232 CHARLES HARLAN ABBOTT 



environment. Because of this fact, such diverse animals as 

 annelid worms, gasteropods, and crustaceans, which are all found 

 in a wide variety of habitats, frequently occur together in the 

 same ecological association. 



The environmental importance of behavior characters is shown 

 further in a discussion by Vestal ('14), in which he compares in 

 detail five sets of characters — structural, physiological, psycho- 

 logical, biographical, and numerical. Vestal concludes that, at 

 least so far as competition with other animals is concerned, 

 'psychological' or, as he preferably calls them in another con- 

 nection, behavior characters, are actually the most important 

 of all. 



In order to discuss intelligently the significance of the photo- 

 taxis of land isopods, a partial ecological analysis of these ani- 

 mals will be given. 



1. Habitat 



A habitat analysis may be made on the basis of a table by 

 Adams ('15, p. 95), which summarzizes for comparative purposes 

 the habitats of prairie and forest invertebrates. The ground 

 stratum of the forest, which land isopods typically occupy, is a 

 region where they are exposed, comparatively speaking, to a 

 large amount of humidity, a small amount of evaporation, and 

 a fairly constant temperature. For animals only slightly 

 adapted to land life, these appear to be the optimum conditions. 



2. Structural and physiological adaptations 



A brief discussion will be given of a few of the characteristics 

 of land isopods which make possible their living in a habitat 

 furnishing the conditions just mentioned. 



a. Respiration. Land isopods have a somewhat generalized 

 crustacean structure, with a hard outer covering of chitin, which 

 prevents evaporation of water from the body. The organs of 

 respiration, as in the aquatic isopods, are modified abdominal 

 appendages (pleopods). The nature of the mechanism of res- 

 piration is still somewhat in doubt, although the subject has 



