The document summarizes the respiratory systems of various chordates. It describes how skin, gills, and lungs have evolved for respiration in different vertebrates. Skin respiration occurs mainly in amphibians like salamanders through their thin skin. Cartilaginous and bony fishes respire using gills consisting of pouches and slits. Lungs evolved from the swim bladder of early fish and are present in tetrapods, with variations in structure between amphibian, reptile, bird, and mammalian lungs.

1. Notes: Zoology- VI Semester, University of Mumbai, India.
Comparative Anatomy of Chordates
Respiratory System of Vertebrates
[Skin, gills of cartilaginous & bony fish, lungs in vertebrates]
Skin (cutaneous) respiration in vertebrates:

Respiration through the skin can take place in air, water, or both
 Most important among amphibians (especially the family Plethodontidae)
 Adult, terrestrial salamanders of the family Plethodontidae, however, rely solely on
cutaneous respiration, as they lack lungs and gills. Species that rely mainly on cutaneous
respiration are typically long, cylindrical in shape, with thin epidermal layers laden with
dense capillary beds that pickup O2 and expel
CO2. Cutaneous respiration is the absorption of
oxygen, and disposal of carbon dioxide, through
the skin e.g. Plethodon spp. The long, cylindrical
shape creates a high surface area to volume ratio,
which enhances the amount of oxygen diffused.
To further promote cutaneous respiration, these
salamanders also have slow metabolisms, costal
grooves that increase the surface area, and the
ability to withstand oxygen debt through anaerobic
glycolysis (energy metabolism). All of these
features make for the successful utilization of
cutaneous respiration in most amphibians. Aquatic
species Giant Salamanders also possess large folds of skin that increase the oxygenabsorbing surface area, thus increasing the oxygen intake.
Plethodon spp.
Frog
Salamander
Gills in cartilaginous and bony fish:
If we look at the pharyngeal region of a vertebrate embryo we find that the wall of the pharynx
develops pouches that go out. The lining of these pouches is endodermal tissue. Between the
pouches we have a piece of tissue that is the visceral arch. It is made of lateral plate mesoderm.
Prof. S. D. Rathod, Associate Professor in Zoology, B. N. Bandodkar College of Science, Thane -400605

2. Notes: Zoology- VI Semester, University of Mumbai, India.
The outer covering of the pharynx is made from ectoderm. The ectoderm bulges in a bit to meet
the pouches. This forms a groove. The endoderm from the pouch induces the ectoderm to form
the groove. Vertebrates have 7 or fewer pouches on each side laterally supported by visceral
arches. Inside the visceral arch we find one of the aortic arches. This short vessel connects the
ventral aorta to the dorsal aorta. There are also nerves in the arches too. As we go up the
taxonomic ladder the number decreases.
The 1st visceral arch gives rise to the jaw of all vertebrates except Agnathans. The 1st closing
plate is lost during development in gilled fish and becomes the tympanum of tetrapods. The 1st
pharyngeal pouch becomes a gill chamber (Agnatha), a spiracle (other fish) or the middle ear
cavity (tetrapods).
The 2nd visceral arch becomes part of the jaw support or moves into the middle ear (columella).
The other visceral arches support gills in fish and in tetrapods they help support the tongue, and
becomes parts of the trachea and larynx. In fishes the gills are derived from the visceral arches.
The mesodermal arch is surrounded by ectoderm on the outer surface and endoderm on the other
surfaces. The arch starts to develop bone or cartilage skeletal structures and muscles. The gill
filaments sit close together and so water has to flow over the filaments to get to the external gill
chamber.
Water flows in with high oxygen content. It comes in contact with blood that has lower oxygen
content. By diffusion oxygen goes from higher -> lower concentration.
Countercurrent exchange - works well if the water is well aerated. Water holds less oxygen
than air, but fish Hb binds to oxygen better than tetrapod Hb.
1. Pouched gills in Agnatha:

