amounts of fluid may be filtered by utilizing the hydrostatic pressure of the blood
vascular system. Glomerular development and filtration is more pronounced in
vertebrate species living in fresh water than in the sea. Such a filtration mechanism,
when combined with a tubular system for reabsorption of essential solutes, constitutes an admirable mechanism for excreting excess water. MARSHALL and SMITH
(1930) concluded that the glomerulus, which is present in most species of all the
major vertebrate groups including the myxinoids, arose as an evolutionary adaptation to life in fresh water. It was assumed that its presence in marine species, such
as the myxinoids, is due to their secondary movement into the sea and is not a
primary situation. ROBERTSON'S interpretation of the glomerulus differs strongly
from that of MARSHALL and SMITH; he believes that such a mechanism must have
been present before such animals could have lived in fresh water, and points out
that such filtration-reabsorption mechanisms exist in man y marine invertebrates
whose ancestors have never been associated with fresh water. Such filtration mechanisms , when followed by selective reabsorption, act as regulators for excretion
of ions in primarily marine species. Studies on contemporary myxinoids (the Atlantic and Pacific hagfishes) suggest that in this group, the nephron is concerned
with regulation of divalent ions rather than water excretion and sodium conservation (MUNZ and McFARLAND, 1965). These authors agree with ROBERTSON,
that the hagfish kidney reflects a primary marine origin for the myxinoids, and
suggests that the Vertebrata originated in the sea, providing of course that cyclostome origins were monophyletic. The recent discovery of a fossil lamprey, Mayomyzon, 200 to 300 million years old, which lacks all myxinoid characters, has led
BARDACK and ZANGERL (1968) to suggest that the origin of the cyclostornes may
indeed be diphyletic as originally suggested by STENSIO . Evidence, based on the
characteristics and relationships of plasma proteins in contemporary cyclostomes
and other modern vertebrates, supports the concept of a petromyzontoid-type of
ancestor (MANWELL, 1963). If this is so, and the petromyzontoids and myxinoids
represent two ph yletic branches of cyclostomes, then osmoregulatory information
about the myxinoids would not be relevant to the speculations on the environment
of the main line of early vertebrates. There are too many "buts" and "ifs" to draw
any firm conclusions. Evidence of vertebrate origins based on contemporary ph ysiological and serological information cannot be conclusively extrapolated back for
400 million years. The interpretation of such data, as reflecting a primary or secondary type of vertebrate osmoregulatory pattern, must remain an opinion, no
matter how well considered.
37
vascular system. Glomerular development and filtration is more pronounced in
vertebrate species living in fresh water than in the sea. Such a filtration mechanism,
when combined with a tubular system for reabsorption of essential solutes, constitutes an admirable mechanism for excreting excess water. MARSHALL and SMITH
(1930) concluded that the glomerulus, which is present in most species of all the
major vertebrate groups including the myxinoids, arose as an evolutionary adaptation to life in fresh water. It was assumed that its presence in marine species, such
as the myxinoids, is due to their secondary movement into the sea and is not a
primary situation. ROBERTSON'S interpretation of the glomerulus differs strongly
from that of MARSHALL and SMITH; he believes that such a mechanism must have
been present before such animals could have lived in fresh water, and points out
that such filtration-reabsorption mechanisms exist in man y marine invertebrates
whose ancestors have never been associated with fresh water. Such filtration mechanisms , when followed by selective reabsorption, act as regulators for excretion
of ions in primarily marine species. Studies on contemporary myxinoids (the Atlantic and Pacific hagfishes) suggest that in this group, the nephron is concerned
with regulation of divalent ions rather than water excretion and sodium conservation (MUNZ and McFARLAND, 1965). These authors agree with ROBERTSON,
that the hagfish kidney reflects a primary marine origin for the myxinoids, and
suggests that the Vertebrata originated in the sea, providing of course that cyclostome origins were monophyletic. The recent discovery of a fossil lamprey, Mayomyzon, 200 to 300 million years old, which lacks all myxinoid characters, has led
BARDACK and ZANGERL (1968) to suggest that the origin of the cyclostornes may
indeed be diphyletic as originally suggested by STENSIO . Evidence, based on the
characteristics and relationships of plasma proteins in contemporary cyclostomes
and other modern vertebrates, supports the concept of a petromyzontoid-type of
ancestor (MANWELL, 1963). If this is so, and the petromyzontoids and myxinoids
represent two ph yletic branches of cyclostomes, then osmoregulatory information
about the myxinoids would not be relevant to the speculations on the environment
of the main line of early vertebrates. There are too many "buts" and "ifs" to draw
any firm conclusions. Evidence of vertebrate origins based on contemporary ph ysiological and serological information cannot be conclusively extrapolated back for
400 million years. The interpretation of such data, as reflecting a primary or secondary type of vertebrate osmoregulatory pattern, must remain an opinion, no
matter how well considered.
37