THE PARASITES OF THE PEAEL OYSTER. 87 



'None of these features are reproduced in either of the Tetrarhynchi encysted in the 

 tissues of the Batistes. The more advanced Larvse from the pearl oyster have arrived 

 at a later stage in development than the larvae found in the Balistes, and, unlike 

 them, seem to belong to that group of the Tetrarhynchidae which form no vesicle. 

 Tin' teeth on the introvert differ in all respects from those of the Tetrarhynchi of the 

 Balistes and from another Tetrarhynehus sp. found in Trygon walga. Those of the 

 two species from the Balistes differ in nearly every detail. It would be rash to make 

 a dogmatic statement as to the future fate of the Tetrarhynehus of the pearl oyster. 

 The trigger or file fishes (Balistes), known to the Tamil fishermen as the Kilathi, 

 have by our own observation been proved guilty of feeding largely on pearl oysters. 

 Fragments of pearl oysters are frequently found in their stomachs, aud altogether 

 they seem to be the most likely host for the further development of the oyster 

 parasite. We do not, however, think that it is at present clear that they are the 

 second or final host. 



Like the pearl oysters, the infested file fishes are by no means confined to the 

 oyster banks in the Gulf of Manaar. Our Ceylon Expedition took them at Trin- 

 comalee and again at Galle ; at the latter place one was taken so infested with young 

 Tetrarhynchi that the mass of their cysts equalled in hulk the whole of the stomach 

 and intestine. A second point is that the form with the large vesicle, T. pinnce, was 

 also found in the tissues of a large Pinna sp., a mollusc more widely distributed 

 around Ceylon than Margaritifera vulgaris. 



The nature of the teeth and their arrangement in T. minimus argues against any 

 relationship between the pearl oyster parasite aud the small Tetrarhynehus found in 

 Tceniura melanospilos, and this want of relationship is corroborated by its very 

 minute size. At present the final host of the pearl-forming Cestode does not seem to 

 be Tceniura melanospilos. Still the adult Tetrarhynchi live almost exclusively in the 

 alimentary canal of Elasmobranchs. Other members of this order which were found 

 in the neighbourhood of the pearl fisheries were Pteroplatea micrura, Bl. and Schist., 

 and Tru<jon walga, Mull, and Heull, T. uarnah (Foesk.) and T. sephen (Forsk.). 

 The last two species were taken by Thurston in the Gulf of Manaar. Day also 

 records T. marginatus, Blyth, T. bleekeri, Blyth, T. hennettri, Mull, and Heull, 

 T. huhlii, Mull, and Heull, T. Iwnbricata, Bl. and Schn., T. zugei, Mull, and 

 Heull, from the Indian and Indo-Malayan seas. Probably one of these is the host ot 

 the final stages of the two Tetrarhynchid metacestoid larvae found in the Balistes, 

 which they certainly eat, and possibly also of the pearl-forming larvas of the Marga- 

 ritifera vulgaris. LlNTON* has described a number of Tetrarhynchi from various 

 species of Trygon. These are Tetrarhynehus (Rhynchobothrium) hispidus, T. (Rh.) 

 longispinis, T. (Rh.) tenuispinis, and T. (Rh.) wagneri, T. tenuis,^ and T. robustus 



* 'U.S. Commission of Fish and Fisheries. Commissioner's Report,' Part XV., 1887. Washing- 

 ton, 1891. 



t The T. lintoni of Vaui.legeard. 



