THE PARASITES OF THE PEARL OYSTER. 85 



ID niillinis. to 15 millims. in length (Plate IT, figs. 28, 2D, and 30). The larva with 

 the cysl is covered by a thin cuticle, beneath which lie some poorly developed circular 

 and longitudinal muscular fibres (Plate 11.. fig. 38). The body of the vesicle is very 

 fluid, consisting of a highly vacuolated parenchyma. In the parenchymatous cells 

 numerous oil drops and calcareous bodies lie. 



Finally we have a very small Tetrarhynchus found amongst the spiral valves of the 

 intestine of a sting-ray, Tceniura melanospilos, Blkk., kindly identified for us by 

 Mr. G. A. BOULENGEE of the British Museum (Plate IV.. figs. 67, 70, 71, and 72). 

 In the stomach of this fish, known locally as the " Pulli-thirikkai, ' two entire and 

 quite unmutilated Batistes were found. Dr. von Linstow has kindly furnished us 

 with a diagnosis of this Tetrarhynchus, which he has named T. mini urns, and this we 

 subjoin in the systematic part dealing with the Cestoda (p. 89). 



Amongst the specimens of Tetrarhynchus minimus found in the Tasniura was a 

 single example of Polypocephalus, a genus established by Braun* for some specimens 

 he described from the stomach of Rhinobatis granulatus, < !uv. 



We have now dealt with the following forms : 



A. In the pearl oyster, Margaritifera vulgaris, Schtjm. 



(i.) Small Cestode larvae in various tissues, some of these form the nuclei ot 

 pearls. These correspond very closely with the larvae found by Monsieur 

 Seurat, and identified by Professor Gtard as belonging to the family 

 Monobothria. 



(ii.) Older larva?, of the genus Tetrarhynchus, in more than one stage, the 

 most mature and the most abundant of which have no sign or trace of a 

 vesicle, the introverts protruded and probably functional, the muscular sacs 

 of the introverts reaching back to the middle of the body, a well developed 

 excretory system with a terminal pore, the posterior end of the body spinous, 

 with sinuous markings, and the rest of the body covered with low warts 

 (Plate II, fig. 20). 



B. In the file or trigger fishes. Batistes mitis, B. undulatus, and B. stellatus 



(iii.) Tetrarhynchus balistidis, n. sp., whose head is closely enveloped in a 

 vesicle which does not enclose the body, the teeth on the introvert are 

 large and few in number, 4 to 6 in a horizontal ring (Plate II., fig. 24) ; 

 a later stage of this form, like it encysted in the sub-peritoneal tissue, is 

 represented by a squarish head with 4 bothria (Plate II., figs. 25 and 26). 

 This stage has quite freed itself from vesicle and "body," but is provided 

 posteriorly with a papilla or protuberance, from which the proglottides 

 will probably arise. 



(iv.) Tetrarhynchus pinnce, n. sp., enveloped in a large vesicle swollen with 



* 'Arbeit. Inst. Wurtzburg,.' IV., 1877-78, p. 297, 



