140 



BNTOZOA. 



To the first question I reply that the object of tunneUing is appa- 

 rently two-fold : first, that the animal may constantly obtain fresh 

 nourishment ; and secondly, because the creature is impelled by 

 instinct to seek out another residence. It may be objected that- 

 neither of these reasons are satisfactory, for the parasite is not sup- 

 plied with digestive organs, neither can it gain access, by itself, to 

 another dwelling. It should, however, be observed that as it ac- 

 quires nutriment by imbibition through the general surface of the 

 body, a change of locality would facilitate growth, and render such 

 imbibition more easy ; whilst, on the other hand, a long stagnation 

 at any one spot would sooner or later bring on cretification and 

 death. It is, certainly, a curious example of an animal perpetually 

 striving to perform an act which it cannot accomphsh ; for, in order 

 to arrive at sexual maturity, it must wait until the sunfish is de- 

 voured by a shark. In regard to the question as to the boring- 

 action giving rise to paia, one cannot, of course, speak with cer- 

 tainty; but from the slowness of the process and the extreme 

 minuteness of the boring-apparatus, I think it very doubtful if the 

 presence of the parasites is even felt at aU. When, however, there 

 are many of them, and when, by their complex and continued 

 movements they have injured the secreting parenchyma of the liver, 

 or any other important viscus, then, doubtless, they give pain, 

 indirectly, by causing the decay of the organ infested and a conse- 

 quent enfeebling of the vital powers of the animal. It is at such a 

 time that the sun-fish would be easily overcome by its natural ene- 

 mies, and the piscine life would be sacrificed in favour of the long- 

 imprisoned guests. So far as our observation extends, these 

 wandering scolices never escape by themselves fi:"om the body of 

 the intermediate host, but are in all cases passively transferred 

 into the alimentary canal of the ultimate entertainer. In the 

 Sharks and Eays the parasites probably acquire sexual maturity 

 very rapidly, and whilst within these animals the proglottides a.nd 

 their contained embryos, doubtless, pass into the water in the 

 ordinary way ; the ova are subsequently swallowed by various 



