up, and hemoglobin disappears from the nerve-tissue. The 
Schizonemertine arrangement appears therefore to be a temporary 
expedient for attracting oxygen to the parts which need it most; 
with this structural arrangement it is instructive to correlate the 
mud-dwelling habits of the worms that possess it. 
Let us turn now to other colouring matters than hemoglobin. 
Within the last few years, and especially owing to the researches 
of Dr. MacMunn, who has demonstrated that the active practising 
physician can aid considerably in the advancement of natural 
knowledge, our acquaintance with colouring matters other than 
hemoglobin has been very greatly increased. 
memoir on the blood of Crustacea (Journ. Physiol. vi. pp. 832—38). The 
statement in the text as to the presence of hemoglobin in special corpuscles 
is more correct, 
