34<5 DIVERSIONS OF A NATURALIST 



I trust that none of my readers may suddenly faint 

 on reading this page, but should be glad to hear of any 

 experience of the kind. It is readily understood when 

 the profound impression produced by the colour of 

 man's blood is considered, that the great inquirer 

 Aristotle and a good many uninquiring people of the 

 present day should overlook the fact that the lower 

 animals have blood. The insects, crustaceans, mussels, 

 clams, snails, and cuttle-fish, and many worms have true 

 blood and a heart and blood-vessels, but in most of them 

 the blood is colourless, or of a very pale blue tint. Hence, 

 like the lymph described in the preceding chapter, it 

 escapes attention, and Aristotle called them all " blood- 

 less animals." The fact is, however, that not only do 

 they possess colourless or pale blue blood, but that the 

 bristle-footed worms (earth-worms and river- worms and 

 marine Annelids) and even the leeches possess bright 

 red blood contained in a complete branching network 

 of blood-vessels, whilst here and there among the other- 

 wise colourless-blooded molluscs and crustaceans and 

 insects we find isolated instances of the possession of 

 red blood. Thus the flat-coiled pond-snail, Planorbis, 

 has bright red blood, so have one or two bivalve clams, 

 so, too, has an insect larva (known to boys as a blood- 

 worm) that of the midge (Chironomus), so, too, have 

 some small fresh-water shrimps, and also a single species 

 of star-fish and one kind of sea cucumber ! 



I explained in the previous chapter that the blood of 

 the vertebrates may well be called hsemolymph, since in 

 them the colourless, slightly opalescent fluid called 

 " lymph " is continually poured through certain openings 

 into the red blood, and mixed with it. In the earth- 

 worm and other lower animals the red-coloured blood, or 

 its equivalent the " haema/' as distinguished from the 



