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 ctegeneration is that of 

 SaccuUna. The young larva of Sacculina is a typical entomostracan 

 crustacean larva which swims about and leads a free Ufe 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 relation is relatively one-sided. 



An interesting example of commensalism is that of the sea cucum- 

 ber and the little fish Fierasjer. This strange little animal inhabits 



