Practical Course in Zoology: Study Guide - T. A. Dauda 2014

Multicellular Animals
Chordates
Subphylum Vertebrates, or Craniates — Superclass Tetrapods — Class Amphibians

General characteristics

The superclass comprises vertebrates that lead a terrestrial existence to varying degrees: amphibians, reptiles, birds, and mammals. Some subgroups within these classes have secondarily become partially or fully aquatic. Respiration is pulmonary. There are two circuits of Blood Circulation. The paired limbs of the pentadactyl type function as multi-jointed levers.

General Characteristics

Amphibians are a relatively small group of the most primitive terrestrial vertebrates. The overwhelming majority of amphibians inhabit either Water or land, depending on their life cycle stage. As a rule, they undergo metamorphosis during their lifetime, transforming from purely aquatic larvae into adult forms that predominantly live out of water.

Consequently, gill respiration is replaced by pulmonary respiration, the Circulatory system changes accordingly, pentadactyl limbs appear, and the sensory organ system undergoes significant modification.

The total number of extant amphibian species is about 2,500, grouped into three orders: Urodela (Caudata), Anura (Salientia), and Apoda (Gymnophiona).

The classical object of study is the frog, a typical representative of the order of tailless amphibians (anurans).

Frog (Rana sp.).

Suitable subjects for study include the moor frog (Rana temporaria), the edible frog (R. esculenta), or the even larger marsh frog (R. ridibunda).

The moor frog is more widely distributed and less dependent on water than the marsh and edible frogs. The Development of frogs proceeds with metamorphosis. The larval stage—the tadpole—lives exclusively in water, possessing gills, a lateral line, and a two-chambered Heart. Frogs feed on insects, their larvae, and other invertebrates.

Materials and Equipment. For every one or two students, the following are required: a freshly killed frog and a dissection kit. Frogs are euthanized 10–15 minutes before the practical Class by placing them in a closed jar containing a cotton ball soaked in ether or chloroform. As they die, frogs secrete a foamy mucus. For convenience, the frogs should be rinsed with cold water.

The work requires: a dissecting pan; forceps; a scalpel; scissors; a Glass tube with a fine curved tip; dissecting needles; waste dishes; pins (10–12 pcs.); wall charts: "Internal Structure of the Frog", "Diagram of the Circulatory System", "STRUCTURE OF THE Brain", "Urinogenital System of the Male and Female".

Assignment. Study the External structure of the frog. Dissect the frog, examine the arrangement of the Internal Organs, separate them, and study their structure. Open the Skull, examine, and sketch the brain.

Identify features shared with fish as well as novel features that developed in amphibians under the Influence of Environmental conditions during evolution. Make the following drawings: "Internal Structure of the Frog", "Diagram of the Circulatory System", and "Structure of the Brain".

Study of the external structure on a live and euthanized frog. Observe the Movements of the frog—it gives the impression of an awkward animal. Its body can be divided into a HEAD, a short trunk, and limbs. The head merges smoothly into the trunk, with no distinct neck. The body is somewhat flattened dorsoventrally and covered with Skin. If you lift the skin with forceps (on the euthanized animal), it separates easily because it is connected to the underlying musculature only in certain areas (the spaces between the skin and Muscles are filled with Lymph fluid and are called lymph sacs).

Notice that the skin is darker on the dorsal side and lighter on the ventral side (compare with a fish); this coloration plays a protective role both on land and in water. Touch the skin—it is soft and moist, as it is covered with mucus secreted by Skin glands. The mucus protects the skin from drying out in terrestrial environments and facilitates the frog's movement in water. Maintaining constant skin moisture is of great physiological significance for the frog, as its skin serves as an accessory respiratory organ.

In the moor frog, a triangular dark patch can be seen on the sides of the head, with its base abutting the eye (temporal spot).

Examine the head of the frog—it is triangular in shape, which ensures easier movement in water and The ability to stay at its surface. The large, prominent eyes are clearly visible, equipped with upper and lower eyelids (fish lack eyelids; they appeared in amphibians in connection with a terrestrial lifestyle). Press on the eyes of the euthanized frog with forceps or a dissecting needle; they easily retract into the eye sockets. The Eyeball helps push food down into the Esophagus. With a fixed neck, the prominent eyes help the animal spot prey and approaching enemies.

