THE BLOOD OF MAGELONA. 3 
becomes foul the tint of the worm becomes fainter and fainter, 
till the thorax is white. I first observed this change while 
working upon worms sent from St. Andrews to me at Oxford. 
The worms had been put into clean sea water on their arrival, 
and had been put aside overnight. Next morning they were 
no longer madder-rose, but white. On pointing this out to 
Professor Lankester, he at once suggested aérating the water 
by squirting air into it; this I did, and soon the madder-rose 
tint reappeared. It was this small but extremely interesting 
change in colour which called our attention to the blood, for it 
was evident that we had to deal with a respiratory pigment of 
a kind unusual in the Chetopoda. 
The fact that the tint of the worm is due to contained blood, 
and not to any pigment in the skin, is readily recognised by 
the unaided eye when the alternating processes of eversion 
and retraction of the ‘ proboscis” or “introvert” is watched. 
So long as the introvert is at rest within the body the thorax 
is coloured; when eversion takes place the tint becomes quite 
faint (Pl. 1, fig. 2),—in fact, frequently the thorax becomes 
white. The blood in the thorax is contained in greatly dilated 
vessels, which block up and obliterate nearly the whole of the 
ceelom (fig. 3); the introvert is a hollow sac traversed by thin 
bundles of retractor muscles, the cavity of the sac being con- 
tinuous with the dilated vessels, so that on eversion nearly the 
whole of the blood in the thorax is driven into the introvert— 
this flow of blood is, of course, the cause of the eversion,—and 
the thorax is more or less completely deprived of its colour. 
The fact that the abdomen is not tinted by the blood to any great 
degree is due to the small size of the blood-vessels in this region. 
The arrangement of these blood-vessels has been minutely 
described by M‘Intosh, and need not detain us at present. 
But the remarks of this author, so far as they relate to the 
blood itself, may here be quoted :—‘‘ The blood is a coagulable 
pale rose-red fluid, containing numerous corpuscles. On 
being shed these group themselves in different clumps. The 
size of the spherules is nearly constant, although variations 
may occur. They exhibit molecular movement, and their 
