90 CEYLON PEARL OYSTER REPORT. 



are the proboscis sheathes from which the proboscides arc protruded (Plate IV., 

 figs. 71 and 72). The projections bear very minute, closely packed hooks, from their 

 apices the proboscides protrude, and these bear larger hooks at wider intervals. 

 There is a regular gradation in the size of the proboscis hooks which is shown in 

 Plate IV., fig. 67. The part of the proboscis which is retracted is arranged in a 

 wavy fashion. The reproductive pore is lateral on the posterior third of each 

 proglottis, but for the most part, only immature proglottides were present. The ova 

 are thin-shelled, spherical, with a diameter of 0'0039 millim. This is the smallest 

 of all species of Tetrarhynclms. 



Habitat: The folds of the spiral valve of the intestine of Tceniura melanospilos, 

 Blkr,, taken off Ceylon, at Trincomalee. 



The very peculiar and regular arrangement of the teeth, and the gradation in 

 their size and shape is a remarkable feature in the Tetrarhynchus from the 

 Tceniura. These features seem to separate it off both from the Tetrarhynchi of the 

 pearl oyster and from those of the file fish, and to bring us to the conclusion that 

 these forms which seemed at first as if they might be stages in one life-history are 

 really independent species, and that our knowledge of the life-cvcle of the parasite 

 which causes the formation of pearls is still incomplete. 



II. TREMATODES OF THE PEARL OYSTER. 



Three species of Trematodes inhabit the pearl oyster, all of them in an immature 

 condition. They all appear to be new species and we have called them Muttua 

 margaritiferoB, Musalia herclmani, and Aspidogaster margaritiferce respectively. 



1. Muttua marguritiferae, n. gen. and n. sp. Plate III., figs. 53, 54, 55, 56 and 57. 



Minute, 0'9 millim. to 0'75 millim. in length. Lanceolate shape, slightly 

 narrower anteriorly, both ends bluntly rounded. Cuticle covered with minute 

 pointed scales which extend over posterior end. Suckers almost equal in size, the 

 posterior lies behind the middle of the body. No prepharynx. Pharynx medium in 

 size, no oesophagus, the links of the alimentary canal extend to the vesicle of the 

 excretory system, i.e., almost to the posterior end of the body. Excretory vesicle 

 Large, transversely placed and somewhat basin shaped, the coils of the excretory 

 tubules are very marked on each side of the pharynx. Genital pore to the right 

 of the posterior sucker. The vagina is plicated, the penis is large. The cercaria 

 stage lias two black eye-spots, one on each side of the pharynx. The characters 

 of the species are those of the genus. 



Habitat: The cercaria stage inhabits the pearl oyster, Margaritifera vulgaris, 

 ScHUM. It is usually found in the gills, and is most frequently met with in those 

 oysters which live on a rocky substratum, such as those of the Muttuvaratu Paar. 



Of the three Trematodes associated with the pearl oyster, this is the only one 

 found in any abundance; the other two species are so rare that no more than some 



