ON NEW LARVAL TREMATODES. 767 



43. On two new Larval Trematodes from the Striped Snake 

 {Tropidonotiis ordinatus sirtalis). By William Nicoll, 

 D.Sc, F.Z.S., Lister Listitute oE Preventive Medicine, 

 London. 



[Received April 9, 1912 : Read May 21, 1912.] 

 (Text-figure 107.) 



Index. 



Page 



Ethology : Two larval Trematodes in the mesenteric fat of 

 Striped Snake {Tropidonohis ordinatus sirtalis). Adult 

 form probably in a bird 767 



Geographical Zoology : North America ; Striped Snake 



infected with two larval Trematodes 767 



Sj'stematic : Cercaria ordinata, sp. ii., and Diplostomum 

 sirtale, sp. n,, from cysts in themesenter^' of the Striped 

 Snake 768 



The occurrence of encysted larval parasites in snakes is evidence^ 

 if such were wanting, that some snakes are eaten by other 

 animals.. What is more important, the character of the parasites 

 may indicate what variety of animal is in the habit of eating the 

 snake in questioii. Conversely, the presence of any particular 

 species of adult parasite in an animal is almost always a sure 

 proof that such animal eats the snake in which the larval stage 

 is found. It is unfortunately in many cases a matter of difficulty 

 to diagnose the systematic characters of a larval pai-asite. In a 

 number of cases, however, it is possible to assign it to a definite 

 genus, rarely to a particular species. 



The two cases to be dealt with here present a certain amount 

 of difficulty. The first larva is evidently a Distomate Trematode,. 

 but beyond that it is impossible to go ; the second larva is just 

 as obviously a Holostomid, and almost certainly belongs to the 

 genus Hemistomum. Such a diagnosis does not lead very far, 

 but it at least enables one to say that in all probability the 

 Striped Snake (IVopidonotics ordinaitis, var. sirtalis) is eaten by 

 some bird, for adult Holostomata are known to occur only in 

 birds. This will possibly be confirmed by direct observation. 



Both forms were met with together in each of three Striped 

 Snakes from North America, which died in the Society's Gardens 

 on the 5th and 20th December, 1910, and on 10th Maixh, 1911. 

 They occurred in enor-mous numbers in the mesenteric fat along 

 the whole length of the intestine. Each was enclosed in a small 

 spherical or ovoid cyst wdth unusually thin and soft walls.. 

 Unlike what is generally found in a Trematode cyst, the wall 

 gave the impression of being a thin membrane instead of the 

 more usual tough chitinous investment. On this account, not 

 only could the larvae be extracted from the cysts without dif- 

 ficulty, but when placed in water they escaped readily of their 

 own accord. When a piece of the cyst-infested mesentery was. 



