72 The Vertebrate Organ Systems 



endocrines secrete hormones which are distributed bv the blood to all 

 parts of the body. 



Just ventral to the midbrain is the pituitary gland. This gland 

 consists of three separate lobes, each with a different function. The 

 anterior lobe controls the growth process, particularly that concerned 

 with the size attained by the individual. It also secretes other hor- 

 mones which exercise a controlling effect over the other endocrine 

 glands. The intermediate lobe of the pituitary secretes a hormone which 

 controls the scattering or concentration of the pigment granules of the 

 chromatophores. The posterior lobe controls the shedding of the skin 

 and the water intake of the skin. 



Another important gland is the thyroid. It is located just posterior 

 to the hyoid cartilage in the throat region. Its secretion controls the 

 general metabolism of the body and is necessary for the transformation 

 of the larva into the adult. 



On the ventral surface of the kidneys are the long yellow adrenal 

 glands. Their secretion affects blood pressure and pigment concen- 

 tration. 



The pancreas which secretes a digestive enzyme has special cells, 

 the islets of Langerhans which secrete a hormone. This hormone con- 

 trols sugar metabolism. 



THE EXCRETORY SYSTEM 



The principal excretory organs of the frog are the kidneys (Fig. 

 20). These are elongated, brown-colored organs lying dorsal to the 

 peritoneum of the coelom in the subvertebral lymph sinus. Through a 

 series of tubules in the kidneys, nitrogenous wastes are filtered from 

 the blood. These tubules unite with others to form larger tubules 

 which finally form the ureter. The ureters, one from each kidney, are 

 slender white tubes which extend behind the peritoneum from the kid- 

 neys to the cloaca. The waste material, urine, may either pass out the 

 anus or be stored in the bladder that opens from the ventral side of 

 the cloaca. 



THE REPRODUCTIVE SYSTEM 



The Female. — The reproductive organs which produce the eggs 

 are the ovaries (Fig. 20). When these large dark bodies become filled 

 with eggs, they occupy almost the entire coelomic space, even displac- 

 ing the other organs. When the eggs are ripe, they break out of the 



