CESTODE PARASITES. 259 



No. 22. 



A DESCEIPTION OF TEN NEW SPECIES OF CESTODE 



PARASITES FROM MARINE FISHES OF CEYLON, 



WITH NOTES ON OTHER CESTODES 



FROM THE SAME REGION. 



By T. SOUTHWELL, A.R.C.Sc. (Lond.), F.L.S., F.Z.S. 

 With three Plates. 



Trawling operations were continued, as usual, during the past year. The area under lease over 

 which these operations were carried out cover the greater part of 700 square miles. At the jjresent time 

 fish are by no means abundant, a fact which is undoubtedly connected with the absence of suitable food. 

 "Shell fish" are remarkably scanty. Five years ago, wlien both oysters and other shell fish were 

 abmidant, fish were much more plentiful. 



All the fish caught have been carefully and systematically exammed for cestode parasites, 

 particularly the Elasmobranchs. No adult cestodes have been found in any Teleosts, but cysts are 

 very numerous in most species, and a few are described in the present paper. The habitat and the larvje 

 of the fdult cestodes described from the pearl banks are generally not known. Most probably this is 

 because they have not lieen sufficiently carefully searched for. 



It is almost certain that the larvae of the adult forms described will be found later in tlio various 

 Crustacea and molluscs. 



No parasites have been found in the flesh of any of the Teleosts examined. All the cj^sts obtained 

 were found in the mesenteries, save in one case, where a few were discovered along with some Trematode 

 redia (which measured about 20 mm. by 8 mm.) on the walls of the air bladder. 



The position of the encysted forms found in Teleosts, with relation to the Ufe-history of the parasite 

 concerned, is very enigmatical. I am confident that in some cases these bony fish are merely parallel 

 hosts, and that the life-history of the parasite is direct from the crustacean or molluscan host to the 

 Elasmobranch. In some cases it may be found later that the mfoction of the Teleost is initial, and that 

 the life-history is direct from the Teleost (particularly if this is a small form) to the Elasmobranch. 

 The fact that no adult cestodes have ever been found in Teleosts lends favour to this theory. 



With reference to the life-history of the pearl-inducing worm itself (Tetrarhynclius unionijactor) 

 no further light has been adduced, but it seems practically certain that the life-history is dhect from the 

 oyster to the various Elasmobranchs whicJi devour them, and that Balistes, Serranus, and possibly other 

 genera of fish represent subsidiary or parallel hosts. 



During an examination of Taniblegam Lake, which I recently made along with Dr. Pearson oi the 

 Colombo Musemn, a few Trygons of various species were collected. In one .species a few adult specimens 

 of Tetrarhynchus unionijactor were obtained. There seems little doubt that the pearl-inducing larvae. 



