P. Granville put into practice between Alum Bay in the Isle of Wight and the Needles lighthouse a method which depends upon conduction through sea water.
Besides the delta of the Po and the large marshy tracts which it forms, there exist on both sides of it extensive lagoons of salt water, generally separated from the Adriatic by narrow strips of sand or embankments, partly natural and partly artificial, but havin openings which admit the influx and efflux of the sea-water, and serve as ports for communication with the mainland.
They can live in brackish and even in sea water.
Many Gastropoda deposit their eggs, after fertilization, enclosed in capsules; others, as Paludina, are viviparous; others, again, as the Zygobranchia, agree with the Lamellibranch Conchifera (the bivalves) in having simple exits for the ova without glandular walls, and therefore discharge their eggs unenclosed in capsules freely into the sea-water; such unencapsuled eggs are merely enclosed each in its own delicate chorion.
Such could hardly be obtained in any other way by those worms that have no special respiratory apparatus, and that live in mud and under stones where the natural supply of freshly oxygenated sea-water is practicaily limited.