ENVIRONMENTAL CLASSIFICATION 



45 



made possible an air-breathing existence in moist situations for the 

 more annelid-like Peripatus and its allies. 



Vertebrates are protected against drying by the stratified structure 

 of their skin, and this is already developed in the fishes. In inverte- 

 brates, with the sole exception of the chaetognathous worms, the 

 epidermis consists of a single layer of cells; in vertebrates it is com- 

 posed of successive cellular layers. The outermost of these layers 

 undergoes adaptive changes even among the fishes. The cells die off 

 with an accompanying development of horn and form a protective 



Fig. 3. — Outer branch of the first abdominal appendage of the land isopod 

 Armadillidium nasutum, with much divided respiratory cavity. After Herold. 



covering for the remainder of the skin. In terrestrial vertebrates the 

 number of layers of cells is increased, and the horny stratum, still 

 single in the amphibians, is formed of more numerous layers of small 

 cells. Protection against evaporation is thus based on structures al- 

 ready available. 



Complete transition to an air-breathing existence became possible 

 to the vertebrates, as to the invertebrates, by means of changes in the 

 breathing apparatus. The gills of fishes, not unlike those of the higher 

 Crustacea, afford a sufficient surface only in the water, as their 

 branches cohere in the air. Of the various types of air-breathing or- 

 gans developed among the fishes, lungs proved most successful. These 

 originate as a sac-like evagination of the anterior part of the alimen- 

 tary canal, just posterior to the hindmost pair of gill clefts. 



