308 THE ZOOLOGIST. 
This power of carrying oxygen may be made clearer to you 
if I remind you that the same property is possessed by nitric 
oxide, and is made use of in the manufacture of sulphuric acid, 
Sulphurous acid (SO?) acting on nitrous acid in the presence of 
steam forms sulphuric acid and nitric oxide. The nitric oxide, 
if put in contact with moist air, takes up oxygen from it, and 
forms again nitrous acid.* In this way a small quantity of nitric 
oxide helps to make a large quantity of sulphuric acid. 
If I put this shortly, I shall put it in the way I particularly 
wish you to remember to-day. Hemoglobin exists in two states 
of oxidation. If this be true of hemoglobin it is, no doubt, true 
of other colouring matters, which with similar tests give different 
absorption-bands ; and, if this be true, then is the converse true 
also. Colouring matters which exist in two different states of 
oxidation are respiratory agents. Before, however, we pass to 
the consideration of other colouring matters, let us first review 
our knowledge of hemoglobin. 
Aristotle’s division of animals into those with and those 
without blood has long since ceased to recommend itself to 
naturalists, but until 1872 they were altogether ignorant of the 
distribution of hemoglobin outside the division which is called 
that of Vertebrates, and to which alone Aristotle assigned the 
possession of blood. In that year Prof. Ray Lankester gave an 
account of his researches, which had extended over more than 
four years, on the distribution of hemoglobin in Invertebrates ; 
additions of various forms have since been made, but only one 
important addition, that of a representative of a different sub- 
kingdom. lLankester found the colouring matter in various 
marine and fresh-water worms, in molluscs, and one larval 
insect; in 1880 M. Foettinger reported its presence among the 
Echinodermata, for he found it in the water-vascular system of 
the Ophiurid, Ophiactis virens ; lately Mr. W. H. Howell has found 
it in red discs in the water-vascular system and the body-cavity of 
a Holothurian. In all but a few of these cases (Solen, Arca, Gly- 
cera, Capitella, Phoronis, Thalassema, Hanungia) the hemoglobin 
is not contained in special corpuscles as it is in ourselves and 
other Vertebrates, but it is dissolved in the fluid blood-plasma.t 
* $O2-+ N203-++ H?O—H?SO0!-+ 2NO. 2NO-+ O=N?0°. 
+ The student interested in the subject will find a list of the animals in 
which hzmoglobin has been found in Dr. W. D. Halliburton’s interesting 
