PARASITISM AND DEGENERATION 193 



101. Parasitic vertebrates. Among the vertebrate ani- 

 mals there are not many examples of true parasitism. The 

 hag-fishes or borers (Myxine, Heptatrema, Polistotrema) are 

 long and cylindrical, eel-like creatures, very slimy and very 

 low in structure. The mouth is without jaws, but forms a 

 sucking disk, by which the hag-fish attaches itself to the 

 body of some other fish. By means of the rasping teeth on 

 its tongue, it makes a round hole through the skin, usually 

 at the throat. It then devours all the muscular substance 

 of the fish, leaving the viscera untouched. When the fish 

 finally dies it is a mere hulk of skin, scales, bones, and 

 viscera, nearly all the muscle being gone. Then the hag- 

 fish slips out and attacks another individual. 



The lamprey, another low fish, in similar fashion feeds 

 leech-like on the blood of other fishes, which it obtains by 

 lacerating the flesh with its rasp-like teeth, remaining at- 

 tached by the round sucking disk of its mouth. 



Certain birds, as the cow-bird and the European cuckoo, 

 have a parasitic . habit, laying their eggs in the nests of 

 other birds, leaving, their young to be hatched and reared 

 by their unwilling hosts. This is, however, not bodily para- 

 sitism, such as is seen among lower forms. 



102. Degeneration through quiescence. While parasitism 

 is the principal cause of degeneration among animals, yet 

 it is not the sole cause. It is evident that if for any other 

 reason animals should become fixed, and live inactive or 

 sedentary lives, they would degenerate. And there are not 

 a few instances of degeneration due simply to a quiescent 

 life, unaccompanied by parasitism. The Tunicata, or sea^ 

 squirts (Fig. 122), are animals which have become simple 

 through degeneration, due to the adoption of a sedentary 

 life, the withdrawal from the crowd of animals and from 

 the struggle which it necessitates. The young tunicate is 

 a free-swimming, active, tadpole-like or fish-like creature, 

 which possesses organs very like those of the adult of the 

 simplest fishes or fish-like forms. That is, the sea-squirt 



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