PARASITISM 353 



It has a cartilaginous skeleton, eyes deeply 

 embedded in the skin, and a mouth so rudi- 

 mentary that it consists of a mere membranous 

 ring furnished with a single tooth in its upper 

 part. The tongue, however, is furnished with 

 two rows of strong teeth. Around the mouth 

 are eight short tentacles. With the tongue as 

 a piston and using the mouth as a sucker, the 

 hag attaches itself to a halibut or other large 

 fish, holds on firmly and gradually bores its 

 way into the body cavity, consuming the flesh 

 of the fish until only the skin, entrails, and 

 cartilaginous skeleton remain. 



It is indispensable to successful parasitic existence 

 that the symbiotic relationship be so adjusted that 

 means are provided for the escape of the parasites or 

 their offspring in order that new generations in new 

 hosts may obtain. With the ecto-parasites this is a 

 simple matter, but with those living within the bodies 

 of the hosts it is more difficult. 



The means of transmission is very varied and in 

 many cases very interesting. With the ocasional 

 parasites, such as the mosquitoes, fleas, bed-bugs, and 

 some of the ticks, the symbiotic union is so indefinite 

 that the host is not fixed. With the lice which actually 

 live upon the host, the transmission of the adult para- 

 sites is the accidental result of the intimate personal as- 

 sociation of the hosts. In case of such emergency arising 

 as the death of the host, the parasites being unable to 

 seek new hosts, remain upon, and die with, or rather 

 after him. 



The intestinal worms discharge enormous numbers 

 of eggs which pass out with the excrement, admission to 

 fresh hosts being a matter of chance. In such cases 

 it is usually at the sacrifice of countless eggs and embryos 

 that one is preserved by entrance into a new host. Thus 

 the eggs falling upon the soil must wait in some cases 

 until an appropriate animal, browsing upon the herbage, 



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