ENVIRONMENTAL CLASSIFICATION 47 



protection against evaporation. Their mucus (in Limax maximus, for 

 example) is often tougher than in the shell-bearing snails, and affords 

 a more effective protection. 



Other animals which have accomplished the transition to land life 

 are a few forms with incomplete protection from evaporation, such as 

 land planarians, nematodes, earthworms, and a number of leeches. 



Protection against desiccation is extremely unequal in the various 

 groups, and their capacity for life in the air is equally variable. One 

 may find every transition from aquatic animals which can exist for a 

 short time out of their native element to forms which live continu- 

 ously under conditions of extreme dryness, without drying out. Even 

 though a sharp line cannot be drawn between them, it seems useful 

 to classify air-breathers into hygric, mesic, and xeric forms on the 

 basis of the extent of their tolerance of aridity. 



Hygric animals include land planarians, earthworms, land leeches, 

 Peripatus, and the air-breathing crustaceans. All these are found, with 

 few exceptions, in situations with very humid air. They protect them- 

 selves against temporary drought by retreating into holes in the earth 

 or beneath stones or logs or similar objects, and, in so far as they are 

 not protected by a dermal armor, by the production of mucus in their 

 skins. Isopods, such as Armadillidium and Porcellio pictus, which 

 live in drier places, are less subject to evaporation on account of hav- 

 ing fewer glands. 31 The air-breathing snails also belong to this cate- 

 gory. They are able to come out of their shells only in moist air, but 

 they can penetrate into relatively dry regions if the air reaches the 

 necessary degree of humidity from time to time, as they are able to 

 retire into their shells and close them off during the dry periods. The 

 thicker their shell and the greater their capacity for existing in a state 

 of suspended animation without food, the more possible it becomes 

 for them to inhabit arid situations. Thus in Germany, Helix (Xero- 

 phila) ericetorum lives on dry and sun-burned slopes, and elsewhere, 

 desert snails extend as far into the steppe and desert as does the occa- 

 sional deposit of dew. These animals can exist for great periods of 

 time without sign of life, in a state of aestivation. While the German 

 grapevine snail (Helix po?natia) can live at most a year in such a con- 

 dition, steppe and desert snails, such as Helix desertorum, have been 

 repeatedly known to revive after more than four years of suspended 

 animation. 32 



Of the terrestrial vertebrates, the amphibia belong to the hygric 

 group. But these, too, are insured against injury by drought by means 

 of special adjustments which enable them to spread into dry regions 

 with only occasional humid periods. Some are able to save up water 



