36 TREMATODES OF THE NORTHUMBERLAND COAST 



presumably the testes, are faintly visible. The curious thing 

 about this worm is that its cyst is distinctly oval and 

 measures o'4o-o*35 mm., and its walls are very thin, whereas 

 Mcintosh's worm came from a spherical cyst with a very 

 thick wall, and measured o"i3 mm. in diameter. This points, 

 I think, to their being different species. Another encysted 

 cercaria was noticed (Plate I., fig. 10) from Carcinus mcenas, 

 the cyst measuring 0*2 mm. across, and quite spherical. Un- 

 fortunately the worm was destroyed in pressing it out of the 

 cyst. The wall of the cyst was thick, and in all probability 

 this was the species described by Mcintosh. 



In the absence of more detailed measurements it is almost 

 impossible to attempt to fit in these larval forms of Spelotrema 

 with any known adults. It is interesting to find that all those 

 so far observed encyst in Crustacea. Villot* found Distonmvi 

 brachysomum, Creplin, which seems to be a species of Spelo- 

 trema, encysted in Anthtti'a gracilis. 



Two cases of the younger stages of what are almost 

 certainly species of this genus have been observed in MoUusca; 

 in both cases the cercarise, which are without tails, occurred 

 in simple round sporocysts. One of these, Cercaria crispata, 

 was found by Pelseneerf in Natica alderi at Boulogne-sur- 

 Mer; the second was found by myself^ in Littorina 7'udis. In 

 the latter case many more cercarise were found in the sporo- 

 cysts, only two or three occurring in Pelseneer's species. 

 Nicoll§ considers that those from Littorina rudis are likely to 

 be an early stage of Spelotrema claviforme (Brandes) from 

 Tringa alpina and Aegialites hiaticiila (Dunlin and Ringed 

 Plover), which is a very small species not exceeding 0*40 mm. 

 in length. 



Thus so far as we know the genus Spelotrema lives in the 

 adult stage in marine birds, the cercaria encysts in Crustacea, 

 and is developed without a tail in sporocysts in a moUusk (in 



'■ Op. cit. 



t Op. cit., p. 171, PI. XI., fig. 33-40, 



% " Notes on Northumbrian Trematodes," op. oit.. p. 6. 



§ Nkoll, Jr„ Op, cit., Ann. and Ma°-. Nat. Hist., 1907, p. 255, 



