The Endocrine System 249 



Little is known of the exact chemical nature of these hormones nor 

 of the possible effects of hypo- or hyperactivity. 



OTHER POSSIBLE ENDOCRINE STRUCTURES 



The Thymus Gland. — In the thorax, just above the heart, is a 

 large glandular structure, the thymus. It is fairly large in young ani- 

 mals, but atrophies after maturity is reached. Because of this size 

 relationship, it has been suggested that perhaps the thymus has some 

 endocrine function related to sexual development and attainment of 

 puberty. This, however, has never been adequately proved ; thus it 

 is safest to regard the thymus as at best a doubtful producer of hormones. 



The Pineal Gland. — The pineal body is an outgrowth from the 

 roof of the diencephalon of the brain, but it does not have nerve cells in 

 it. It is quite conspicuous, and has been known to anatomists for many 

 centuries. There have been attempts to ascribe some function or sig- 

 nificance to this structure. At one time it was assumed to be the 

 seat of the soul, but this is an illusive function for a mere biologist 

 to prove ! Because of its histological structure, it has at times been 

 assumed to be an endocrine gland ; this, too, has not been proved. 



A more reasonable explanation is that it represents a vestigial 

 organ. This is based upon the fact that it is homologous with the 

 third eye of certain primitive fish and reptiles. 



INVERTEBRATE HORMONES 



Although relatively little is known about invertebrate endocrine 

 systems, it has been clearly demonstrated that important hormones 

 do exist. 



In insects it has been definitely shown that the metamorphosis from 

 larva to adult is under hormonal control. This growth and differentia- 

 tion hormone appears to be secreted during the entire larval or nymphal 

 period. It is responsible for the molts that occur during growth. There 

 is also some evidence that in a few species reproductive processes are 

 in part controlled by special hormones. 



Among the crustaceans (crabs, crayfish, shrimp, etc.) an important 

 gland, the sinus gland, is located at the base of the eyestalk. Secretions 

 from this gland have been demonstrated to be important in inhibiting 

 molting, in controlling color by dispersion of pigment, and in stimulat- 

 ing the eye for light adaptation. 