Fig. 123 External structure of a male frog (Rana temporaria):

1 — nostrils; 2 — nuptial pad on the digit (males only); 3 — tympanic membrane; 4 — forearm; 5 — upper arm (brachium); 6 — cloacal opening; 7 — thigh; 8 — shank (crus); 9 — heel region of the FOOT; 10 — webbing.

A pair of nostrils, closed by cutaneous Valves, is located anterior to the eyes (Fig. 123). Insert a dissecting needle into the nostrils: the nostrils open into the Oral Cavity via the internal nostrils, or choanae (recall the nostrils in fish). The appearance of internal nostrils in amphibians is associated with adaptation to breathing atmospheric air. Observe the breathing of a live frog. Note that the frog does not open its Mouth when breathing. By observing through a magnifying glass, you can see that while the floor of the oral cavity lowers, the frog's nostrils are open (air rushes through them into the buccopharyngeal cavity); when the floor of the oral cavity rises, the nostril valves close, and air is forced into the Lungs.

When the frog is underwater, the nostrils are closed by valves. Air is forced back out of the lungs due to the deflation of their elastic walls. Puncture the tympanic membrane with a dissecting needle, open the frog's mouth, and locate the openings of the Eustachian tubes in the corners of the mouth.

Locate the openings of the Middle ear behind the eyes (in the euthanized frog), covered by the tympanic membrane (Fig. 124). In male frogs of Rana ridibunda and R. esculenta, vocal sacs (resonators) are present behind the ears in the corners of the mouth, which inflate like bubbles when croaking. In R. temporaria and other frogs, resonators are located beneath the throat skin or are absent.

Open the frog's mouth and run forceps or a finger along the upper and lower jaws and the palate—small, conical, uniform Teeth are present only on the upper jaw and the anterior part of the palate (vomerine teeth).

Fig. 124. Head of a frog:

1 — mouth; 2 — external naris (nostril); 3 — upper eyelid; 4 — lower eyelid; 5 — tympanic membrane (eardrum); 6 — external vocal sacs (resonators); 7 — Tongue; 8 — choanae; 9 — openings of the Eustachian tubes; 10 — glottis; 11 — vomerine teeth; 12 — eye.

The function of the frog's teeth is to hold prey—specifically, moving insects which it catches with its tongue. Take the tongue with forceps and pull it gently. You will easily find that the fleshy frog's tongue is attached by its anterior end to the edge of the lower jaw, while its posterior end is free and can be flipped out.

Note that the posterior end of the tongue is forked, and its two small lobes are covered with mucus that helps retain captured insects. By pressing the lower jaw firmly downwards, just behind the tongue deep within the oral cavity, you can spot a small tubercle. Touch its center with a dissecting needle and locate the narrow glottis. Just behind this tubercle lies the opening leading into the esophagus.

Examine the frog's forelimbs—they are shorter than the hindlimbs and play a secondary role (used for sitting and jumping). A forelimb consists of three segments: the upper arm (brachium), forearm (antebrachium), and hand (manus) with 4 digits. The hindlimb is likewise formed of three segments: the thigh, shank (crus), and foot (pes) with 5 digits. On an anesthetized frog, it is clear that the digits lack claws. The hindlimbs play the primary role in locomotion both in water and on land. A web is stretched between the digits of the hindlimbs. Locate the cloacal opening on the dorsal side between the bases of the hindlimbs. Learn to distinguish males from females. On the first digit of the male's forelimb, There is a thickening—the nuptial pad—which is important during mating and is especially well-developed in the spring. The female's abdomen is broad, whereas the male's is narrow and drawn in. Males of the aforementioned frog species also possess vocal sacs.

Dissection. Study the internal anatomy using a freshly dissected frog, a preserved wet mount of dissected male and female frogs, and a diagram of the circulatory system.

Take an anesthetized or immobilized frog, place it on its back, and pin its stretched limbs to the bottom of the dissecting tray. Lift the skin at the lower part of the abdomen with forceps and make a transverse incision; then use scissors to cut the skin along the midline all the way up to the lower jaw along the entire body. At the level of the forelimbs, make transverse cuts to the right and left. Turn back the skin flaps to the sides, snipping with a scalpel the Connective Tissue bridges linking the skin to the muscles. Pin the skin to the bottom of the tray. Pay attention to the rich network of Blood Vessels branching out within the skin (which is associated with cutaneous respiration in frogs). Examine the muscles, noting the almost complete absence of segmentation, which is so well pronounced in fish. This Muscle differentiation is driven by limb development and complex movements. The most powerful muscles are concentrated in the region of the hindlimbs.