6 - 15 pairs of gill pouches
 Pouches connected to pharynx by afferent branchial (or gill) ducts & to exterior
by efferent branchial (or gill) ducts
 Water flows from mouth to gill pouch and exits via an external pore.
 The latter occurs when the mouth is involved in feeding. Muscular contraction
expels water.
2. Septal gills in Cartilaginous fishes:
 5 ‘naked’ gill slits
 Anterior & posterior walls of the 1st 4 gill chambers have a gill surface
(hemibranch). Posterior wall of last (5th) chamber has no hemibranch.
 Interbranchial septum lies between 2 hemibranchs of a gill arch
 Gill rakers protrude from gill cartilage & ‘guard’ entrance into gill chamber
Prof. S. D. Rathod, Associate Professor in Zoology, B. N. Bandodkar College of Science, Thane -400605

3. Notes: Zoology- VI Semester, University of Mumbai, India.

2 hemibranchs + septum & associated cartilage, blood vessels, muscles, & nerves
= holobranch
3. Opercular gills in Bony fishes (teleosts):

Usually have 5 gill slits
 Operculum projects backward over gill chambers
 Interbranchial septa are very short or absent
Lungs in vertebrates:
Swim bladder & origin of lungs The ancient archetype of the lung was the
swimbladder in fishes. The swimbladder is
homologous in position and structure with
the lungs of the higher vertebrate animals.
Most vertebrates develop an out-pocketing
of pharynx or esophagus that becomes one
or a pair of sacs (swim bladders or lungs)
filled with gases derived directly or
indirectly from the atmosphere. Similarities
between swim bladders & lungs indicate
they are the same organs. Vertebrates
without swim bladders or lungs include
cyclostomes, cartilaginous fish, and a few
teleosts (e.g., flounders and other bottomdwellers).
Prof. S. D. Rathod, Associate Professor in Zoology, B. N. Bandodkar College of Science, Thane -400605

4. Notes: Zoology- VI Semester, University of Mumbai, India.
Swim bladders:
May be paired or unpaired. During development, a pneumatic duct that usually connects to the
esophagus is formed. The duct remains open (physostomous) in bowfins and lungfish, but
closes off (physoclistous) in most teleosts (see above fig.). Serve primarily as a hydrostatic
organ (regulating a fish's specific gravity). Gain gas by way of a 'red body' (or red gland); gas is
resorbed via the oval body on posterior part of bladder.
Lungs:
Higher iertebrate respiratory organs include the lung. Lungs develop from the pharynx. Lungs
arise in the embryo as an endodermal diverticulum from the ventral wall of the pharynx. The
diverticulum soon divides into two parts, which form right and left lungs. A windpipe or trachea
connects the lungs with the pharynx. Anterior part of the trachea is modified into the larynx. The
larynx communicates with the pharynx by a slit like opening the glottis. The laryrnx functions as
sound producing organ in tetrapods except in birds. The birds have their sound-producing organ
known as 'syrinx'. The trachea bifurcates into two branchi. Each primary branchus further
divided inside the lungs as secondary bronchi, tertiary bronchi and bronchides. The bronchides
are connected to the alveoli.
Amphibian lungs







2 simple, long, spindle shaped, semitransparent, elastic, delicate, thin walled and sac
like structures.
Internal lining may be smooth or have simple sacculations or pockets.
Air exchanged via positive-pressure ventilation. In the Amphibia, however, there are
not even ribs developed, or, if they exist at all, they are such mere rudiments.
The lungs communicate with the buccal cavity through the laryngeotracheal chamber.
The laryngeotracheal chamber opens in the floor of the buccal cavity by means of
glottis. A pair of lungs is located in the anterior region of coelom on the dorsolateral
sides of the heart. The outer surface of the lungs is lined by visceral peritoneum.
The inner surface of the lungs is projected in the lumen as large number of irregular
and radially arranged folds. The space between two consecutive folds of the inner
surface of the lungs forms an alveolus.
The Alveoli greatly increase the inner respiratory surface of the lungs.
Prof. S. D. Rathod, Associate Professor in Zoology, B. N. Bandodkar College of Science, Thane -400605

5. Notes: Zoology- VI Semester, University of Mumbai, India.
Reptilian lungs


Simple sacs in Sphenodon & snakes
Lizards, crocodilians, & turtles - lining is septate, with lots of chambers & subchambers. The lungs of Reptiles are two capacious membranous sacs occupying a
considerable portion of the visceral cavity, which, as there is no diaphragm as yet
developed, cannot properly be divided into thorax and abdomen, as it is in
Mammalians. From the internal surface of the walls of each lung membranous septa
project inwards, so as partially to divide the interior of the organ into numerous
polygonal cells, which are themselves subdivided into smaller compartments in a
similar manner. This structure is well seen in the lung of the Tortoise.
 Air exchanged via positive-pressure ventilation as well developed ribs assist in
respiration.
Prof. S. D. Rathod, Associate Professor in Zoology, B. N. Bandodkar College of Science, Thane -400605

