THE PAEASITES OF THE PEARL OYSTER. 83 



terminates bluntly and is armed over the posterior moiety with stiff cilia-like hairs 

 (Plate N.. fig. 20). 



The internal structure of the body is obscured by the massing of innumerable 

 calcareous corpuscles in the cortical region. No details of the excretory system 

 could be made out, save the presence of the main longitudinal trunks, and of 

 a well developed terminal contractile vesicle opening to the exterior at the 

 posterior extremity (Plate 11., figs. 1!) and 20), and a few loops in the head (Plate II., 

 fig. 21). 



Conspicuous within tin- second quarter ot the body lie the four great proboscis- 

 sacs. Each proboscis passes forwards to its point of emergence between the cephalic 

 suckers (Plate II., fig. 22). < >ne of their hooks is shown in Plate II., fig. 2:3. 



The cuticle possesses a certain surface ornamentation, consisting of tiny mammilla- 

 tions or tubercles, irregularly disposed (Plate II., fig. 20), except upon the caudal 

 portion, where the markings assume a meandering Greek pattern of graceful and 

 intricate curves. The latter are possibly the expression of a post-mortem shrinkage, 

 they were not observed in all cases, and are exaggerated in our figure. 



II. Cestodbs in the File or Trigger Fishes. 



In looking for the second host of the pearl oyster parasites, it was natural to 

 examine the species of Batistes, the Trigger or File fishes. It has been asserted, and 

 also contradicted, that these fishes feed largely on pearl oysters and other molluscs. 

 We ha \e. however, confirmed the truth of the statement, having on many occasions 

 found pearl oyster shells in the stomachs of Balistes taken on the banks. The 

 presence in the body of both B. mitis and B. stellatus of numerous Tetrarhynchid 

 cysts led to the hope that a further stage in the life history of the parasite upon 

 which jewellers are so greatly dependent had been discovered. A more minute 

 examination however, renders the connexion between the parasites of the pearl 

 oyster and those of the file fish a doubtful one. 



We have found so far in the Balistes two Tetrarhynchi belonging in our opinion to 

 two distinct species, and these we have named Tetrarhynchus balistidis and T. pinnce 

 respectively. The latter is ensheathed in a large bladder-like vesicle, four or five 

 rimes the length of the scolex. and perhaps twice as broad, the whole somewhat 

 resembling a Lima-bean. The other form, T. balistidis, is also ensheathed in a vesicle, 

 but of somewhat different form ; it has, for instance, little space between the inner and 

 outer wall, and is simply a double membrane closely applied to the head of the 

 scolex. It does not arise from the extreme end of the scolex, and so envelop the 

 whole scolex, but it arises a little way behind the head and is folded forward as a 

 woman might turn a short cape over her head. The result is that in this second 

 form the scolex projects behind the vesicle (Plate II., fig. 24), whilst in the first 

 the vesicle projects behind the scolex (Plate II., figs. 31 and 32). 



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