=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, attaining to the more perfect or sexual form on reaching the interior of some vertebrate. Thus _Distoma endolabum_ 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 (_Gammarus 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 _Planorbis corneus_, secondly through certain flies and gnats (_Ephemera_, _Perla_, _Chironomus_), and finally arrives within certain species of bats. _Distoma nodulosum_ Zed. inhabits firstly _Paludina impura_, secondly certain fishes (_Cyprinus Acerina_), and lastly the common perch. The sporocyst of _Distoma macrostomum_ inhabits _Succinea putris_, pushing itself up into the tentacles, which become unnaturally distended (Fig. 23). While in this situation it is swallowed by various birds, such as the thrush, wagtail, and blackbird, which are partial to _Succinea_, and thus obtains lodgment in their bodies. _Amphistoma subclavatum_ spends an early stage in _Planorbis 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 _Amphistoma_, which thus becomes established in the frog’s stomach.[142] [Illustration: FIG. 23.--A Trematode worm (_Leucochloridium paradoxum_ Car.) parasitic in the tentacles of _Succinea putris_ L. × 20 (after Baudon).]