96 NATURAL SCIENCE. Feb.. 



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than do red muscles, for the contraction of the latter is prolonged 

 and sustained. This is in itself a curious fact, for the theory of 

 increased oxidation in the red muscles would lead one to expect that 

 such muscles would contract more rapidly and not more slowly than 

 the pale. It may, of course, be said that the haemoglobin provides a 

 store of oxygen which is used up during the long-continuing con- 

 traction of the red muscles, but it is questionable whether the in- 

 creased blood-supply to such muscles is not in itself sufficient for this 

 purpose. In the rabbit, indeed, according to Meyer, the association 

 between the colour and the other characters is not very close, some 

 red muscles corresponding in character to the ordinary type of pale 

 muscle. In this connection, we may note the well-known fact that 

 insects possess two kinds of muscle, distinguished both by colour and 

 by histological peculiarities; in this case, the extensive development 

 of the tracheal system forbids the idea of the presence or absence of 

 a special respiratory pigment. 



Again, in fishes the red muscles are frequently those connected 

 with the skin, and are certainly not remarkable for great activity. 

 Krukenberg (7) suggests that in this case the haemoglobin is of special 

 importance in the respiration of the muscle, and is necessary on 

 account of the superficial position of such muscles and the slowness of 

 the circulation in fishes. In the fish Luvarus, however, he remarks 

 that, besides the pale muscles and the red, there are muscles of an 

 intermediate shade which he calls " halbroth." Are we to suppose 

 that these muscles have only half as great a need of oxidation as 

 those which are bright red ? This question of the quantity of 

 haemoglobin is very important in the consideration of function. 



As to haemoglobin in invertebrates, we have the same hypothesis 

 and the same difficulties. It is said that haemoglobin is especially 

 necessary to Lnvibricus on account of its peculiar habitat ; that its 

 presence in the head-slits of nemerteans is essential for the oxidation 

 of the brain ; that it is present in the muscles of the buccal mass of 

 Littorina because these muscles are especially active ; and so on. On 

 the other hand, many large marine worms have no haemoglobin, what- 

 ever their habitat, and many gasteropods have none in their buccal 

 muscles. Can we suppose that these muscles are less active in the 

 limpet, the snail, and many others than in Littorina ? Again, the 

 amount of haemoglobin present in the tissues of invertebrates seems 

 to be usually small, so that the amount of oxygen which it can take 

 up must likewise be small. Not that haemoglobin in invertebrates 

 has no function, but there is not much evidence to support the 

 view that it is invariably of supreme importance to the organism 

 in which it occurs. If this were so, we should surely find that 

 haemoglobin, when once acquired by the members of a group, would 

 be retained by all their descendants, however widely they might 

 diverge in other respects; and the irregular distribution of haemoglobin 

 among the invertebrate groups is contrary to this supposition. 



