196 ESSENTIALS OF ZOOLOGY 



third group is that of the hormones which affect, in particular, the 

 growth and development of the organism. Another general method 

 of classifying substances of this nature terms those substances 

 which serve as excitors or accelerators ''hormones" and those which 

 mhihit or depress activities as ''chalones. " Illustrations of each of 

 these will be found in the discussion of the individual organs and 

 their functions. 



Hormones are found in plants as growth hormones, in the inverte- 

 brate animals, and are best known in vertebrate animals. The follow- 

 ing examples will serve to demonstrate the nature of endocrine func- 

 tion in invertebrates: Earthworms whose testes are completely de- 

 stroyed, do not develop the clitellum, the band-like organ which func- 

 tions during and following copulation. In BonelUa, another annelid 

 worm, there seems to be a relation between sex determination and cer- 

 tain hormones, in that the week-old larvae are indifferent sexually, 

 but those that attach to the proboscis of the female parent become 

 males due to some agent received there. All others develop to become 

 normal females. Among the crabs there is a parasite which attacks 

 and destroys the gonads, and it is found that the parasitized male 

 crab will take on distinctly female characters due to the lack of some 

 humoral agent lost with the destruction of these glands. Too, using 

 radium to destroy the gonads of Asellus, another crustacean, seems 

 to indicate that the development of the brood pouch is controlled by a 

 substance produced in the normal ovaries. Molting and metamorpho- 

 sis both are regulated by hormones in insects. The supraesophageal 

 ganglion seems to produce a substance which initiates pupation activi- 

 ties in moths. The endocrine glands of vertebrates which are best 

 known and most clearly understood at the present time are the thy- 

 roid gland, the parathyroid glands, the suprarenal bodies, the pitui- 

 tary body or hypophysis, the thymus, the gonads, and the pancreas. 



The Thyroid Gland 



The thyroid gland is the most familiar of the endocrine organs 

 to the layman. It is a body of two lobes of about the size of wal- 

 nuts, slightly flattened, placed one on each side of the upper part 

 of the trachea just below the larynx, or voice box, the two lobes 

 being connected by a saddle-shaped isthmus. The thyroid is well 

 supplied with blood, receiving in proportion to weight, three and 

 one-half times as much blood as the brain; this permits an easy 

 access of the hormone to all parts of the body. The thyroid is 



