PEARL PRODUCTION. 



21 



various stages of the Cestode larvae both from the centre of decalcified pearls and also 

 free in the tissues of the pearl oyster, but left it an open question whether the sub- 

 globular younger larvae belong to the same life-history as the elongated older forms 



A. 



B. 



0. 



D. 



E. 



F. 



Fig. 4. 



Series illustrating the connection between the Cestode larvse found in the pearl oyster. 

 A, B, C, and D represent stages that commonly occur, E is the hypothetical connecting link, and 

 F is a young TetrarhyncJiiis, copied from fig. 31, of Plate II., in the "Report on the Parasites of the 

 Pearl Oyster " (Part II.). Since this figure was made, a still younger Tetrarhynchm, very slightly 

 more advanced than is shown in E here, has been found in the liver of the pearl oyster (see text, 

 p. 22, and Plate III., fig. 10). 



which are young Tetrarhynchids. If our arrangement ot the stages observed in the 

 tissues of the pearl oyster is correct, and if all these larvae belong to the same species, 

 then the interpretation we have given above brings us to the conclusion that the 

 larger of our two globular larvae belongs to the worm which SHIPLEY and HORNELL 

 described as Tetrarhynchus unionifactor in 1904. Figs. 1 to 8 on Plate III. show 

 most of the common stages we have found, and in regard to which there can scarcely 

 be any doubt (1) that they all belong to the same life-history, and (2) that they are 

 young Tetrarhynchids leading on to the stages shown in figs. 10 and 11. 



If we distinguish the genus Rhynchobothrius from Tetrarhynchus by the possession 

 of only two bothridia, then the correct name of the species becomes Rhynchobothrius 

 unionifactor (SHIPLEY and HORNELL). The adult condition of this species is found in 

 the large ray Rhinoptera javanica, M. and H. (see this vol., p. 65). In addition to these 

 larger larvae there is, however, a smaller form of globular larva (Plate II., fig. 19, &c.) 

 which we meet with in the tissues of the pearl oyster, and which probably belongs 

 to a distinct species of Cestode. The two forms of larvae are seen side by side in 

 fig. 17 on Plate II., and, as shown there, the larger (B) is about six times the diameter 

 of the other (A). The two are, however, closely related forms and in similar stages. 

 In both there is the same anterior invagination with the central papilla various 

 stages in the formation of which are shown in figs. 20, 21, 22 on Plate II. and 

 figs. 1, 2, 6 and 8 OD Plate III. There are the same cuticular spines round the margin 

 of the invagination in both, and the same histological structure in the body wall of 

 the vesicle and the future scolex. In size, SEURA.T'S larvae approach more nearly to 

 our smaller form ; but differ from both in proportions and details of structure. 



Although we have examined sections of several hundred of these parasites from. 



