88 HERBERT E. DURHAM. 
that, taking into consideration the undoubted outwandering of 
similar pigment-cells from the free surface (fig. 3), we must 
believe that those seen in the madreporic tubes are likewise 
travelling to the exterior. It is noteworthy that free wander- 
cells are usually present in the madreporic tubules both in 
Asterids and in Echinussphera. Sometimes the pigment, 
probably owing to the death and decay of the cells containing 
it, never reaches the exterior, and masses become formed in 
various regions and organs of the body: such masses have 
been frequently described (e.g. Hamann). They consist of 
pigment-granules, some free with nuclei scattered here and 
there, others definitely located within corpuscles. It would 
seem as if the protoplasm of the cells became exhausted before 
its duties had been entirely fulfilled. It appears that the 
older the specimen the more abundant the pigment; it is 
possible that other observers have chiefly confined their sections 
to young individuals in which the process is not so marked, 
and therefore has been overlooked. 
III. ConsIDERATIONS ON THE ABOVE. 
We have now to consider the importance of these two 
processes :— 
A. The reaction to minute foreign bodies. 
B. The use of wander-cells in getting rid of effete material 
from the system. 
A. In a very large number of animals, of one kind and 
of another, it has been frequently shown that small foreign 
particles are ingested by amoeboid cells (vide Metschnikoff, 
49, &c.) ; but as regards the ultimate fate of these particles, 
except in the case of mammals, our knowledge is extremely 
limited: in the known cases most of our information on the 
matter has been obtained from observations upon man. For 
instance, it is now very well known that in persons who are 
exposed to a dusty atmosphere (townspeople, miners, stone- 
masons, &c.) the lungs are found to be deeply pigmented, the 
