The Sunfish as a Host. 89 



e( host," arises out of the circumstance that the joints cannot 

 become mature until the parasite finds its way into the ali- 

 mentary canal of the " host " it is ultimately destined to occupy. 

 It is a curious example of an animal perpetually striving to 

 perform an act which cannot be accomplished by its own un- 

 aided powers, for our Tetrarhynchus must wait until a shark or 

 other large fish attack and devour the sun-fish before it can 

 gain access to the stomach and intestines of the final lc host." 

 Although the mature condition of the joints or proglottides of 

 Tetrarhynchus reptans at present remain unknown, it is quite 

 certain that, no matter how long the immature strobila remains 

 within the sunfish, it cannot attain sexual maturity until it is 

 transferred in the manner pointed out. This law in the deve- 

 lopment of tapeworms appears to be universal, and as such 

 was, we believe, first recognised by Yon Siebold. Another 

 and more familiar illustration of its application is seen in the 

 development of the Gysticercus fasciolaris in the liver of the 

 mouse. It is not uncommon, in old mice especially, to find 

 this cysticercus developed into a strobila or tapeworm several 

 inches in length, whilst it is still coiled within the liver, but if 

 the joints be examined, none of them will be found to contain 

 reproductive organs, or even indications of them ; as soon, 

 however, as the cat — the final " host " of the parasite — swal- 

 lows the mouse, the cysticercal condition of the immature tape- 

 worm immediately disappears, and fresh joints are formed, 

 which become sexually mature; in this state the strobila is 

 recognised under the title of Taenia crassicollis. In regard to 

 the question as to whether the boring of the young tapeworms 

 through the flesh and viscera of the first " host" gives pain or 

 otherwise, we cannot of course speak with absolute certainty, 

 but from the slowness of the process and the extreme minute- 

 ness of the boring apparatus, we think it very doubtful if the 

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

 there are many of them, and they have by their complex and 

 long-continued movements injured the secreting structure of 

 the liver, we think they give pain indirectly, as it were, by 

 causing the decay of that organ and the consequent enfeebling 

 of the vital powers of the " host." It is at such a time that 

 the sunfish would be easily overcome by its natural enemies, 

 and the piscine life would thus be sacrificed for the advantage 

 of the long-imprisoned guests. The Tetrarhynchi are passively 

 transferred into the alimentary canal of their final " host," and, 

 so far as our observations extend, we know of no instance 

 where the young tapeworms escape by themselves from the 

 body of the first "host" before they complete their final 

 development. 



