PARASITISM AND DEGENERATION 193 
101. Parasitic vertebrates—Among the vertebrate ani- 
mals there are not many examples of true parasitism. The 
hag-fisbes 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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