In this order of the class Pisces
the integument is devoid of scales or bony plates.
The spinal column consists of a thick persistent notochord
enveloped in a sheath, but devoid of vertebral centra. The
neural arches and the ribs may be represented by cartilages,
and there is a distinct skull presenting cartilage at least in its
base, and retaining many of the characters of the foetal cranium
of the higher Vertebrata
. The notochord terminates in
a point in the base of this cartilaginous skull behind the pituitary
body; and the skull is not movable upon the spinal column.
There are no jaws; but the palatopterygoid, the quadrate,
the hyomandibular, and the hyoidean apparatus of higher Vertebrata
, are imperfectly represented (Fig. 30, f, g, h
some genera a basket-like cartilaginous apparatus strengthens
the walls of the oral cavity; while, in others, such a framework
supports the gill-sacs.
possess neither the pectoral nor the
pelvic pair of limbs, nor their arches. Horny teeth may be
developed upon the roof of the palate, or upon the tongue, or
may be supported by peculiarly developed labial cartilages.
|Fig 30. - A, the skull of a Lamprey, viewed from the side;
B, from above:-a, the ethmovo
merine plate; b, the
capsule; c, the auditory capsule; d, the neural arches of
spinal column; e, the palatopterygold portion; f, probably,
the metapterygoid. or
superior quadrate, portion, and g, the
inferior quadrate portion, of the subocular arch; h,
process; i, lingual cartilage; k, inferior, l lateral. prolongation
of the cranial cartilage; 1, 2, 8, accessory labial cartilages; m,
branchial skeleton. The spaces on
either side of 1 are
closed by niembrane.
The alimentary canal is simple and straight, and the liver is
not sac-like, but resembles that organ ia other Vertebrata
The heart has the usual piscine structure, consisting of a
single auricle preceded by a venous sinus, a single ventricle,
and an aortic bulb, all separated from one another by valves.
This heart is contained in a pericardium, the cavity of which
communicates with that of the peritonaeum.
the portal vein is rhythmically contractile.
The cardiac aorta, which is continued from the bulb, distributes
its branches to the respiratory organs. These consist
of antero-posteriorly flattened sacs, which communicate directly
or indirectly, on the inner side, with the pharynx, and, externally,
with the surrounding medium.
In the Lamprey there are seven sacs, upon each side, which
open externally by as many distinct apertures. Internally,
they communicate with a long canal, which lies beneath the
oesophagus and is closed behind, while anteriorly it communicates
freely with the cavity of the mouth (Fig. 33, Pr
|Fig. 31. - Side and upper view of the brain of Petromyzon
fluviatilis, and an
inner view of the membranous labyrinth
of P. marinus. The
following letters refer to
the flgnres of the brain:
I., the olfactory nerves, narrow
anterior prolongations of the
(A); B, the prosencephalon; C, the
thalamencephalon; D, the mesencephalon;
E, the medulla oblongata; F, the fourth
ventricle; e, the narrow band which
is all that represents the cerebellum; G, the
spinal cord; II., theoptic; III.,
the oculomotorius; IV., the patheticus; V., the
trigeminal; VI., the abducens;
VII., the facial, and
the auditory; VIII., the glosso-
pharyngeal and pneumogastric;
IX., the hypoglossal
nerves; 1, 1', 2, 2', sensory
and motor roots of the first two
spinal nerves. In the figure
of the membranous
lahyrinth: k, the auditory nerve; a, the vestibule; c, the two semicircular
which correspond with the anterior
and posterior vertical canals of other ertebrata; d, their union and common
opening into the vestibule; b, the ampullae.
The kidneys are well developed, and have the ordinary vertebrate
structure, while the ureters open behind the rectum.
The brain, though very small, is quite distinct from the
myelon, and presents all the great divisions found in the higher
Vertebrata—that is to say, a fore-brain, mid-brain, and hind-brain.
The fore-brain is further divided into rhinencephala,
solid prosencephalic lobes, and a thalamencephalon; the hind-brain,
into metencephalon and myelencephalon (Fig. 31).
The auditory organ is simpler than in other fishes, possessing
only two semicircular canals and a sacculated vestibule in
the Lamprey. In Myxine
the whole organ is represented by
a single circular membranous tube, without further distinction
into canals and vestibule.
differ remarkably, not only from the
fishes which lie above them, but from all other vertebrate animals,
in the characters of the olfactory organ, which consists of a sac placed in the middle line of the head, and having a single,
median, external aperture. In all other Vertebrata
are two nasal sacs. In the Lampreys, the nasal sac terminates
blindly belovr and behind, but in the Hags (Myxine
), it opens
into the pharynx. In no other fishes, except Lepidosiren
the olfactory apparatus communicate with the cavity of the
The reproductive organs of the Marsipobranchii
plates suspended beneath the spinal column, and Ihey have no
ducts, but shed their contents into the abdomen, whence they
pass out by an abdominal pore. In the early stages of their
development the Lampreys present some singular resemblances
to the Amphibia
. They also undergo a metamorphosis, the
being so unlike the parent, that it was, until
lately, regarded as a distinct genus-Ammocoetes
the young Lampreys never possess external branchial filaments
are inhabitants of both fresh and salt
water. The Myxinoids are remarkable for their parasitic habits the Hag boring its way into the bodies of other fishes,
such as the Cod. No fossil Marsipobranchii
are known. This
circumstance may, in part, be due to the perishableness of
their bodies; though horny teeth, like those of the Lampreys,
might have been preserved under favorable circumstances.