Mollusca 135 



general type is amazingly conservative in structure, yet plastic 

 in its adaptations to different climates. 



Another species of Agriolimax, the A. agrestis of Europe, has 

 become naturalized in certain gardens in Boulder. It is quite a 

 pest, and seems to have a particular liking for choice orchids. 



Agriolimax agrestis Linnaeus Agriolimax campestris Binney 



This species is larger than the native one, when well grown about 

 35 mm. long, usually pale reddish or grayish, and often with gray 

 or black spotting. The most distinctive feature, however, is the 

 possession of milky slime. Specimens of A. agrestis received 

 from the Colorado Agricultural College were collected by J. H. 

 Newton of Paonia, where they were (1924) injuring ripe tomatoes. 

 Two other foreign slugs, brought in with plants, have been 

 observed in Boulder gardens. One is Limax maximus, a very 

 large species with the mantle or covering of the anterior part of 

 the body prettily spotted or marbled. The other, called Milax 

 gagates, is of moderate size, with a sharp keel or ridge down the 

 back, especially noticeable when the animal is at rest. Two 

 other kinds of slugs have been found in a greenhouse at Boulder, 

 but have no permanent place in the fauna. 



FRESH WATER SNAILS 



Fresh water snails may be divided into two great groups 

 according to their mode of respiration. Undoubtedly the more 

 primitive method is by gills or processes of the body, absorbing 

 the oxygen which is dissolved in the water. The original molluscs 

 were certainly aquatic, and presumably marine; but many millions 

 of years ago some of them, just as with the vertebrates and ar- 

 thropods, discovered the land, gradually becoming adapted to 

 terrestrial life. Their subsequent history also shows a curious 

 parallelism. Among the vertebrates, certain terrestrial groups 

 produced members which took to the waters, reversing the original 

 sequence of events. Doing so, they could not return to the old 



