86 CEYLON PEARL OYSTER REPORT. 



aqueous parenchyma. The vesicle covers the whole head and such body 

 as exists, and in relation to the scolex recalls the Cysticercus stage of 

 Tcenia. The oval cysts lie embedded in the sub-peritoneal tissues. The 

 introverts of this form have enormous numbers of weak hooks which arise 

 from ^-^ shaped bases. There are but 2 bothria (Plate II, figs. 28, 29, 



30, 31, 32, 36, 377 38). 

 (v.) Tetrarhynchus minimus, v. Lins., from the spiral valve of the intestine of 

 Tceniura melanospilos, Blkr., a fish which eats Balistcs (Plate IV., figs. 67, 

 70, 71, 72). 



C. In the sea 



(vi.) A planarian-like larva, which in the structure of its calcareous corpuscles 

 and its cuticle recalls the embryonic forms described above. 



What is the relationship of these six forms ? 



To begin with the last, the planarian-like larva found swimming by lateral 

 undulations for it is unciliated certainly resembles the youngest forms found in the 

 pearl oyster. The indications of the invaginated head and the presence of calcareous 

 corpuscles strengthen the resemblance. On the other hand, we as yet know of no 

 other Tetrarhynchid which has a free-swimming larva. Tetrarhynchi make their way 

 into their first host as embryos still encased in egg-shells. The Bothriocephalidse have 

 however free-swimming larvae, but these swim by cilia. On the whole, we think it 

 probable that this larva is the first stage in the life-history of the pearl-forming 

 organism, but until it has been observed to enter the Margaritifera vulgaris, and 

 until a more minute examination by sections proves the precise nature of the larva, 

 it would be unwise to be dogmatic. 



The young larvae within the oyster have a two-fold fate, (i) either they become 

 the nuclei around which their own sarcophagus is secreted, or but this is most 

 improbable (ii) they may grow into the older Tetrarhynchus larva?. The first have 

 no further interest for their race. They perish but to form " pearls of great price " 

 for which men risk their lives, and, as dried-up mummies set in a costly sheath, they 

 serve to deck the crowns of kings and the necks of fair women. Had Cleopatra, 

 when she dissolved her Orient pearl in vinegar, examined the residue with a lens 

 instead of drinking it, she would doubtless have found a shrivelled, dried-up particle, 

 the mummy of a tape-worm larva, around which the pearl had been deposited. 



The Tetrarhynchi found in the pearl oyster seem to lack but reproductive organs 

 to be adults. The following features in these forms are of importance in considering 

 their alleged relationship with the encysted forms found in the Batistes: the 

 presence of the 2 lappets, each sheltering 2 bothria ; the absence of any kind of 

 vesicle; the position of the muscular sheaths into which the introverts can be 

 withdrawn, the posterior end of these lies about half-way along the body ; the warty 

 markings on the skin ; and the ciliated posterior end of the body. 



