Before the hagfish lineage separated from that of
the vertebrates, a number of additional anatomical changes had evolved.
These included a hypothalamic portal system, adrenal glands, thyroid follicles,
extrinsic eye muscles, a heart composed of an atrium, ventricle, and sinus
venosus, heart valves, a closed circulatory system, regulation of cardiac
output, a pancreas, a gall bladder, nephrons in the urinary system, and
keratins in the epidermis.
Two jawless fish are known from the Early Cambrian. Myllokumingia
(depicted in the above photo) had a head and trunk, a dorsal fin, a ventral
fin (that might have been paired), 5-6 gill pouches, around 25 muscle
segments (myomeres), a pharynx, an intestine, a notochord, and perhaps
a pericardial cavity. With such features it is similar in complexity to
the most primitive fish alive today, the hagfish, but perhaps slightly
more advanced (Shu, 1999).
THE ENDOCRINE SYSTEM
A portal system developed in the vicinity of hypothalamus and pituitary
(Hoar, Vol. 2). The pituitary secreted growth hormone (Chuzhanova, 2000),
structures known as follicles formed within the thyroid tissue (Hoar,
Vol. 2), B and D cells were present in the endocrine pancreas (Youson,
1999), the blood supply to the endocrine pancreas increased (Jansson,
1998), cells existed which corresponded to those of the adrenal cortex
in vertebrates (Hoar, Vol. 2), and steroid hormone receptors evolved (Thornton,
2001). Enkalphin and POMC genes produce signaling molecules for the nervous
and endocrine system. The duplication of the single ancestral gene preceded
occurred in the ancestors of the craniates (Danielson, 1999).
THE MUSCULAR SYSTEM
Early craniates possessed different types of muscle cells including both
fast and slow twitch muscles (Hardisty, p. 357). Myotomes from the head
region of jawless and cartilaginous fish embryos suggest that the ancestral
craniate condition included pro-otic somites whose muscles produced the
muscles which move the eye. The first pro-otic somite formed the superior
rectus, inferior rectus, internal rectus, and inferior oblique and was
innervated by the oculomotor nerve. The second pro-otic somite formed
the superior oblique and was innervated by the trochlear nerve. The third
pro-otic somite produced the external rectus and was innervated by the
abducens nerve (Weichert, 1970). Early craniates also developed hypobranchial
musculature (Romer, p. 288).
THE CARDIOVASCULAR SYSTEM
Early craniates possessed a heart with three separate chambers: the sinus
venosus, a single atrium, and a single ventricle. The sinus venosus was
the site where the contraction of the heart was initiated, (which is also
true of the embryonic hearts of all vertebrates) (Torrey). The heart was
composed of cardiac muscle (Kardong, p. 462), and its wall could be divided
into an epicardium, myocardium, and endocardium (Webster, 1974, p. 129).
Three heart valves (sinoatrial, atrioventricular, and semilunar heart
valves) permitted only a one-way direction of flow between heart chambers
(Kardong, p. 462).
Cardiac output could be varied: catecholamines stimulated moderate increases
in the contraction rate and catecholamines, ANP, and neuropeptides regulated
other aspects of circulatory physiology (Forster, 1997).
Although the early craniates possessed a circulatory system which was
close to being a completely closed system like that of higher vertebrates,
they possessed a system of venous sinuses and plexuses which were reminiscent
of the open circulatory system of many invertebrates (Hardisty, p. 242).
Craniates increased the number of blood vessels which serviced skeletal
The blood vessels of early craniates included external and internal carotid
arteries (with internal carotids supplying the brain), celiac, anterior
mesenteric, posterior mesenteric, renal, and gonadal arteries. The number
of aortic arches was reduced (Romer).
The first blood cells formed in yolk sac and are nucleated (the conserved
vertebrate condition even in mammals). The enzymes used by neutrophils
included alkaline phosphatase (Hine, 1990). Early craniates possessed
prothrombin molecules homologous to those of vertebrates (Banfield, 1994).
Early craniates modified carbonic anhydrase to increase its catalytic
efficiency (Tufts, 2003).
THE LYMPHATIC SYSTEM
Although primitive craniates possessed thin walled vessels of vascular
plexuses which functioned as primitive lymphatic vessels, there would
be no true lymphatic vessels until gnathostomes (Hardisty, 249). The early
craniates possessed cells homologous to the vertebrate thymus (Romer 448).
The "spleen" was represented by diffuse tissue associated with
the gastrointestinal tract (Torrey, 1979).
Primitive craniates possessed a protein known as CLP (complement-like
protein) which functioned in immune defenses was structurally similar
to a mammalian complement protein (Hanley, 1992). Although the complement
system is an important component of the adaptive immunity of higher vertebrates,
its original function was one of innate immunity in primitive craniates.
Primitive craniates possessed complement proteins which functioned in
the lectin pathway, including C3 which functioned in opsonization (the
coating of a foreign object to promote its phagocytosis; Zarkadis, 2001).
Early craniates possessed heterodimeric immunoglobulins (similar to both
antibodies and T cell receptors) which were secreted into the plasma as
part of the immune response (Varner, 1991).
All fish, beginning with early craniates, possessed homologs of the macrophage
migration inhibitory factor (MIF) (which is similar to a number of vertebrate
enzymes, including some known in nematodes (Sato, 2003a).
THE DIGESTIVE SYSTEM
Extensions of the dorsal mesentery around the gastrointestinal tract formed
the mesentery, mesocolon, and greater momentum (the absence of the mesentery
in lampreys is a secondary loss) (Romer, p. 318-9). Early craniates possessed
a pancreas although the pancreatic tissue was dispersed, similar to the
vertebrate condition in which multiple units form the embryonic pancreas)
(Romer, p. 393-5). The liver no longer functioned in enzyme production
and food absorption as it had in primitive chordates (Romer, p. 390).
Craniates developed a gall bladder to store bile (Weichert, 1970, p. 194).
During the formation of the mouth, only the posterior portion of the stomodeum
(the anterior embryonic invagination which unites with the digestive tube)
contributed to mouth (Kardong, p. 490). There was a reduction of the number
of pharyngeal pouches and cartilage surrounded the pharynx (Weichert,
1970, p. 214). Early craniate digestive systems used a number of enzymes
which would also be utilized by higher vertebrates such as chitinase,
trypsin, aminopeptidase, and carboxypeptidase (Stevens).
In early craniates, the archinephritic duct united a series of excretory
tubules and the archinephros released waste into the cloaca (Romer, p.
404). The kidney was composed of nephrons as in higher vertebrates and
these nephrons consisted of a glomerular (Bowman's) capsule and regions
homologous to the neck and proximal segment I of the renal tubule (Hoar,
1969, Vol. I). The early craniates utilized anterior excretory tubules
known as the pronephros; although these develop in vertebrates they are
non-functional and degenerate during embryonic development (Romer, p.
The craniate urinary system was capable of reabsorbing filtered glucose
and secreting ions (Hoar, 1969, Vol. I, p. 99).
THE REPRODUCTIVE SYSTEM
Sermatogonia and oogonia were originated in the embryonic yolk sac (rather
than the gonads), as is typical in vertebrates (Romer, p. 421). The gonads
not segmented, as in many invertebrates (Weichert, p. 280).
Keratins are a family of proteins which are best known for their structural
role in forming the epidermis, hair, scales, feathers, horns, and antlers
of vertebrates. Several keratin genes existed in the first craniates.