THE CEYLO.V I'EAr(I--OVSTKK. 287 



from lialistes) ; uud he claims that they are " exactly .similar to 

 the encysted Tetrarhynchid found in the oyster " — /. e. the true 

 Tfiirarhj/nchnH uaionifactor, except tliat they are slightly larger. 

 Mr. .Southwell is satisfied that they are derived from the oyster.s 

 eaten hy /julixtes, and thinks that they are derived from 

 hoth the genuine Tetrarhy rich ids in the Oyster's intestine and 

 from the glohidar cysts in its tissues. Jle considers that if 

 liaiinteH is eaten by an Elasmobranch, the young Avorms become 

 julult ; but I)(0'ihUh is not a necfissfiry ho.st, it is merely a " carrier." 



Johnstone {2%h) confirms the view that the Teleostean host is 

 a collatciral one, and not a noirnal stage in the life-history, by his 

 recent researches on the European Teirarhynchus erriucceus. 

 The adult stage of this worm occurs in various species of Skates 

 and llays, and the first liost is pi-obably some Invertebrate. 

 Johnst/)ne regards the fre<^uent occurrence of a lai-Aal form of 

 this worm in Teleosts as a " cul-de-.sa<! " stage, due to the first 

 ho.st being eaten V)y the wrong fish ; and as normally leading no 

 fartlier, V)ut ending in degeneiation. 



Mr. Southwell has further shown (38) that the adult Tutra- 

 rhynchuH an ionif actor occurs also in the Shark, 0'in<jlymoHtoroa 

 concolor. This fish was doctored Avith Male-feiri anrl castor-oil, 

 and subsefjuently fed on pearl-oysters ; but Mr. Southwell d<jes not 

 claim that the TfArarhynrJii were actually derived from these 

 pearl-f>y.sters, though he is incline*! to think they were. The 

 .same experiment wns subsequently repeated [Southwell (41)1 

 and y'ftrarhynrhuH ii/iiinni factor was again found in Ginglymostoma 

 rov color -^ and while Mr. Southwell admits that his results are not 

 altogether conclusive, it .seems liighly probable that the infection 

 was in fact induced by his feeding experiment. There is nothing, 

 however, to show that the adult tapeworms in Ginfjlymostmna 

 were derived from the globular cysts in the oysters ; it seems 

 more proV)able that they were derived from the Tetrarhynchi 

 in the oy.ster.s* inte.stines. 



The.se elaborate experiments are, of course, chiefly of academic 

 intere.st, in the ab.sence of proof (1) tliat the Tetrarhynchi ure ii 

 later stage of the globular cysts, and (2) that the latter are 

 concerned in pearl-formation. 



To sum up, then, the gap between {a) the resting scolex 

 enclosed in its tough fibrous cy.st in the c;onnective tissues of the 

 l^eail -Oyster and strongly suggesting by analogy with other 

 foi-ms — notaVjly Seurat's larva — a young Tylocephalum, awaiting 

 and ready for its final host to devour the tissues which contain it, 

 and {h) the equally expectant, but much larger, Telrarhynchu^ 

 unloaifactor in the wall of the oy.ster's inte.stine, ha« not yet been 

 bridged. 



I set out below diagrainmatically the conclusions of Jlerdman, 

 Hornell, Shiplev, and Southwell, as to the probable life-hi.story 

 of these parasites, and also, for purposes of cornpari.son. inv own 

 attempt at an interpretation of the facts. 



