METAZOA III. VERTEBRATES I (Fishes and Amphibians)
The fishes are the oldest vertebrates. The first appeared about 500
million years ago, were small, had a bony shell and
had no mandible. Its mouth was circular and adapted to filter
the food. Then the armor-plated fishes with mandible appeared,
then the sharks that have barely changed ever since, and finally,
the fishes with bones. Because
of respiring through the gills they have not been able to colonize
the terrestrial environment. A little group called Lungfish, thanks
to having its swim bladder connected to the outside, can live out
of the water, crawling and jumping on wet mud with its pectoral
fins. When it is dry and the mud begins to dry, these fish are sinking
in the mud and survive by taking an advantage of the inferior moisture.
The Lungfish give us a clue about how the lungs and limbs of animals that
walk on land appeared.
amphibians appeared about 370 million years ago and some
even exceed three meters in length. Thanks to having pulmonary respiration
system they were the first vertebrates to colonize the terrestrial
environment, but as its lungs are very simple, they still need the
cooperation with the cutaneous respiration system. This means that
its skin is naked and humid, and this means that
they can only live in very humid environments. Moreover, by having an
external fertilization, they need water to be able to reproduce.
In conclusion, amphibians can live out of the water but only in humid
environments and in those where from time to time heavy rains occur so
they can be able to reproduce, therefore, they are not fully independent
from the aquatic environment.
Activities to do. Read the explanations on the general characteristics
and classification of Fishes and perform Multiple Choice Test 18.1 and
Relate pictures with names 18.1. Then read the text on the amphibians
and perform Multiple Choice Test 18.2, Relate pictures with names 18.2
and Crossword 18.
1. The vertebrates. Are the metazoans that have the spinal
column, meaning the articulated bone structure that protects nerve
cord or spinal cord. The name "vertebral column" is most appropriate
for bipedal vertebrates like us humans and birds, but for the fishes and
quadrupedal vertebrates, it is also customary to use the name of dorsal
have a closed circulation, which means that the blood is always
flowing inside the ducts called vessels (arteries, veins and capillaries).
There are known around 50,000. They differ into 5 groups of vertebrates
are: fishes, amphibians, reptiles, birds and
mammals. This chapter discusses the first two.
They are the vertebrates that have skin with dermic scales and
which swim in the water using fins.
They are vertebrates the that have wet and uncovered skin (without
scales, feathers nor hair) and have four legs in a lateral position
for walking and swimming.
They are the vertebrates that have skin with epidermic scales
and have four legs in a lateral position (except snakes,
which does not have legs) so they usually move by crawling on
They are the vertebrates that have skin with feathers, the extremities
that form two wings, which allow most of them to fly, and the
back extremities that constitute two legs that allow them to
They are the vertebrates that have skin with hair and suckle
their youngs. Most have four legs to walk, a few have fins
for swimming, like whales, and a few have
wings to fly like bats.
. The fishes. They are the aquatic vertebrates that
have fins to move, the skin provided with dermic scales
(such as sardines) or dermic denticles
(as in sharks) or without them (as in the
lamprey), they breathe through gills,
have a variable internal temperature and they usually reproduce
by eggs which do not supporte the desiccation. Fish scales are
dermic and are covered with a transparent and mucous skin that outside
of the water dries. They are the earliest known vertebrates, appeared
in the Paleozoic, around 500 million years ago.
1. External morphology.
There are three parts: head, trunk and tail.
It has two nasal ofirices, mouth and two lidless eyes.
It has four paired fins (the two pectoral fins and the
two ventral fins) and 2 unpaired fins (dorsal fin
and anal fin). It also presents a line of nerve endings that
pick up the vibrations of water (lateral line). The trunk goes
from the head to the sewer that is a little open cavity inside which
can be differentiated the genital opening, excretory orifice and anal
opening or anus.
