Limnaeidae taken from a great depth of water, _e.g._ from 130 fathoms in the lake of Geneva, have been examined by Forel.[272] The pulmonary sac is full of water, but there is no transformation of organs, no appearance of a branchia, to meet the changed circumstances of their environment. Doubtless a good deal of respiration is done by the skin; being soft and vascular, it respires the air dissolved in the water. Forel cites cases of _Limnaea_ living at much shallower depths, which come to the surface once, and then remain below for months. The oxygen of this supply must soon have become exhausted, and the animals, discontinuing for a time the use of the pulmonary chamber, must have respired through the skin. Shallow-water _Limnaea_, according to the same authority, remain beneath the surface during cold weather; when warm weather returns they rise to the surface to take in a supply of air. Since the water at great depths is always very cold, there is no need for the _Limnaea_ living there to rise to the surface at all. It is a curious fact that _Limnaea_, which have been respiring by the skin for the whole winter, should suddenly, on the first warm days of summer, take to rising to the surface and breathing air. But exactly the same phenomenon is shown in the case of _Limnaea_ from great depths. Placed in an aquarium, they _immediately_ begin rising to the surface and inspiring air; in other words, they experience instantaneously a complete transformation of their respiratory system. In _Onchidium_, a land pulmonate which has retrogressed to an amphibious or quasi-marine mode of life, there is no organ which represents the pulmonary or branchial cavity, the so-called lung being only a cavity of the kidney. Respiration is, however, conducted by the skin as well, and by the dorsal papillae.[273]