A good book to read on weekend mornings with a cup of coffee in one hand and the iPad in the other and when the house is quiet, because only I and the cat are yet awake and the only disturbance is an occasional bird passing by the window to land on the feeder whose view is otherwise obstructed by the curtain is Carl Semper's Animal life as affected by the natural conditions of existence from 1881.
Semper's book is as informative as a 19th century science book can get and what makes it even more interesting than most others from that period is that he too studied snails and wrote about them. He also thought about the adaptations of semi-terrestrial animals and the differences between them and their aquatic and terrestrial relatives.
In most of the cases here adduced, the organisation of the animal appears, so far as we know, to be entirely that of a creature living and breathing in water, or only very slightly modified. The Orchestidae [amphipods; beech hoppers], Nemertidae [terrestrial nemertines], [terrestrial] Snails, and [terrestrial] Leeches show not the smallest difference from their nearest allies living in water...This is quite correct. I know from my own research that there are species of semi-terrestrial snails that do not seem to have any morphological adaptations that enable them to live outside of the water and which their fully aquatic relatives lack. Possible physiological adaptations notwithstanding, the only specialized adaptation the semi-terrestrials seem to have is behavioral. But more on that some other time.
Several related posts have appeared on this blog before. Two of them are here and here.