VERTEBRATES FROM A CRUSTACEAN-LIKE ANCESTOR. 413 



cutting as a whole, so that all parts of it had undergone 

 the same preliminary treatment. These sections show that the 

 heart and large blood-vessels in connection with it are full of 

 blood-corpuscles of normal character, amidst which it requires 

 careful search to find a single pigment-bearing corpuscle; 

 while, on the other hand, the blood spaces of the branchiae, 

 especially the upper ones, are so loaded with corpuscles bear- 

 ing masses of pigment of all shapes and sizes as to give 

 the appearance of an injection of a pigmented mass into the 

 branchial vessels. 



In fig. 18 I have drawn a group of corpuscles as they 

 appeared in a section of one of the upper branchial blood 

 spaces, a and h are normal blood-corpuscles ; in the others the 

 nucleus is more or less concealed by the dark-yellow brown irre- 

 gular clumps of pigment : the colour is not shown in the figure. 



These pigmented blood-corpuscles are found lying singly or 

 massed together, especially in the upper branchial vascular 

 spaces, often in close contiguity to the pigmented walls of 

 these spaces ; and it is frequently difficult to decide whether a 

 mass of pigment belongs to a blood-corpuscle or to the pig- 

 mented walls. 



Further, the comparison of these sections with those of a 

 fresh-caught Ammocoetes of the same size shows that the former 

 is much more pigmented than the latter, and that the increase 

 of pigment is due to the accumulation of pigment in those 

 places where pigment is always found, viz. in the blood spaces 

 of the branchiae, and other places where stagnation is liable 

 to occur. The result, then, of preventing metamorphosis by 

 placing the animal in a condition of impaired nutrition — a con- 

 dition which, according to Hunter, is the one most favorable 

 to passive blood destruction in health — is to increase largely 

 the passive destruction of the red blood-corpuscles in those 

 places where the blood is liable to stagnate, and concurrently 

 to increase the deposition of pigment in those places. This 

 thickening of the pigmented walls and septa of the blood 

 spaces naturally must lead to a diminution of their size, and 

 so to a quickening of the current through them, and con- 



VOL. XXXI, PART III. NEW SEE. E E 



