66 CEYLON PEARL OYSTER REPORT. 



saucers or a pile of developing ephyrse ; when they deepen a little, they have one, 

 rarely two, transverse creases in their cuticle, but as they get to be as deep as they 

 are broad, the number of these creases has very much increased, and the posterior end 

 of the body is quite crinkled. 



The proboscides are armed with hooks which are spirally arranged ; the hooks are 

 not very hooked, and the angle is slight ; further, the hooks are all shaped alike and 

 are all about the same size. They are very small. 



The two bothridia are comparatively shallow, but during life their edges are 

 obviously very mobile, and they may deepen or become shallower as occasion arises. 

 Their outline is roughly triangular, one angle being anterior. The angles are very 

 rounded, and the deepest part of the botbridium lies in the posterior angles. 



We have in these forms, undoubtedly, the mature generation of the larval form 

 we described and named T. unionifactor in the tissues of the pearl-forming oyster, 

 Margaritifera vulgaris, Schum. In the structure of the head, the lappets with 

 bothridia, the arrangement, shape and size of the hooks on the young and the old 

 animals closely resemble one another. There is no doubt that the immature 

 T. unionifactor is swallowed by RJnnoptera jaranica when it eats the oysters, as it 

 undoubtedly does, and that the tapeworm becomes mature in the intestine of the fish, 

 that it lays eggs, and that these, somehow or other, make their way into the 

 pearl oyster. Whether some of these become the little Cestode larvae around which 

 the pearls are deposited is still largely a matter of conjecture ; if they do, they perish 

 in a costly coffin. It is certain, however, that many of the young of T. unionifactor 

 escape entombment and grow into the larval forms described in Part II. of these 

 Reports. If we could find quite young larval forms of this T. unionifactor, and if on 

 comparison with the forms which make the pearl they appeared to us to be identical, 

 we should have solved the problem of pearl-formation, at any rate in the Ceylon seas. 

 It seems increasingly probable that the pearl-forming Cestode is a T. unionifactor, 

 lmt this has not yet been proved. 



We described the species from the larva as we had no adults at our disposal ; we 

 now add a few more features taken from the adult. 

 The diagnosis of Tetrarhynchus unionifactor is : 



Head and proboscides as in the larva (see Part II. of these Reports, p. 88). Length, 

 1*5 millims. to 3 millims. Head and body stout. Neck containing the much-coiled 

 proboscis sheaths, and the proboscis bulbs 1"5 millims. to 2 millims. in length. 

 I i-enital pores irregularly right and left. Anterior proglottides shallow and saucer-like, 

 with projecting edges, lmt about the middle of the body the proglottides hardly over- 

 lap at all, and the right and left sides form a straight line. There is, however, 

 especially anteriorly, a tendency to be crinkled. 



The larval form is found in the tissues of the pearl oyster, Margaritifera vulgaris, 

 Srni'M.. and possibly encysted in the pearls. The adult lives in the stomach of 

 Rhinoptera javanica, Mulleu and Hkxlic, a great Ray which feeds on oysters. 



