98 How Animals Changed 



perature, moisture, light, but at the same time prevents the same 

 animal from developing the more flexible reaction through thought 

 as man knows it. Thought will be forever prohibited to the insect 

 line because it is conditioned on a large mass of properly organized 

 nervous tissue. The exoskeleton of the insect positively prevents the 

 development of a land insect of over an ounce or two in weight." 

 A complete exoskeleton in arthropods is inimical to the develop- 

 ment of a large brain. "Besides the skeleton use, it may have devel- 

 oped as an armor in the water or as a protection on tidal beaches." 

 An exoskeleton gives an insect a certain degree of passive resistance, 

 but, as it is a land animal, limits it to a small size and restricts its 

 "reserves of excess tissues." It is stronger than an endoskeleton, 

 gives great surface for muscle attachments, and is excellent for 

 preventing desiccation. The presence of an exoskeleton has left in- 

 sects plastic in some respects; e.g., it has permitted the evolution of 

 metamorphoses. 



Animals such as certain nematodes and dipterous larvae (Thorpe, 

 1930; Hinman, 1932) are able to live in media which differ mark- 

 edly from their own body fluids because the cuticles on the out- 

 sides of their bodies are practically impermeable. Animals with 

 such cuticles may be aquatic or terrestrial, and the presence of such 

 protective coverings perhaps is to be looked upon as a fortunate 

 accident which permits their possessors to live in media which would 

 be fatal to less protected animals, rather than adaptations which 

 have developed to fit certain environments. 



The respiratory functions of the integument in aquatic habitats 

 are served by various types of more or less elaborate gills. Often 

 these are protected in enclosed cavities through which water circu- 

 lates. Such cavities may enclose the whole body of an animal, as in 

 certain polychaete worms, clams, ostracods, chironomids, and caddis- 

 fly larvae, or merely the respiratory organs, as in shrimps and active 

 snails. In air-breathing and terrestrial animals capillaries are often 

 spread over thin, moist membranes, which cover enclosed branchial 



