282 DR. H. LYSTER JAMESON ON 



Tetrarhi/nchus form, with its complex proboscides, cannot be said 

 to have been satisfactorily bridged. 



Herdman found the Tetrarhynchus foi-m much scarcer than 

 the Tylocephalum form, and it appears from his Report, Part V. 

 p. 22, that the lutio of the latter to the former is about 200 : 6. 

 Shipley and Hornell (Report, Part II. p. 79) give the ratio of 

 globular larvfe to undoubted Tetrarhynchi as 100 : 1. 



Prof. Herdman's suggested explanation of this, namely, that the 

 globular parasite only occasionally advances to the Tetrarhynchus 

 stage, requires, it seems to me, a greater efFoit of the imagination 

 than the hypothesis that the two worms are distinct forms. 



It is hard to conceive of conditions that would lead a small 

 minority of Tylocej)hali(,m ludificans or T. minus to leave their 

 tough fibrous cysts in the peripheral tissues, and migrate to the 

 intestine, there to take on the Tetrarhynchus form. It seems to 

 me much easier to regard these as two (or three) distinct species, 

 and their simultaneous presence in one host as a case of parallel 

 infection. 



In his latest paper (42, p. 129), Southwell, speaking of these 

 undoubted Tetrarhynchids, says : — 



" These are by no means rare, and are in almost every 

 case confined to a particular part of the wall of the gut, about 

 one inch from the anus and on the terminal part of the gut. 

 They often occur in clusters of three or four. They are 

 small (about 1 mm.), but appear to be adult in every way, 

 save that strobilization has not commenced. This encysted 

 joung Tetrarhynchid is quite dissimilar to the globular cysts 

 found in the same oyster. In the latter case, the larvfe are 

 so young that the Cestodian characters are but ill defined. 

 In the former case, a normal and full-grown Tetrarhynchid 

 head is present. No stage or stages have been found inter- 

 mediate between them, and the evidence that they are both 

 stages in the life-history of the same parasite rests on circum- 

 stantial evidence and on evidence obtained by feeding 

 experiments." 



And with reference to these feeding experiments, which are 

 referred to below (p. 287), and in wdiich Tetrarhynchi were found 

 in Sharks that had been fed on pearl-ovsters, Mr. Southwell says 

 (p. 130):- 



" The mere fact that the adults were obtained by feeding 

 is in itself almost sufiicient to prove that they are the 

 adult of the pearl-inducing worm, for it is difficult to believe 

 that their occurrence in the Gingly mo stoma was a mere 

 coincidence each year.*' 



I think there is very good reason to believe that Southwell 

 did, in his feeding experiments, actually transmit Tetrarhynchus 

 unionifactor from the Oyster to the Elasmobranch, but it is 



