In this article we will discuss about the endocrine system of toad.
The endocrine (endos = within; krine = separate) system includes a number of ductless glands which secrete chemical substances called hormones (hormao —excite) directly into the blood stream. They are thus distinguished from the exocrine glands, such as liver, which pour out their secretions externally through ducts.
The hormones are carried by the circulating blood to every part of the body. They control the function of distant organs and regulate the activities of the animal as a whole. Moreover, any single hormone does not usually function alone; it either stimulates or depresses the production of some other hormone.
Thus, the endocrine glands act as a team which is responsible for the chemical co-ordination of the vital processes. It is for this reason that the endocrine glands, though small in size and scattered in distant parts of the body, are recognised as an organ system.
The endocrine system shares with the nervous system the important task of coordinating the parts of the body. The nervous system exerts its control through nerves which are directly connected with all the body structures. The endocrine system does the same job by a more simple method; its means of control is purely chemical, the hormones being carried to distant parts of the body by the blood stream.
Glands under Endocrine System:
The various glands included under this system are:
(7) Gonads, and
The pancreas plays a dual part by giving off both exocrine and endocrine secretions. Similarly, the gonads manufacture germ cells and at the same time release sex hormones. The spleen, although it is a ductless gland, has no endocrine function.
1. Thyroid Glands:
These are a pair of small oval structures located on the floor of the buccal cavity, one on each side of the hyoid apparatus. The glands are deeply concealed and often overlooked.
They produce a hormone, called thyroxine, which regulates general metabolism in the adult and metamorphosis in the tadpole. When small tadpoles are fed with thyroid gland substance, they stop growing, undergo an early change, and are quickly transformed into tiny toads. Ecdysis, or periodic shedding of the horny layer of the skin, is also regulated, at least partly, by the activity of the thyroid gland.
2. Parathyroid Glands:
Parathyroids (para = near) are yellowish, ovoid bodies lying near the thyroid, upon the external jugular veins. They regulate the amount of calcium salts in the blood stream.
3. Thymus Glands:
These are small yellowish structures situated on either side of the head just behind the tympanum. Functioning in young tadpoles, the thymus regulates growth and retards premature formation of germ cells. The glands are lost by atrophy in adult toads. Nowadays, the thymus is not regarded as an endocrine organ.
4. Pituitary Body:
This is a glandular structure attached to the hypophysis and located on the floor of the diencephalon. The gland has two lobes: an anterior lobe and a posterior lobe. The posterior lobe of the pituitary body secretes a hormone called pituitrin which regulates the intake of water by the skin.
Considering the fact that a toad hardly drinks water by the mouth, this is perhaps the only means for the intake of water. Moreover, pituitrin enables the toad to change the colour of its skin by shifting the position of the chromatophores.
The anterior lobe of the pituitary secretes a number of hormones. In tadpoles and young toads, it secretes a growth-stimulating hormone and a thyroid-stimulating hormone. It has been proved conclusively that the anterior pituitary, in collaboration with the thyroid, regulates the process of metamorphosis by which a tadpole is changed into a toad.
In adult toads, the anterior lobe secretes:
(i) A gonad-stimulating hormone which hastens the liberation of germ cells,
(ii) A metabolic hormone which stimulates metabolism in general, and
(iii) Several other hormones for regulating the activity of other endocrine glands. It is for this reason that the anterior pituitary is regarded as the captain of the endocrine team.
5. Adrenal Glands:
These are narrow orange-coloured bands on the ventral surface of the kidneys. The outer portion or cortex of the gland produces a hormone called cortin which is essential for life. Removal of both adrenals is followed by the death of the animal in the course of a week.
The inner portion or medulla of the adrenal secretes another hormone called adrenaline which regulates the activity of the involuntary muscles by stimulating the autonomic nervous system. It controls blood pressure and stimulates metabolism in general.
Scattered within the substance of the pancreas are special groups of cells called islets of Langerhans. The cells of the islets are quite different from the ordinary pancreatic cells which manufacture digestive enzymes.
Moreover, they are not connected with the pancreatic duct in any way. The secretion of the islets, the insulin, is poured directly into the blood stream. Insulin regulates carbohydrate metabolism and favours storage of glycogen in the liver.
The testes and the ovaries, in addition to manufacturing germ cells, produce sex-hormones which regulate the development of the secondary sexual characters. The male sex-hormone is known as the testosterone. Swelling of the thumb pad, inflation of the vocal sac, and the clasping habit of the male toad are regulated by the male sex-hormone.
The endocrine secretion of the ovaries is known as the oestradiol. It controls the behaviour of the female toads at the time of breeding. In the male toads, a peculiar structure called Bidder’s organ is located at the anterior end of each testis.
It has been proved, that if the testes are removed, Bidder’s organs will soon develop into functional ovaries. A complete reversal of sex may thus be induced, the transformed Bidder’s organs helping the process by secreting female sex-hormone.
The spleen is the only ductless gland which does not secrete any hormone. It is a small dark-red structure of spherical shape which is situated in the mesentery near the junction of the ileum and the rectum. The spleen serves as a storehouse for blood and manufactures one kind of leucocytes. Moreover, old and worn-out erythrocytes are destroyed in the spleen.