The same technique is applied to dissecting the muscular body wall (insert the scissors shallowly so as not to damage internal organs). When cutting, lift the abdominal body wall with forceps. Along the line of the incision, you will encounter the bones and Cartilage of the Pectoral Girdle—these should be cut through. The longitudinal incision of the muscular wall must run alongside the midline to avoid damaging the abdominal vein and causing Hemorrhage. Continue the incision up to the Sternum, then lift it with forceps and cut through it. Pin the flaps of the muscular wall to the bottom of the tray along with the skin.

After dissection, pour water into the tray (this will make the internal organs much more clearly visible). If the water becomes discolored by blood leaking from wounded vessels, drain it and pour in fresh water.

Examine the overall arrangement of the internal organs. Draw the outline of the frog's body and fill it in as you study each organ system. The frog's body cavity is not divided into thoracic and abdominal cavities, but for convenience of study, you may conditionally mark this boundary on your drawing.

Locate The Heart in the thoracic region of the body (Fig. 125), which continues to beat in anesthetized frogs. The lungs are situated to the right and left of the heart, and sometimes beneath it.

When inflated with air, the lungs appear as large, light-gray sacs; when deflated, they look like small, slate-gray pouches. In the dissection of a sexually mature female, the Ovaries immediately catch the eye—they are large, dark-colored, multichambered sacs that conceal the other organs. Remove the Ovary on the left side by cutting the mesentery that Supports it. Push the oviducts (long white tubes) aside to examine the underlying internal organs.

Find the large Liver on either side of the heart, consisting of two lobes—right and left; the left lobe is bifurcated. Lift the posterior edge of the liver; beneath it, a bluish-green sphere—the Gallbladder—is visible. The elongated Stomach is partially covered by the left lobe of the liver, and the duodenum originates from it.

Reflect the loop of the duodenum and The Stomach to the right side and locate the Pancreas. Examine the intestine: the duodenum transitions into the Small Intestine, which forms several loops, and the small intestine seamlessly continues into the Large Intestine. The intestine terminates in the cloaca. Note that the intestine is suspended by a mesentery through which blood vessels branch. You can reveal the mesentery by gently parting the loops of the intestine with forceps. At the posterior end of the small intestine, the Spleen—appearing as a small, rounded body—is suspended from the mesentery. When examining the intestine, take care not to make a common mistake: in females, the long, convoluted oviducts located along the sides of the Abdominal cavity can easily be mistaken for the intestine.

Fig. 125. Internal anatomy of a frog:

1 — heart; 2 — lung; 3, 4 — liver; 5 — gallbladder; 6 — stomach; 7 — pancreas; 8, 9 — small intestine; 10 — large intestine; 11 — spleen; 12 — cloaca; 13 — Urinary Bladder; 14 — its opening into the cloaca; 15 — Kidney; 16 — Ureter; 17 — its opening into the cloaca; 18 — ovary; 19 — fat body; 20, 21 — oviducts; 22 — uterine region of the oviduct; 23 — opening of the oviduct into the cloaca; 24 — dorsal aorta; 25 — posterior vena cava; 26 — carotid artery; 27 — aortic arch; 28 — pulmonary artery.

Locate the bilobed, thin-walled urinary bladder situated ventrally at the very terminus of the rectum. Its walls often collapse during dissection.

Following a general familiarization with the arrangement of the internal organs and sketching the internal structure of the dissected frog, you should proceed to dissect and study individual Organ Systems in greater detail.

The Digestive System begins with the oral cavity. Unlike fish, the frog possesses Salivary Glands, so food is moistened with saliva. The posterior part of the oral cavity transitions directly into the Pharynx, forming the oro-pharyngeal cavity. From the oro-pharyngeal cavity, food passes through a short esophagus into the stomach, and subsequently into the duodenum, into which the ducts of the previously examined liver and pancreas empty.

Prise out and isolate the intestine by untangling its loops and cutting the mesentery. Examine its regions. The small intestine is relatively short; it is the site where the final Digestion AND ABSORPTION of food take place. Undigested residues pass into the large intestine and then into its terminal section—the rectum, which receives the ducts of the Urogenital System and Functions as the cloaca.

