THE CEYLON PEAHL-OYSTER. 283 



difficult to escape the conclusion that the worms found in Ginyly- 

 mostoma were derived from the Tetrarhynchus lai'vse in or around 

 the alimentary canal of the Oysters, and not from the globular 

 Tylocephala in the other tissues, to which Southwell refers when 

 he speaks of the " pearl-inducing worm." To dispute this view, 

 it would be necessary to demonstrate that the Tetrarhynchus -stnge 

 was not present in the Oysters used. 



Shipley remained throughout sceptical about the identity of 

 the supjoosed pearl -forming larvae with Tetrarhynchus unioni factor. 

 In Part II. of Herdman's Report, p. 86, he says it is most 

 improbable that the young larvje grow into the Tetrarhynchus 

 larva. In their report on the Cestode and Nematode Parasites 

 from the Marine Fishes of Ceylon, Shipley and Hornell say 

 (Pt. V. p. 66) :— 



"It seems increasingly probable that the pearl-foi-ming 

 Cestode is a T. unionifactor, but this has not yet been 

 proved." 



Shipley and Hoi'nell, in Herdman's Report, Part V. p. 98, offer 

 the following hypothetical life-history : — 



" Of the given number of larv.ie which enter at a veiy 

 early stage into the body of the Oyster a certain number arrive 

 in the mantle and other tissues, acquire an ectodermal sac and 

 there encyst, and find a costly grave in the developing pearl." 

 [The ectodermal sac around these parasites is so far purely 

 hypothetical and has never been demonsti'ated. — H. L. J.] 

 " I'he remainder, however, reach the alimentary canal and 

 grow and flourish thei'e. When they attain the dimensions of 

 thestages described in Part II. they leave the alimentary canal 

 and encyst, usually upon the outer surface of the intestine. 

 Now they are too big for enclosure in a pearl, and they can 

 wait without anxiety for the advent of their second host 

 {Rhinoptera javanica)., within whose intestine they rapidly 

 become sexually mature." 



It would seem to the present writer much simpler to set aside, 

 for a while, the hypothesis that Tylocephalum ludificans and 

 2\ minus are younger stages of a Tetrarhynchus, and to seek for 

 their adult stages ainong the members of the genus Tylocephalum, 

 or allied types described as new genei'a, occurring in oyster-eating 

 Elasmobranchs. Shipley and Hornell have already described a 

 number of these, which I give below : — 



^^ Tylocephalum {Tetragonocephalimi) trygonis (Report, Part 

 III. p. 51 and Part V. pp. 48 k 83). Hahltat : intestine of 

 Trygon tvalga and Aetohatis narinari. Diameter of head 

 0"03 mm. 

 * Tylocephalum (Tetragonocephalum) a'etohatidis (Report, Part 

 III. p. 52 and Part V. p. 48). Intestine of Aetohatis 

 narinari. Diameter of head TS mm. 



