direction. It is not clear if such changes in thyroid function influence osmoregulation, but OLIVEREAU (1948) has demonstrated increased glandular activity
when teleost fishes are moved from the sea to fresh water, and HICKMAN (1959)
has noted a similar change when the starry flounder is moved from fresh water
into solutions of increased salinity. Treatment of fish with thyroid and antithyroid
preparations also may influence their salinity 'tolerances' and 'preferences,' but the
sites of action, and normal ph ysiological significance of these observations are not
clear (M . FONTAINE, 1956). HOAR (1959) has suggested that the thyroid may aid
fishes in adapting to altered environmental situations and stresses, including osmotic changes, a concept, which in my opinion, makes good sense. We should,
however, reserve judgement as to whether such changes have adaptational significance, remembering that in other endocrine glands, like the adrenal cortex and neurohypophysis, changes that appear to be irrelevant physiologically can be initiated
by non-specific stresses.
4. The Parathyroid Glands and Ultimobranchial Bodies
These endocrine tissues are concerned with the regulation of calcium metabolism.
Embryologically they originate from the region of the branchial (gill) pouches ; the
four parathyroid glands are derived from the third and fourth pair of pouches, and
the ultimobranchial bodies from tissue near the last pair. The parathyroid glands
make their initial phyletic appearance in the amphibians and are absent in the fishes ,
while the ultimobranchial bodies are present in all vertebrates, with the apparent
exception of the cyclostomes. The parathyroid glands lie on either side of the
th yroid gland, while in non-mammalian vertebrates the ultimobranchial bodies lie
in the pericardial region. In mammals the latter tissues are present within the
th yroid tissue, where they make up the parafollicular or 'C' (for calcitonin) cells.
The secretion of the parathyroid glands is a polypeptide made up of 83 amino
acids and is called parathorrnone (GREEP, 1963) . The ultimobranchial hormone
(th yrocalcitonin or calcitonin) of mammals was identified by Cosr et al. in 1962
and has since also been found in representatives of the rest of the tetrapods and
in the chondrichthyean and bony fishe s (Cor-n, 1968). Mammalian calcitonin is
a polypeptide, which consists of 32 amino acids (MAcINTYRE, 1968).
Parathormone elevates the plasma-calcium concentration, while calcitonin lowers it. They thus have an opposing action, which is probably physiological, and
is predominantly mediated by their effects on the rate of calcium reabsorption from
bone. Thus, parathormone facilitates resorption of calcium from bone while calcitonin reduces it. In addition there ma y also be actions on other tissues, including
the kidney .
a) Relation to Osmoregulation
Calcium is an important mineral constituent of the body, both with respect to
structure and to its essential role in many biochemical reactions. It is necessary
for the adequate functioning of all cells, but is not usually considered to be an 0563
when teleost fishes are moved from the sea to fresh water, and HICKMAN (1959)
has noted a similar change when the starry flounder is moved from fresh water
into solutions of increased salinity. Treatment of fish with thyroid and antithyroid
preparations also may influence their salinity 'tolerances' and 'preferences,' but the
sites of action, and normal ph ysiological significance of these observations are not
clear (M . FONTAINE, 1956). HOAR (1959) has suggested that the thyroid may aid
fishes in adapting to altered environmental situations and stresses, including osmotic changes, a concept, which in my opinion, makes good sense. We should,
however, reserve judgement as to whether such changes have adaptational significance, remembering that in other endocrine glands, like the adrenal cortex and neurohypophysis, changes that appear to be irrelevant physiologically can be initiated
by non-specific stresses.
4. The Parathyroid Glands and Ultimobranchial Bodies
These endocrine tissues are concerned with the regulation of calcium metabolism.
Embryologically they originate from the region of the branchial (gill) pouches ; the
four parathyroid glands are derived from the third and fourth pair of pouches, and
the ultimobranchial bodies from tissue near the last pair. The parathyroid glands
make their initial phyletic appearance in the amphibians and are absent in the fishes ,
while the ultimobranchial bodies are present in all vertebrates, with the apparent
exception of the cyclostomes. The parathyroid glands lie on either side of the
th yroid gland, while in non-mammalian vertebrates the ultimobranchial bodies lie
in the pericardial region. In mammals the latter tissues are present within the
th yroid tissue, where they make up the parafollicular or 'C' (for calcitonin) cells.
The secretion of the parathyroid glands is a polypeptide made up of 83 amino
acids and is called parathorrnone (GREEP, 1963) . The ultimobranchial hormone
(th yrocalcitonin or calcitonin) of mammals was identified by Cosr et al. in 1962
and has since also been found in representatives of the rest of the tetrapods and
in the chondrichthyean and bony fishe s (Cor-n, 1968). Mammalian calcitonin is
a polypeptide, which consists of 32 amino acids (MAcINTYRE, 1968).
Parathormone elevates the plasma-calcium concentration, while calcitonin lowers it. They thus have an opposing action, which is probably physiological, and
is predominantly mediated by their effects on the rate of calcium reabsorption from
bone. Thus, parathormone facilitates resorption of calcium from bone while calcitonin reduces it. In addition there ma y also be actions on other tissues, including
the kidney .
a) Relation to Osmoregulation
Calcium is an important mineral constituent of the body, both with respect to
structure and to its essential role in many biochemical reactions. It is necessary
for the adequate functioning of all cells, but is not usually considered to be an 0563