CHAP. II.] THE BLOOD. 135 



changes into a peculiar dark green. When a quantity of perivisceral fluid 

 containing corpuscles in the dingy brown state is placed in the vacuum 

 of the mercurial gas pump, it rapidly recovers its normal colour. Thus the 

 colouring matter of these corpuscles is readily oxidised and deoxidised, and 

 there is considerable probability that it may have a respiratory function. 

 However, on account of the small number of brown corpuscles in the fluids 

 of the Urchin, it is impossible to make satisfactory analyses of the evolved 

 gases by means of the blood pump, nor has any attempt to isolate the 

 pigment yet succeeded. 



That this brown substance is nearly related to the purple colouring 

 matter of the shell of many urchins, as well as to the yellowish-brown 

 (biliary 1) pigment of the intestinal epithelium, is made evident by adding a 

 mineral acid to their alcoholic solutions. All three immediately assume a 

 green tint, very similar to that of the integument of the dead Urchin. 

 Moreover, when a morsel of any of the highly pigmented tissues of 

 Spatangus purpureus, for instance, the ovary, is slightly torn with needles, 

 purple spots appear at the injured points, and, under the microscope, 

 the brown corpuscles may be watched, one by one changing into purple. 



Lemon-yellow amoeboid corpuscles are also found, though sparingly, 

 in the fluids of certain of the regular Sea-urchins (Dorocidaris, Arbocia), and 

 are exceedingly abundant in the perivisceral fluid of the Spatangoidea. 



The greatest variety of colour is to be seen in the contents of the intes- 

 tinal vessels of /Spatangus, in a single preparation of which may be seen 

 brown, purple, green, lemon-yellow, and indigo-blue amoeboid corpuscles, 

 together with vast numbers of peculiar greyish vesicles of very variable size, 

 from that of a micrococcus up to more than that of a coloured corpuscle. 



