CHAP. II.] THE BLOOD. 129 



SECT. 7. CHARACTERS PRESENTED BY THE BLOOD OF 

 INVERTEBRATE ANIMALS. 



It has already been stated that with very few exceptions *, the 

 blood of all vertebrate animals is characterized by the possession of a 

 red colour which is due to the presence within it of coloured 

 corpuscles, which in all classes but one (that of the Mammalia) are 

 nucleated. In addition to the coloured corpuscles, we have seen that 

 the blood always contains a much smaller number of colourless cells, 

 consisting of nucleated masses of protoplasm, endowed with con- 

 tractility, and presenting many of the essential features of independent 

 elementary organisms, and it has been incidentally remarked that 

 there appears to be a much greater uniformity in the shape and size 

 of the colourless than of the coloured corpuscles of the blood of 

 different classes of Vertebrates. 



When we pass from the vertebrate to the invertebrate sub-king- 

 doms we find that in all those organisms in which a differentiated 

 blood-vascular system exists, the contained liquid presents floating in it 

 nucleated masses of protoplasm closely resembling the colourless cells 

 of vertebrate blood, but is generally, though not invariably, free from 

 all representatives of the coloured corpuscles. In the immense majority 

 of invertebrate animals this intra-vascular liquid is colourless, or 

 presents a yellowish tint, though in a small minority it is coloured red, 

 or green, or blue. Generally, however, the colour is diffused through 

 the liquor sanguinis if it is not actually dissolved in it. 



In the colourless liquid contained in the vascular system of most 

 Invertebrates, we have probably a liquid which discharges only one 

 half of the functions of the vertebrate blood which serves merely as a 

 common medium, supplying liquid and solid matters to the various 

 tissues and organs, and washing away from them products of waste 

 and decay, which it discharges through the agency of, or at, the various 

 excretory organs. The other half of the functions of the vertebrate 

 blood, the respiratory,, are probably scarcely represented by the 

 colourless blood of Invertebrata. 



Such blood possesses, probably, no arrangement whereby the 

 oxygen of the medium external to the body can be stored up by it, 

 at certain points, to be carried away to tissues and organs far removed 

 from that medium and then given up. 



The respiratory exchanges in creatures provided with such blood 

 probably take place by processes of diffusion directly between the 

 tissues of the organism and the medium which it inhabits, and 



1 It is a matter of dispute whether the blood corpuscles of Amphioxus contain 

 haemoglobin. According to Bay Lankester they do not. In Leptocephalm we have at 

 any rate a fish whose blood is certainly free from haemoglobin. (Lankester: "A 

 Contribution to the Knowledge of Haemoglobin. " Proceedings of Royal Society, Vol. xxi. 

 (1872) p. 71 et seq. 



G. 9 



