Zoology is studied as a ‘pure science’ (knowledge gaining) and it has application in other branches such as euphenics, eugenics, biotechnology, bioenergetics, bioinformatics, etc. As applied science, it has tremendous scope in agriculture, aquaculture, animal husbandry, human health, diseases, ...
10 Zoology
8
Carotid arch Systemic side. The conus arteriosus bifurcates into two
arch
branches and each of it divides into three aortic
Pulmocutaneous
Right
arch
arches namely carotid, systemic and
precaval
pulmocutaneous. Blood from the heart is
Left precaval
Right distributed to all parts of the body by the branches
atrium Left
atrium of aortic arches. Three major veins collect blood
Conus from the different parts of the body and carry it to
arteriosus
the sinus venosus. Circulation is described as
Ventricle
‘Incomplete Double’ circulation.
Portal Systems: Special venous connections
Post caval between the intestine and liver as well as lower
parts of the body and kidneys are present in frog.
Fig. 4.17 Frog heart
These are called hepatic and renal portal veins,
respectively.
Blood: Blood is composed of plasma and cells. The blood cells are erythrocytes,
leucocytes and thrombocytes. RBCs are nucleate. WBCs are nucleated amoeboid
cells which help in defence. Thrombocytes help in haemostasis (stopping of bleeding).
Lymphatic system: In addition to the blood vascular system, frog has a lymphatic
system. The lymphatic system consists of lymph, lymph channels and lymph nodes.
Lymph is different from blood. It lacks RBCs and contains fewer proteins, than those
in the blood.
6. Integrating System
Control and co-ordination of body systems is highly developed in frog. It includes
both nervous and endocrine systems. The chemical integration of various organs of
the body is achieved by the hormones, which are secreted by the endocrine glands.
The prominent endocrine glands found in frog are pituitary, thyroid, parathyroid,
thymus, pineal body, pancreatic islets, adrenal glands and gonads.
The nervous system is organised into a central (brain and spinal cord), peripheral
(cranial and spinal nerves) and autonomous (sympathetic and parasympathetic)
nervous systems.
Brain is enclosed in a bony structure called brain box (cranium). The brain is
divided into fore brain, mid brain and hind brain. The fore brain includes two olfactory
lobes (concerned with sense of smell), two cerebral hemispheres (control of voluntary
actions) and diencephalon (perception of heat, cold, pain and integrity of autonomous
system)
Mid brain is represented by a pair of optic lobes (corpora bigemina). The optic
lobes are associated with the sense of sight. The hind brain consists of cerebellum
(maintains equilibrium) and medulla oblongata (controls involuntary movements).
Medulla oblongata passes out through the foramen magnum and continues as spinal
cord which is enclosed in the neural canal of the vertebral column. The spinal cord
acts as the ‘middle man’ between the brain and effectors.
7. Special senses
Frog has sense organs such as the organs of touch, taste, smell, sight and hearing.
The well-organised structures among them are eyes, internal ears and the rest are ‘cellular
aggregations’ around nerve endings. The receptors of touch (sensory pupillae) occur
in the skin. Organs of taste are called taste buds that lie on small papillae of tongue.
The organs of smell are a pair of nasal chambers.
The organs of sight are a pair of eyes located in the orbits of the skull. Eyes are
protected by eyelids. The upper eyelid is immovable. The lower eyelid is folded into a
transparent nictitating membrane, which can be drawn across the surface of the eye.
The retina of the eye contains both rods and cones. Cones provide ‘colour vision’ and
rods are helpful in ‘dimlight vision’.
Ear is useful for hearing and balance. It consists of a middle ear closed externally
by a large tympanic membrane (ear drum) and a columella that transmits vibrations
to the inner ear. The inner ear consists of a utriculus with three semicircular canals
and a small sacculus.
8. Excretory System
The elimination of the nitrogenous wastes and the maintenance of water and salt
balance are performed by a well-developed excretory system. It consists of a pair of
kidneys, ureters, a urinary bladder and a cloaca.
