PHYSIOLOG\ OF THE INVERTEBRATA. 133 



nature has not been determined. The corpuscles of Phyto- 

 l)hagous blood are amcoboid. 



Coaijubitioii. — " The blood clots after a very variable period 

 of time, but generally darkens in about five minutes, 

 ultimately forming a solid black clot which is due to oxida- 

 tion. If blood be sealed in a tube, the small quantity of 

 oxygen present will form a thin black film on the surface of 

 the blood, and the action then ceases." Mr. Poulton has 

 shown " how blood can be kept indefinitely without clotting 

 in a section of tube with a cover-glass over one end, and the 

 other cemented to a glass slide." He has kept " the blood of 

 Pij'jcvi'ii Bucephalus in this way for a month, quite unchanged, 

 and on then breaking off part of the cover-glass a thick 

 black crust was formed on the surface, while the blood 

 beneath became translucent instead of clear and transparent. 

 On removing the crust a second thin one was formed, but on 

 removing this, no further coagulation 'took place. If in 

 sealing up blood, or placing it in a tube section, a bubble of 

 air is accidentally included, coagulation takes place round 

 the bubble, but not elsewhere. This black substance is the 

 normal clot, for the injured places on larv^ which have 

 healed are always black, notably the horns of SpMnx larvie 

 which have been nibbled ofE by others of the same species. 

 The coagulation takes place after the addition of water, or of 

 a saturated solution of neutral salt (sodium sulphate). 

 The occurrence of a reducing agent in the blood appears to 

 be very remarkable, but it is possible that the substance is 

 capable of again yielding up its oxygen, and so acting as a 

 carrier. It has been observed that if fresh blood be added to 

 that which is turning black on the surface, the black clouds 

 are redissolved. If this be not so, it is difficult to see how 

 the blood can be the internal medium for the supply of 

 oxygen in these animals, and one is tempted to the supposi- 

 tion that in the tracheal system we have a means for the 

 supply of oxygen direct to the tissues." Another suggestion 

 which occurred to I'oulton was that "the coagulation is a 