Respiratory organs—the lungs—appeared in amphibians in connection with their transition to terrestrial life. Insert a glass pipette into the glottis, inflate the lungs through it, and examine their structure. Air enters through the nostrils into the oro-pharyngeal cavity, from there into the laryngotracheal chamber, and into the lungs. Frogs lack Bronchi. The lungs have a primitive structure. Pulmonary respiration in the frog is supplemented by a highly developed cutaneous respiration. Consequently, the frog's skin is naked, scale-free, moist, and supplied with an extensive network of blood vessels.

Due to the transition of amphibian ancestors to land and the development of pulmonary respiration, The structure of the heart also became more complex. It consists of two atria and a ventricle, alongside a sinus venosus and a conus arteriosus. The circulatory system features two circuits: systemic (greater) and pulmonary (lesser), though they are not completely separated.

The frog's circulatory system is difficult to examine in its entirety during routine dissection in Practical Classes; only the most accessible PARTS OF THE circulatory system are visible here, while a detailed study is carried out using an injected specimen and a diagram. Pull the pericardial sac aside with forceps, cut it open with scissors, and observe the beating of the heart. Locate the atria and the ventricle: the atria are darker in color, with thin, soft walls, whereas the ventricle is paler and its walls are much more muscular. Press gently with a needle on the atria and then on the ventricle. Pin the frog's heart by piercing it with a needle and anchoring it, through the frog's body, to the bottom of the dissecting tray. This puts tension on the conus arteriosus, systemic aorta, aortic arches, pulmocutaneous, and carotid Arteries. For a more thorough inspection and study of the arteries, use fine scissors to trim away the surrounding Tissues and muscle bundles, and free the blood vessels using a sharp needle. Remove the pin, lift the heart with forceps, flip it upward, and examine the sinus venosus (a dark, thin-walled sac) and the Pulmonary Veins.

Make a transverse cut through the atria and ventricle with a scalpel, and examine the ventricular and atrial walls. Cut the ventricle lengthwise and note the wrinkled appearance of its inner wall.

Study the systemic and pulmocutaneous circulations using the diagram and the injected specimen. The truncus arteriosus, originating from the right side of the ventricle, extends into the truncus aortae, which divides into right and left branches. Three vessels branch off from each side. Thus, the ventral aorta gives rise to three pairs of arterial trunks, or aortic arches.

The pair of vessels closest to the heart is the pulmocutaneous arteries, which belong to the pulmonary circuit. During ventricular systole, predominantly venous blood from the right side of the ventricle is pumped into the pulmocutaneous arteries and directed to the lungs and skin. Following Gas Exchange in the pulmonary capillaries, oxygenated blood returns via the pulmonary veins to the left atrium. The pulmonary circuit thus begins in the ventricle, passes through the lungs, and ends in the left atrium. The second pair of aortic arches comprises the systemic arches, which curve around the heart on the right and left sides. They give rise to the subclavian arteries supplying the forelimbs. Posterior to the heart, the systemic arches turn backward and enter the abdominal cavity, where they unite to form the unpaired dorsal aorta extending along THE Vertebral Column. Branches from the aorta supply all bodily organs with blood. Posterior to the Kidneys, the aorta bifurcates into two common iliac arteries carrying blood to the hindlimbs. Mixed blood enters the aortic arches from the ventricle. Consequently, all Organs of the frog's body, except for the head, receive mixed blood.

The third pair of vessels consists of the slender and short carotid arteries. They branch off from the aortic trunk and deliver arterial blood to the head from the leftmost part of the ventricle.

In the systemic circuit, venous blood from the head and trunk returns to the heart via the paired anterior venae cavae and the unpaired posterior vena cava, emptying into the sinus venosus and subsequently into the right atrium. Arterial blood from the skin flows through cutaneous veins likewise into the anterior venae cavae, which empty into the sinus venosus. Therefore, the blood entering the right atrium is not purely venous, but contains an admixture of arterial blood.

The pathway of blood flowing from the internal organs to the venae cavae and further into the right atrium is best examined in detail using the diagram. The systemic circuit,

therefore, begins with the second and third pairs of vessels (the systemic arches and common carotid arteries) originating from the ventricle, and terminates in the right atrium. The SYSTEMIC AND PULMONARY circuits in the frog are not completely segregated because the blood partially mixes within the heart ventricle.

