How Animals Changed 95 



inlet. If the first tetrapod were to succeed on land, the sense organs 

 of the fish would have to undergo considerable modification, for, 

 while the lateral line organs would be no longer required, the audi- 

 tory, optic, and olfactory centers would gain a high importance, 

 demanding in some cases fundamental changes in the structure of 

 the organs. If the head were as flat as that of many frogs, special 

 muscles to raise the eyes above the surface of the skull would be 

 needed if the eyes were to be at all eflicient. Lastly, the loosely hung 

 jaw of the majority of teleosts would have to be firmly fixed to the 

 brain case." % v * \ 



Land animals have more or less effective, and often elaborate, 

 mechanisms for conserving water, regulating temperature, main- 

 taining internal fluids at optimum concentrations, respiration in air, 

 reproduction without water, and locomotion and sensation in air. 

 How such adaptations have developed will be considered in the fol- 

 lowing sections. 



The following classification of animals in relation to environment 

 is proposed by Harms QC Dragendorff (1933) : 



I. Aquatic Animals: positively hydrotactic. 



a. Passively Homoiosmotic, Stenohaline Animals: marine. 



1. Plankton Animals. 



2. Swimming Animals. 



3. Animals Living on Ground. 



4. Animals Living in Ground. 



b. Passively Poikilosmotic, Euryhaline Animals: little adapted brack- 

 ish water animals. Classes 1 to 4 as under a. 



c. Actively Poikilosmotic, Euryhaline Animals: specifically adapted 

 brackish water animals: Classes 1 to 4 as under a. 



d. Actively Poikilosmotic Animals, Euryhaline to Homoiosmotic Ani- 

 mals: estuaries and intertidal zones. Classes 1 to 4 as under a. 



e. Actively Homoiosmotic Animals: fresh-water animals, including 

 marine teleosts, dipnoans?, ganoids, selachians, and hemicraniotes. 

 Classes 1 to 4 as under a. 



II. Air Animals: positively aerotropic. 



a. Actively Poikilosmotic to Actively Homoiosmotic. Animals with 

 Well-Developed Skin Organs to Prevent Desiccation (skin 

 glands) ; in moist (80-90%) air. 

 1. Flying Animals: e.g., flying frog. 



