288 
GEOFFliEY SMITH. 
that in this respect also the infected individuals resemble the- 
adult normal females, because the latter do not attain to the 
same size as the normal males, and this is no doubt due to 
their comparative poverty in the reserve substance, glycogen,, 
from which growth aud the repair of tissue is derived. 
Before going on to formulate a theory of the action of the 
Sacculina roots in stimulating the fatty and depressing the 
glycogenic fnnction of the liver, it is important to realise that 
the facts summarised above afford evidence of a physiological 
process of considerable general interest. We see that the- 
Sacculina roots demand and absorb a large quantity of fat 
from the crab, and, on the other hand, there is no evidence 
of their demanding or taking up glycogen. Now, it might 
be supposed that in consequence the crab’s liver would be 
drained of fat by the parasite, while it would contain an 
excess of glycogen which is not abstracted by the parasite;, 
yet, as a matter of observed fact, we find the exact 
opposite taking’ place : we find that the extra demand 
on the fat made by the parasite is met by an excessive- 
formation of fat in the liver, while the absence of demand for 
glycogen is responded to by a suppression of the glycogenic 
function. This is plainly a process of physiological regulation,, 
an extra demand being met by an excessive supply. 
It is exactly here that we observe an analogy between the- 
physiological regulation of the metabolism in crabs infected 
by Sacculina and the phenomena of regulation met with in 
immunity phenomena in general. It is not indeed surprising 
that we should meet with such an analogy, because in both 
cases we are detiliug with the reaction of an organism to a 
parasite. 
In immunity to bacterial diseases, whatever may be the 
nature or place of origin of the immune substances, it is at 
any rate clear that we are presented with the formation in 
excess of substances which are being linked on to and fixed 
by the parasite. Whether these substances originate from, 
the tis3u.es attacked by the parasite, or from the phagocytes 
which attack the parasite, at any rate an over-production of 
