PARASITISM, ETC., ON THE SEA-SHORE 147 



A possible explanation of this alternation of hosts was put 

 forward earlier in this chapter, viz. that the present inter- 

 mediate host may be the original one and that, as evolution 

 proceeded, later stages of the parasite came into association 

 with other and higher forms. If this is correct, then it is 

 not difficult to see that the chances of such a thing happening 

 are greatest at the point where two or more distinct faunas 

 meet and interact. In other words, the conditions for 

 parasitic linkages of this kind are nowhere more favourable 

 than on the sea-shore. 



Thus it is, no doubt, that great numbers of parasitic 

 flat-worms, particularly " flukes," have their intermediate 

 stages in shore- dwelling molluscs. The life-history of the 

 common Distomum hepaticum, the cause of the sheep 

 disease known as " liver-rot," which is fainiliar to every 

 zoological student, may be taken as closely resembHng the 

 life-histories of those Trematodes which are connected with 

 the shore. It should be noted, however, that while sporo- 

 cysts and cercariae have been recorded from numerous shore 

 animals, the complete life-history has in many cases not been 

 worked out. 



The common periwinkle Littorina littorea harbours 

 Cercaria linearis in the kidneys and mantle-cavity and 

 Echinostomum secundum and C. lophocerca in the digestive 

 gland, while the whelk {Buccinum iindatum) has three species 

 affecting this latter organ (Lebour, 1913— 16). 



The cockle has Bucephalus haimeanus throughout the 

 body, Gymnophallus inargarit<^ under the umbo, a third 

 species in the visceral mass and a fourth in the foot. The 

 first-mentioned form, which is a larval stage of the adult fluke 

 Gasterostomum gracilescens of the " angler," may entirely spoil 

 the cockle for edible purposes. Small aduh Trematodes are 

 also to be found in cockles and two species are known to 

 occur in the. mussel. The tissues of the edible and shore 

 crabs are often crowded with the cysts of the fluke Spelotrema 

 excellens. The digestive glands of infected molluscs look 

 unhealthy, and are of a grey or sickly yellow, or of a pecu- 

 liarly unnatural orange colour. For the most part the 



