Ill WORMS PARASITIC IN MOLLUSCA 6 1 



Parasitic Worms, Mites, etc. — A considerable number of 

 the Trematode worms pass one or more of the stages in the 

 cycle of their development within the bodies of Mollusca, attain- 

 ing to the more perfect or sexual form on reaching the interior 

 of some vertebrate. Thus Distoma endolahum Duj. finds its first 

 intermediate host in Limnaea stagnalis and L. ovata^ its second 

 in L. stagnalis^ or in one of the fresh- water shrimps (Crammarus 

 pulex\ or in the larvae of one of the Phryganeidae (^Limnophilus 

 rhombicus)^ attaining to the sexual form in the common frog. 

 Distoma ascidia v. Ben. passes firstly through Limnaea stagnalis 

 or Planorhis corneus, secondly through certain flies and gnats 

 (^JEJphemera^ Perla^ Chironomus^^ and finally arrives within certain 

 species of bats. Distoma nodulosum Zed. 

 inhabits firstly Paludina impura^ secondly 

 certain fishes (Cyprinus Aeerind)^ and lastly 

 the common perch. The sporocyst of Dis- 

 toma macrostomum inhabits Succinea putris, 

 pushing itself up into the tentacles, which 

 become unnaturally distended (Fig. 23). Fig. 23. — a Trematode 

 While in this situation it is swallowed by ;:ZitZTclrfv^. 

 various birds, such as the thrush, wagtail, sitic in the tentacles of 

 and blackbird, which are partial to iSuccinea^ ^^^^^ Baudon) 

 and thus obtains lodgment in their bodies. 



Amphistoma suhclavatum spends an early stage in Planorhis con- 

 tortus, after which it becomes encysted on the skin of a frog. 

 When the frog sheds its skin, it swallows it, and with it the Am- 

 phistoma, which thus becomes established in the frog's stomach.^ 



The common liver-fluke, which in the winter of 1879-1880 

 cost Great Britain the lives of no less than three million sheep, 

 is perhaps the best known of these remarkable parasitic forms of 

 life. Its history shows us, in one important particular, how 

 essential it is for the creature to meet, at certain stages of 

 its existence, with the exact host to which it is accustomed. 

 Unless the newly-hatched embryo finds a Limnaea truncatula 

 within about eight hours it becomes exhausted, sinks, and dies. 

 It has been tried with all the other common pond and river 

 Mollusca, wdth Limnaea peregra, palustris, auricularia, stagnalis, 

 with Planorhis marginatus, carinatus, vortex, and spirorhis, with 

 Physa fontinalis, Bithynia tentaculata, Paludina vivipara, as 

 1 A. Lang, Ber. Naturf. Ges. Freib. vi. 1892, p. 81. 