The kidneys are elongated, flat and dark red coloured structures situated a little
posteriorly in the body, one on either side of the vertebral column. They are covered by
peritoneum on their ventral (lower) surface only (retroperitoneal). An adrenal gland is
found along the mid ventral surface of each kidney. The functional units of the kidney
are uriniferous tubules or nephrons. From the posterior outer margin of each kidney,
, 11 Zoology
0
a mesonephric or Wolffian duct arises. The ureters act as ‘urino-genital ducts’ in
males because both urine and male gametes pass through them. However, in the females,
the oviducts and ureters are separate. Ureters open into the cloaca by sphinctered
apertures. A thin walled urinary bladder, present ventral to the rectum, opens into the
cloaca. The chief nitrogenous waste in frog is urea, hence, it is said to be ‘ureotelic’.
Do you know - urea is mostly formed in the liver and excreted by the kidney.
Vasa
efferentia 9. Reproductive System
The male reproductive system consists of a pair
Fat bodies of yellowish and ovoid testes, which are attached
to the kidneys and dorsal body wall by a double
Bidder’s
canal fold of peritoneum called mesorchium. Each testis
Adrenal is composed of innumerable seminiferous tubules
gland
which are connected to form 10 to 12 narrow
Rectum
tubules, the vasa efferentia. They enter the
kidneys and open into the Bidder’s canal which is
Cloaca
connected to the ureter through transverse canals
Cloacal
of the kidney. The urino-genital ducts of both the
aperture sides open into the cloaca.
Urinary
bladder The female reproductive system consists of a
4.19 Frog : Male reproductive system pair of ovaries, oviducts and cloaca. The ovaries
are attached to the kidneys and dorsal body wall
by a double fold of peritoneum called
Oviduct
mesovarium. The ovaries have no functional
connection with the kidneys. The oviducts are
Ovary
two long, white and convoluted tubes. Their
anterior ends form funnel-like openings called
Ostia. The posterior ends of oviducts enlarge into
ovisacs before they open into the cloaca
separately.
Ureter
During amplexus, the mass of eggs and the
mass of sperms released by the female and the
male are called spawn and milt respectively.
Ovisac
Cloaca Fertilization is external and takes place in water.
Urinary Cloacal Development involves an aquatic gill breathing,
bladder aperture
herbivorous larval stage called tadpole, which
Fig. 4.20 Frog : Female reproductive system metamorphoses into an air breathing, carnivorous
, Animal Diversity - II 111
adult frog. Tadpole of frog resembles a fish pertaining to taking in water and respiring
through gills5.
Majority of the tetrapods possess an amnion during their development. Frogs are
also tetrapods. Do their embryos too possess amnion? If not why?
10. Economic Importance of Frogs
Frogs are beneficial to mankind because they are insectivorous and help in reducing
the insect population. In some countries such as China and Japan, the muscular thighs
of frogs are used for human consumption.
4.6.2. Reptilia (L. reptilis - creeping)
Reptiles are ectothermic (organisms that regulate their body temperature largely
by exchanging heat with their
surroundings; cold-blooded) Embryo
Amnion
amniotes. The extinct Shell
labyrinthodont amphibians
gave rise to reptiles during the Chorion
carboniferous period. They
emerged as the dominant Allantois
vertebrate group during the
Yolk sac
Mesozoic era (Golden age of
reptiles). Most reptiles lay water
tight ‘cleidoic’ eggs (covered by
porous calcareous shell - to
Fig. 4.21 Amniotic egg
allow the passage of air to
provide oxygen) on land with extra embryonic membranes namely amnion, allantois,
chorion and yolk sac, which make the egg an independent ‘life support system’. Thus,
the ‘amniotic egg’ helped the reptiles to abandon ties with their ancestral aquatic habitat.
In addition to cleidoic eggs, the evolution of ‘dry scaly skin’ to prevent water loss, two
pairs of pentadactyl limbs with clawed digits for moving on land, pulmonary breathing
and internal fertilization are the key adaptations that led to the success of reptiles. The
class reptilia includes the extinct dinosaurs (terrible lizards) and the extant chelonians,
crocodilians, snakes, lizards and Sphenodon. The study of poikilothermic tetrapods
is called herpetology.
What changes in the design of egg and embryo allowed reptiles to lay eggs on
land? Why their eggs are often called ‘amniotic eggs’?
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