340 DIVERSIONS OF A NATURALIST 



conjunction of hfema and lymph, which in the more 

 primitive earth-worm and sea-worm have neve effected 

 a junction ! In some closely allied marine worms, how- 

 ever, a junction of these two is effected in another way. 

 We know that in the Vertebrates the red blood corpuscles 

 are formed by detached bits of the same tissue, which 

 becomes converted into capillaries, the finest blood- 

 vessels. Now in several marine Chaetopods or bristle- 

 footed worms (Glycera, Capitella, etc.) the tissue which 

 should form the blood-vascular system and its red liquid 

 blood, changes its mode of growth ; it never forms blood- 

 vessels at all, but divides into free red (haemoglobinous) 

 cells or red blood corpuscles, which float in the lymph of 

 the coelom. There is no blood-vascular system produced 

 in these worms, but the " cells " of the tissue which would 

 in other worms form blood-vessels break up into red 

 corpuscles, which, mixing with the lymph, bring it into 

 the condition of " haemolymph," identical with the blood 

 of Vertebrates ! 



In the molluscs, snails, whelks, oysters, clams, and 

 cuttle-fishes there is a further variation. The same two 

 fluids and two systems of spaces are present as in the 

 earth-worm, but the ccelomic space and fluid have been 

 nearly blocked up and obliterated by the swelling-up and 

 great size of the proper haemal vessels. Only in rare 

 cases is the blood of molluscs coloured red by haemo- 

 globin, usually it is of a pale blue colour. There is still 

 left a pericardial coelom, a space around the heart, 

 and from this some fine lymph-holding vessels ramify 

 amongst the tissues, but the chief spaces in the body 

 are dilated parts of the true haemal system. In Insects 

 and Crustacea (say cockroach and lobster) this process is 

 carried still further. The great coelom, so well developed 

 in the Cha^topod worms, and the Sea-urchins and 