6. Notes: Zoology- VI Semester, University of Mumbai, India.
Avian lungs


Modified from those of reptiles:
Air sacs (diverticula of lungs) extensively distributed throughout most of the body.
Functionally, these 9 air sacs can be divided into anterior sacs (interclavicular, cervicals,
& anterior thoracics) & posterior sacs (posterior thoracics & abdominals). Air sacs have
very thin walls with few blood vessels. So, they do not play a direct role in gas exchange.
Rather, they act as a 'bellows' to ventilate the lungs. Arrangement of air ducts in lungs ---> no passageway is a dead-end.
 Air flow through lungs (parabronchi) is unidirectional. Parabronchial lungs of birds are
subdivided into large numbers of extremely small alveoli or air capillaries. The avian
respiratory system is partitioned heterogeneously, so the functions of ventilation and gas
exchange are separate in the air sacs and the parabronchial lung, respectively. Air sacs act
as bellows to ventilate the tube-like parabronchi.
Mammalian lungs:

Multi-chambered & usually divided into lobes about 1-6 lobes. Sometimes right lung has
more lobes than left lung and hence becomes asymmetric (e.g. rabbit, human).
 Respiratory system in mammals starts with nose, leading into the pharynx and is drawn
into the larynx and then the trachea. The epiglottis is found within the larynx. This
structure prevents food and drink passing into the respiratory system. When swallowing,
the larynx is pulled up and the epiglottis flaps back to block the entrance of the larynx.
The larynx is connected with trachea of lung. The trachea contains C-shaped cartilage
rings which prevent the tube collapsing due to the change of pressure. It divides into 2
tubes with smaller diameter called bronchi. The bronchi further divide into bronchioles.
The bronchioles terminate with alveoli (100µm in diameter) which are the site of gas
exchange.
Prof. S. D. Rathod, Associate Professor in Zoology, B. N. Bandodkar College of Science, Thane -400605

7. Notes: Zoology- VI Semester, University of Mumbai, India.

The pulmonary alveoli, thereby the lungs, just like the lungs of the Carnivora of the
mammals, may have 3-5 hundred million pulmonary alveoli.
 Air flow is bidirectional: From nose, the air passes into the pharynx ↔larynx ↔trachea
↔ primary bronchi ↔ secondary bronchi ↔ tertiary bronchi ↔ bronchioles ↔ alveoli
 Air exchanged via negative pressure ventilation, with pressures changing due to
contraction & relaxation of diaphragm & intercostal muscles. The greater the partial
pressure of O2 in alveolar air the more O2 will dissolves in blood (Henry's Law).
The compound evolution of the lung can illustrate this case (Fig. below).
The ancient archetype of the lung was the swimbladder in fishes (Fig A),
"The swimbladder is homologous in position and structure with the lungs
of the higher vertebrate animals". The lung of the amphibians is very
simple, being merely a pair of sacs with thin walls, such as the lungs of
salamander (Cynops orientalis) the lungs of the frog (Rana) (Fig. C) and
the toad (Bufo bufo gargarizans) (Fig. D). The evolution from the lung of
the salamander through that of the frog (Rana) up to the lung of the toad
(Bufo bufo gargarizans) can be accomplished with only a few grades of
compound. The lung of the reptiles (Fig. E) is relatively more developed.
Many small lacunae are separated out inside the lung and the gas exchange
area is thus enlarged. The cells of the reptiles further develop toward new
individuals and further undergo multi-grade compound and following
gastrulation, thereby forming the dense spongy lung of the birds with
millions of alveoli (Fig. F). The pulmonary alveoli, thereby the lungs, just
like the lungs of the Carnivora of the mammals, may have 3-5 hundred
million pulmonary alveoli.
Prepared by
Mr. S. D. Rathod.
Prof. S. D. Rathod, Associate Professor in Zoology, B. N. Bandodkar College of Science, Thane -400605