It goes from the serwer to the end, which is where the tail fin
2. Internal morphology.
They have a spinal column or dorsal column (bone
or cartilage) that protects the spinal cord. They have a
ventral heart with a single atrium and a single ventricle,
so the circulation is simple, meaning that the blood giving a full
circle around the body passes only once through the heart. Many have a
swim bladder taht is able to inflate and deflate (flotation function).
The males have two testicles and the females have two ovaries.
The majority of the species is oviparous. Both parents release
gametes outside, the fertilization occurs in water (external fertilization).
After some time and the embryonic development being completed, the egg
emerges and the small fish called the fry comes out.
There are also the
ovoviviparous species. They have the internal fertilization
and the egg remains and hatches inside the mother. For example the gambusia
and aquarium fishes "guppy". There
are also viviparous species. In them the fertilization is internal
and the embryo gets nourishment from the mother via blood. For example
blue shark and great
There are three classes:
of the Finshes
Are the eel shaped but without jaws, scales and paired
fins. The mouth is round with teeth and the skeleton
is cartilaginous. Example: the lampreys,
are the animals that using mouth stick to other fishes and and bleed
them to the death.
fishes. They have jaws, boneless cartilaginous skeleton,
dermic denticles in the skin and the caudal fin with the
upper lobe well developed, but have no lids, that is why lateral
gill fissures are visible. Examples: white
shark, tiger shark and mantas.
They have jaws, the bony skeleton with fish bones, dermic
scales on the skin, caudal fin with two similar lobes and lids
that cover gills. Many have an expandable swim bladder
that allows them to go up and down buoyancy. Examples: hake,
scad, carp and trout.
pictures with names
3. The Amphibians. Are the vertebrates that as an adult have
four limbs (tetrapods), uncovered, thin and moist
skin, that can live outside of the water as they breath through lungs,
and which like fishes require water to reproduce because of fertilization
process (external fertilization).
1. External morphology.
The body of some amphibians such as frogs
and toads have only two parts (head
and trunk) and others, such as salamanders
and newts, have three parts (head,
trunk and tail).
It has two eyes with eyelids (one upper, one lower, which is
mobile, and transparent nictitating membrane that covers the entire
eyeball when the amphibian is submerged), two lateral tympanic membranes
and the mouth with a forked, hurling tongue and very weak
and equal teeth.
It has four limbs. The upper ones have four fingers and
the lower one have five fingers, which in many species, are united
by interdigital mambranes (adaptation for swimming).
2. Internal anatomy. They have two very simple and little
effective lungs (pulmonary respiration), it is why they
also need to capture the oxygen through its thin and wet skin (cutaneous
respiration). In order to keep the skin moist they secrete mucous
substances and sometimes, to repel predators, also a toxic one.
The heart has two atrias, one that receives blood from the lungs
and another that receives blood from the body, and only one ventricle
where both are mixed, which means that they present the incomplete
and double circulation (double means that the blood while giving a
full circle around the body passes through the heart twice, and incomplete
means that in the heart, the blood rich in oxygen and the blood poor in
oxygen are not completely separated but mix).
The fertilization is external (1).
The fertilized eggs are deposited in the water and are fixed with
mucous substances (2). The eggs have the
permeable cover that would dry in the aerial environment. After 2 or 3
months larvae called tadpoles are born, they breathe through gills
and do not have limbs so they move using a tail (3).
Then, they undergo a metamorphosis: the tail is reduced, the four
legs shown, gills disappear (4) and the lungs
are formed, which is why the individuals can leave the water (5).
4. Classification. The two main groups of amphibians are:
of the amphibians
They have no tail in the adult stage. The second pair of legs is
adapted to jump and they are capable of emitting sounds.
Are the frogs and toads
(they are like frogs but with shorter front legs).
The adults have a long body and a tail. The limbs are short and
are adapted to crawl. Examples are the salamandres
(black colour with yellow spots) and newts
(have a dorsal crest acting as teh fin)
pictures with names