Cut the esophagus near the pharynx and the rectum anterior to the cloaca, remove the digestive organs from the body cavity by trimming the mesentery, and, having straightened the intestine, place them in the dissecting pan next to the dissected frog.

Excretory system. Kidneys—spindle-shaped, dark red structures—are visible along both sides of the vertebral column. As in fish, these are primary, or trunk, kidneys (mesonephros). Ureter ducts extend from them to open into the cloaca, whereas the urinary bladder opens into the cloaca via a separate orifice. In some dissected frogs, narrow yellowish strips—the Adrenal Glands (Endocrine glands)—can be observed on the ventral surface of the kidneys facing the body cavity.

Reproductive System. Examine the reproductive organs of male and female frogs. If the dissected frog is a female, after studying its reproductive organs, you should also examine those of a male using a dissected frog from a neighboring dissecting pan.

Locate the FEMALE REPRODUCTIVE ORGANS: the ovaries and oviducts. The ovaries appear as lobed sacs of varying size and color; they are suspended on both sides of the vertebral column (by a specialized fold called the mesentery) from the dorsal wall of the abdominal cavity. The oviducts are long, convoluted tubes beginning near the Base of the lungs with a small open funnel (ostium). The posterior ends of the oviducts widen slightly before opening into the cloaca. Examine these expanded regions, known as uteri, where eggs accumulate prior to being spawned. During maturation, eggs rupture through the ovarian wall into the body cavity and pass from there into the oviduct funnels (Fig. 126). Fertilization in frogs is external and takes place in water.

Fig. 126. Urogenital system of a female frog:

1 — oviduct funnel; 2 — oviduct; 3 — Uterus; 4 — cloaca; 5 — urinary bladder; 6 — ovary; 7 — kidney; 8 — fat body; 9 — ureter.

Locate the Testes—they are yellowish-white or dark, bean-shaped bodies (Fig. 127). If a Testis is pulled upward with forceps, numerous efferent ductules connecting each testis to the kidney will become visible. Passing through the kidney, these ductules empty into the ureter. Thus, in the frog, the Ureters also function as vas deferens (Wolffian ducts). Examine the Seminal Vesicles, which are dilations of the ureters just before they enter the cloaca. Compare the structure of the excretory and reproductive systems in frogs and fish.

Fig. 127. Urogenital system of a male frog:

1 — testes; 2 — fat body; 3 — kidneys; 4 — Wolffian duct; 5 — Seminal Vesicle; 6 — cloaca; 7 — urinary bladder; 8 — posterior vena cava; 9 — efferent ductules.

Nervous system. Remove the excretory and reproductive organs from the frog's body cavity, and examine the vertebral column consisting of 9 vertebrae, along with the paired Spinal Nerves extending laterally from the spine and toward the hindlimbs as thin, white, glossy threads.

To study the brain, turn the frog dorsal side up and pin its limbs to the bottom of the wax dissecting pan once again. Remove the skin from the head and upper back, and cut through the vertebral column at the base of the skull. Bend the head downward, insert the tip of the scissors into the foramen magnum, incise the skull, and make two oblique cuts toward the eyes, reaching almost to the nostrils. Using forceps, bend the cut edge upward and break or cut it off. Rinse the brain exposed in this manner with water from a pipette. Examine the frog's dorsal brain using a hand lens. It is very small relative to the frog's large head. It comprises the same regions as the fish brain, but is distinguished by a much larger Forebrain (Fig. 128). Locate the olfactory lobes (which receive the olfactory nerves)—they lie anterior to the cerebral hemispheres, with no distinct boundary separating them. Posterior to the hemispheres, examine the Diencephalon, which has a diamond-like shape (absent in fish). Posterior to the rounded optic lobes of the Midbrain (which are relatively smaller than those in fish), lies the Cerebellum, also less developed than in fish, reflecting the lower mobility of frogs. Behind the cerebellum, the Medulla Oblongata is clearly visible, continuing posteriorly into the Spinal Cord.

Fig. 128. Frog brain:

A — dorsal view; B — ventral view; C — lateral view; 1 — cerebral hemispheres; 2 — olfactory lobes; 3 — Olfactory nerve; 4 — diencephalon; 5 — optic nerves; 6 — infundibulum; 7 — Pituitary Gland (hypophysis); 8 — midbrain; 9 — cerebellum; 10 — medulla oblongata; 11 — spinal cord.