BULLETIN OF THE UNITED STATES FISH COMMISSION. 39 



and caudal fins presented a peculiar densely mottled appearance, due to 

 the aggregation of pigment cells in the vicinity of the cysts. The velar 

 flaps within the anterior portion of the mouth were also infested, as well 

 as the floor and roof of the mouth below the tongue, the inner surface 

 of the opercles, and the anterior faces of the gill arches. The cysts were, 

 on the whole, most numerous on tJie fins, embedded in the interradial 

 membranes. 



From the fact of our finding the parasite encysted it is evident that it 

 is not an adult form, but that it is part of the life-cycle of some species 

 which infests in great numbers one or two other hosts, iu which it under- 

 goes its complete development and metamorphosis. It is also in the 

 highest degree probable that it is in fact a Platyelminth or Flat worm, 

 belonging to the group Trematoda, which are almost all truly parasitic, 

 presenting a remarkable life-history and exhibiting a true alternation of 

 generations in the course of its migrations from host to host. The animal 

 becomes sexually mature in the intestine of some vertebrate host, where 

 it discharges its eggs into the faecal matters of the intestine. These ova are 

 then expelled with the faecal matters, and, finding their way into the water, 

 there hatch out as a ciliated larva, after which it loses its cilia ; soon 

 afterwards it enters the body of a snail or other moUusk, where it grows 

 into a sexless individual, in the hollow sac-like interior of which a sec- 

 ond generation of asexual individuals quite different from the first are 

 developed from the walls of the sac, provided with tails for the purpose 

 of propulsion. The sac or " nurse" in which these tailed forms devei- 

 ope then ruptures, and the tadpole-like forms escape which are known 

 as Cercaria. These then swim about iu the water until they find a 

 proper host, into the skin of which they bore, at the same time losing 

 their tails and becoming encysted, as we have observed to be the case 

 with the creature infesting the skin of the Gunner. The next step in 

 their development is the adult sexual state ; this develops directly 

 from the tailless larvae inclosed in the cysts, such as are found in the 

 €;pecimens before us. If another fish should swallow the infested Gun- 

 ners, the embryos of the parasites would leave their cysts in the skin 

 of the latter and develop into fluke-like parasites, which would very 

 probably find their way into the vessels of the digestive apparatus and 

 liver of their new host, where they would finally become mature or 

 capable of producing eggs. If infested Gunners were imperfectly cooked 

 and eaten by man, he would become the final host in which the worm 

 would reach maturity. After a more or less prolonged stay in the final 

 host, the adult parasites are expelled, and are as a rule within the lim- 

 its of this groui) of a flattened or depressed form with a naked soft skin 

 and provided with a mouth, the intestine branched and ending in nu- 

 merous caecal diverticula, with ventral suckers, sometimes armed with 

 rings of hook-like chitinous organs. In the mature condition they are 

 hermaphroditic. 



Gyrodaetiflns is a genus of Trematodes which often infests gold-fifsh 



