472 PARASITES OF ANIMALS 



asked " what is the object of this perpetual tunnelling/' and 

 " does the boring cause suffering to the host," I reply : 

 " The object of tunnelling is apparently twofold ; first, that 

 the parasite may constantly obtain fresh nourishment ; and 

 secondly, that it may acquire another residence." It furnishes 

 an example of a parasite perpetually striving to perform an act 

 which it cannot accomplish ; for, in order to arrive at sexual 

 maturity, it must wait until the sunfish is devoured by a shark. 

 In regard to the question as to the boring action giving rise 

 to pain, one cannot, of course, speak with absolute certainty. 

 When there are many parasites occupying the liver, or other 

 important viscera, then, doubtless, they create pain, and cause 

 decay of the organs infested ; thus they enfeeble the vital 

 powers of the host. At such a time the sunfish would be 

 easily overcome by its natural enemies, and be the first to 

 succumb in the struggle for existence. These wandering 

 tetrarhynchoid scolices never escape the body of the inter- 

 mediate host until they are passively transferred into the 

 alimentary canal of the ultimate entertainer. In the sharks 

 and rays they acquire sexual maturity. From these animals 

 the proglottides pass into the water in the ordinary way. The 

 ova are subsequently swallowed by sunfishes and other inter- 

 mediate hosts, within whose stomachs the six-hooked embryos 

 are liberated, and the scolices become developed in the ordi- 

 nary manner. As obtains in Cysticercus fasciolaris of the 

 mouse the scolex of Tetr. reptans becomes taenioid. I have 

 seen the liver of an adult sunfish so infested by these para- 

 sites that the whole organ might be fitly described as a 

 mere bag of worms, the immature strobiles being inextricably 

 coiled together and defying separation. One of the parasites 

 which I removed from this particular fish is preserved in the 

 Hunterian Collection. 



In reference to the nematoids of fishes I can say but 

 little. They are excessively abundant ; sexually-immature filariaa 

 being found in almost every marine fish that one examines. 

 Even at our dinner and breakfast tables nothing is more 

 common than to observe the little Filaria piscium spirally 

 coiled within the tissues of herrings, -haddocks, cod-fish, and 

 whiting. All the sexually-immature nematoids are, as it were, 

 waiting to be passively transferred to their ultimate hosts. 

 These final bearers are usually either fishes, birds, cetacea, or 

 seals. Amongst fresh-water fishes the Cuc.ullanida play an 



