198 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
species of host. Thus the liver fluke in the adult condition lives in the 
gall bladder of the sheep, while the early larvae live within the body 
cavities of a species of land snail. The transfer from host to host in 
, this case must be a procedure involving many chances of failure to a 
very few chances of success, and, in adaptation to these vicissitudes, 
the number of eggs and larvae produced by a single adult individual 
runs up into the millions. 
The classic case of extreme parasitic degeneration is that of 
Sacculina. The young larva of Sacculina is a typical entomostracan 
crustacean larva which swims about and leads a free life for a time, 
but soon attaches itself by means of its antennae to a hair pit of a crab, 
a small hole in the latter’s armor. The internal tissues of the larva 
then undergo degenerative processes and are reduced to an almost 
fluid mass of embryonic cells, which flow through the hair pore of the 
crab, and into the latter’s lymph spaces. The small mass of cells then 
rounds up and is carried about with the circulation of the crab’s blood 
until it comes to a favorable place of lodgment, usually the wall of the 
intestine just back of the stomach. Here it flattens out and sends 
rootlike branches almost all over the crab’s body, like a malignant 
tumor in its invasion of foreign tissues. ‘The unbranched part of the 
parasite is little more than a sac of reproductive organs, and these 
produce eggs and sperms, which unite to produce larvae. By this 
time the host is killed and, with the decay of its body, the larvae escape 
into the sea water ready for a brief period of free life before attacking 
another host. 
_ Almost every group of animals and most of the groups of plants 
have their parasitic representatives and every degree of parasitism 
and the accompanying degenerative changes are to be found. Of 
course, it is an open question whether parasitism causes degeneration 
or whether degenerating creatures take refuge in parasitism; but in 
either case the adaptive features of the situation are obvious. 
Commensalism.—If parasitism be defined as an association 
between two organisms in which one (the parasite) lives at the expense 
of and to the detriment of the other (the host), commensalism may be 
defined as an association in which the two organisms exist in close 
association without any positive detriment to either. In some cases 
the claim is made that the association is mutually beneficial, but as a 
rule the condition is relatively one-sided. 
An interesting example of commensalism is that of the sea cucum- 
ber and the little fish Fierasfer. This strange little animal inhabits 
