88 CEYLON PEARL OYSTER REPORT. 



consist of hardly anything more than the head, but the little piece of body shows 

 some slight traces of the markings at the hinder end of the large larval form (see 

 Part II., Plate II., fig. 20). The arrangement of the lappets, the proboscides, the 

 proboscis sheath and the proboscis tubes are similar to those of the older larvae, and 

 so, as far as it could be made out, and that was by no means completely, was the 

 shape and arrangement of the teeth in the proboscides. 



Assuming for we have as yet no absolute proof that the youngest form of 

 T. unionifactor forms the nucleus around which the pearls are deposited, we have in 

 this lately found larval form an explanation of how the species is preserved. Of the 

 given number of larvae which enter at a very early stage into the body of the Oyster 

 a certain number arrive in the mantle and other tissues, acquire an ectodermic sac 

 and there encyst, and find a costly grave in the developing pearl. The remainder, 

 however, reach the alimentary canal and grow and flourish there. When they attain 

 the dimensions of the stages described in Part II., they leave the alimentary canal 

 and encyst usually upon the outer surface of the intestine. Now they are too big 

 for enclosure in a pearl, and they can wait without anxiety for the advent of their 

 second host (Rhinoptera javanica) within whose intestine they rapidly become 

 sexually mature. It is not entirely impossible that these Tetrarhynchids are different 

 species, though at present the evidence is in favour of the two being different stages of 

 the same species. If they are different species, the smaller probably corresponds 

 with the smaller pearl-forming larvae described in the previous paper (this vol., p. 22). 



Further specimens of the Trematode Aspidogaster margaritiferce were also collected. 



II. NEMATODA. 



Professor M. STOSSICH, of Trieste, whose untimely death has deprived us of a most 

 helpful friend, has supplied us with the names and in some cases with short descriptions 

 of the few Nematodes collected on this occasion. 



AETOBATIS NARINAItl (EuPHBASEN). 



Spiropterina scillicola, v. BEN. 

 Aetobatis narinari is a new host for this species. 



CARCHARJAS MULLERI, MULL, and HENLE. 

 Ascaris, sp. 



An embryonic form too young to be identified. 



