Ill WORMS PARASITIC IN MOLLUSCA 6 I 



cycle of their developmeut within the bodies of MoUusca, attain- 

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

 of some vertebrate. Thus Distoiiuc endohihmn. Duj. finds its 

 first intermediate host in Liriinaea stagnalis and L. ovata, its 

 second in L. stagnalis, or in one of the fresh -water shrimps 

 (Gammarus 2nclex), ov in the larvae of one of the ritrijganeidae 

 {Limno'pliil'us rhoi/ihicus), attaining to the sexual form in the 

 common frog. Distoma ascidia v. Ben. passes firstly through 

 Lininaea stagnalis or Flanurhis corneus, secondly through certain 

 files and gnats {Eijlieniera, Perla, Chironuiivits), and finally arrives 

 within certain species of bats. Distoma nodidosuiii Zed. inhabits 

 firstly Paludina inqrwra, secondly certain 

 fishes {Cyprinus, Acerina^, and lastly the 

 common perch. The sporocyst of Distoma. 

 ttiacrostomum inhabits Suceinea putris, 

 pushing itself up into the tentacles, which 

 become luaiaturally distended (Fig. 23). 

 AVhile in this situation it is swallowed l)y 



various birds, such as the thrush, wagtail, ^^"'- 23. -A Trematode 

 and blackbird, which are partial to buccinea, ^yaradoxum Car.) para- 

 and thus obtains lodgment in their bodies. f '° l"^ *^'« tentacles of 



*^ Succinea pntris L. x 20 



Am2)h'istoma suhclavatum spends an early (after Baudon). 

 stage in Flanorhis contortus, after which it 



becomes encysted on the skin of a frog. When the frog sheds 

 its skin, it swallows it, and with it the Anvpliistonia, which 

 thus becomes estal)lished in the frog's stomach.^ 



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

 cost this country 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, liow 

 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 end:)ryo finds a Limnaea truncatula within 

 about eight hours it l)ec()mes exhausted, sinks, and dies. It has 

 been tried with all the other common pond and river Mollusca, with 

 Limnaea feregra, palustris, auric ul aria, stagnalis, with Flanorhis 

 marginatus, carinatits, vortex, and sjnrorhis, with FJnjsa fontinalis, 

 Bitliynia tentaculata, Faludina vivijMra, as well as with Succinea 

 putris, Limax agrestis and maximus, Arion ater and hortensis. 



1 A. Lang, Bcr. Naturf. Gcs. Frcih. vi. 1S92, p. 81. 



