204 FLEAS, FLUKES AND CUCKOOS 



cystogenous glands. This fluid rapidly hardens into an impermeable cyst 

 wall. As Hydrobia is found in large numbers feeding upon Z'^siera, the 

 larvae when they emerge frequently encyst on the plant itself. The geese 

 and ducks become infected by ingesting them along with the Z^siera or 

 by accidentally swallowing the minute snails which are encrusted with 

 Catatropis cysts. 



A few gulls are always to be seen dipping into pools, fishing in 

 the httle gulhes on the saltings, or sitting in small flocks along the 

 edge of the water waiting for the turn of the tide (Plate XL). No 

 group of birds seems more heavily infested with worms and at least 

 twenty species of trematodes have been recorded from the black-headed 

 gull alone. One of the great groups of flukes, the Plagiorchioidea, is 

 well represented in the gulls. The larvae of these flukes develop in so- 

 called sporocysts — which are morphologically somewhat different from 

 rediae — and although there are many exceptions to the rule, the 

 majority employ arthropods as the second intermediate host. They have 

 trowel-shaped cercariae armed with a minute javelin-like stylet with 

 which they pierce the softer portions of the host's integument. 



In the intestine of the black-headed gull and various other 

 crustacea-eaters such as dunlin, godwits, turnstones, sandpipers, 

 plovers, oystercatchers and curlews we find various flukes of the 

 family Microphallidae, which are characteristic of the saltings and the 

 shore. Several different species use Hydrobia and Littorina as the first 

 intermediate host and the shore crab {Carcinus maenas) and Gammarus 

 and other amphipods such as sand-hoppers as the second host. In the 

 case of Maritrema oocysta (formerly known as M. humile) the cercariae cut 

 short the complicated life-cycle, for they never emerge from Hydrobia, 

 but cast off their tails and their stylet and encyst within the snail. Thus 

 the redshank, which serves as the final host, becomes infected by eating 

 the mollusc. 



Another group of plagiorchid trematodes which infests an incredible 

 variety of bird hosts are the oviduct flukes (Prosthogonimus) . These 

 worms have attracted a great deal of attention, for their presence in the 

 domestic fowl greatly reduces egg-laying. Sometimes they get caught 

 up in the egg during its development and any trematode the reader may 

 find cooked up with his breakfast is almost sure to be this species. Up 

 to date the known life-cycles all follow a similar pattern. The cercariae 

 emerge from a freshwater snail and swim about in the water. Accident- 

 ally they are drawn into the rectal respiratory chamber of a dragon-fly 



