Faunal Diversity (Zoology): Protista (Paramecium, Plasmodium), animal phyla up to class, earthworm, frog

(d) Sex appeal

The labelled organ X is the vocal sac of the male frog. Its croaking sound is a mating call that attracts the female frog, so it indicates sex appeal (readiness to mate).

(a) Defense, offence, and adhesion

Trichocysts are small spindle-shaped organelles lying just beneath the pellicle of Paramecium. On stimulation they discharge a thread-like structure that helps in defence against enemies, in offence by paralysing prey, and in adhesion by anchoring the organism to a substratum. They play no role in reproduction, nutrition or osmoregulation.

Transverse binary fission is a type of asexual reproduction in which a unicellular organism divides into two nearly equal daughter cells by a constriction that cuts across the short (transverse) plane, i.e. at right angles to the long axis of the body. It is the usual method of asexual reproduction in Paramecium when food and temperature are favourable.

Process in Paramecium:

1. The animal stops feeding. The micronucleus divides by mitosis into two; the macronucleus elongates and divides by amitosis into two.
2. A new cytostome (oral groove) and, if needed, new contractile vacuoles begin to form for the posterior daughter.
3. A transverse constriction appears across the middle of the body and deepens inward.
4. The cytoplasm is cut into two halves (cytokinesis); each half has one micronucleus and one macronucleus.
5. The two daughter cells separate. The anterior one (proter) keeps the old oral apparatus; the posterior one (opisthe) completes the missing organelles. Each grows to full size and repeats the process, one fission taking about 6 to 8 hours.

Pancreas of the frog (2 marks)

The pancreas is a flat, irregular, yellowish gland lying in the mesentery between the stomach and the duodenum. It is a mixed gland. Its exocrine part secretes pancreatic juice, which reaches the duodenum through the common bile (hepatopancreatic) duct. Its endocrine part, the islets of Langerhans, secretes insulin and glucagon into the blood. The pancreatic juice contains enzymes for digesting carbohydrates, proteins and fats, so the pancreas is central to intestinal digestion.

Role in intestinal digestion (3 marks)

1. Neutralisation of chyme: The acidic chyme from the stomach is neutralised by the alkaline (bicarbonate-rich) pancreatic juice. The medium becomes alkaline (about pH 8), which is required for pancreatic and intestinal enzymes.
2. Carbohydrates: Pancreatic amylase (amylopsin) converts starch and glycogen into maltose. Intestinal maltase then converts maltose into glucose.
3. Proteins: Trypsinogen is activated by enterokinase of the intestinal juice into trypsin, which activates chymotrypsinogen into chymotrypsin. These break proteoses and peptones into smaller peptides. Carboxypeptidase removes terminal amino acids, and intestinal peptidases complete the digestion to amino acids.
4. Fats: Bile salts emulsify fats into fine droplets. Pancreatic lipase (steapsin) then hydrolyses them into fatty acids and glycerol.
5. Absorption: The end products (glucose, amino acids, fatty acids, glycerol) are absorbed by the villi into the blood capillaries and lacteals.

Earthworms (Pheretima) are hermaphrodite (bisexual) but cross-fertilisation is the rule, because the testes and ovaries mature at different times (protandry). Reproduction takes place in the breeding season, mostly in the rains.

Copulation (2 marks)

1. Two worms come together with their ventral surfaces in contact, the anterior ends pointing in opposite directions.
2. The male genital papillae (segment 18) of each worm are pressed against the spermathecal pores (segments 5/6 to 8/9) of the other. The clitella are joined by a mucous sheath.
3. Sperms from the seminal vesicles pass out through the male pores and travel along the seminal grooves into the spermathecae of the partner. Both worms exchange sperm at the same time and store it in the spermathecae.
4. The worms then separate. Eggs are not fertilised at this stage.

Cocoon formation (2 marks)

1. A few days later the clitellum (segments 14 to 16) secretes a ring-shaped mucous girdle around the body and a tough, albuminous cocoon wall inside it.
2. The girdle slips forward. At segment 14 it receives ova from the female pore. At segments 5 to 9 it receives the stored sperms of the partner.
3. Fertilisation occurs inside the cocoon. The girdle slips off over the head, and its ends close to form a lemon-shaped cocoon.
4. The cocoon is left in moist soil. It holds one to a few fertilised eggs, and young worms hatch in about 2 to 3 weeks.

Diagram (1 mark)

Draw two worms lying side by side in opposite directions, labelling the clitellum, spermathecal pores, male genital papillae, and arrows showing the transfer of sperm. A second sketch shows the clitellum with the cocoon moving forward over the anterior segments.

Excretion in earthworm (1 mark)

Earthworm is mainly ureotelic, excreting urea (with some ammonia). Wastes are removed from the coelomic fluid and blood by coiled excretory tubules called nephridia. Three kinds are present: septal, integumentary and pharyngeal. Integumentary and pharyngeal nephridia open outside or into the buccal cavity. Septal nephridia open into the intestine.

Structure of septal nephridium (4 marks)

Septal nephridia occur in rows on both faces of the septa, from the 15th segment backwards. Each is a coiled tube with these parts:

1. Nephrostome: a ciliated, funnel-shaped opening lying in the coelom of the segment in front of the septum. Its cilia drive coelomic fluid and wastes into the tube.
2. Neck: a short ciliated tube leading from the funnel to the body.
3. Body (main coiled tube): a ciliated, much-coiled tube (with a straight, a spiral and a terminal loop) that lies in the septum. It is richly supplied with blood capillaries. Useful materials such as water and salts are reabsorbed, and wastes from the blood are added.
4. Terminal duct: the body opens into a terminal duct that runs along the septum and discharges into a longitudinal septal excretory canal, which opens into the intestine through an intestinal nephridiopore. Water is reabsorbed again in the intestine.

Pathway: coelomic fluid, nephrostome, neck, coiled body, terminal duct, septal canal, intestine, anus.

Labelled diagram (3 marks)

 coelom (segment in front)    septum    segment behind
        ___                     |
   nephrostome (ciliated) ---> neck
                                |
            coiled body (with capillaries)
                                |
                          terminal duct
                                |
                  septal excretory canal --> intestine

Label: nephrostome, neck, body, terminal duct, septum, blood capillaries, intestine.

(b) Plasmodium

Malaria is caused by the sporozoan Plasmodium (e.g. P. vivax, P. falciparum) and spread by the bite of the female Anopheles mosquito. Entamoeba causes amoebic dysentery and Trypanosoma gambiense causes sleeping sickness.

(c) Nephridia

Earthworms excrete through coiled tubular organs called nephridia (septal, pharyngeal and integumentary types). Malpighian tubules belong to insects, flame cells to flatworms and green glands to crustaceans.

(b) Skin (Cutaneous)

During hibernation the frog lies buried in mud with a very low metabolic rate, so its oxygen need is small. It breathes through its moist, highly vascular skin by cutaneous respiration instead of the lungs.

(b) 14, 15, 16

In a mature earthworm (Pheretima) the clitellum is a thick, glandular band covering the 14th, 15th and 16th segments. It secretes the cocoon in which eggs are laid.

(b) Osmoregulation and excretion

Water enters the freshwater Paramecium by endosmosis. The contractile vacuole collects the excess water and some dissolved waste and expels it, keeping the water balance of the cell and preventing it from bursting.

Macronucleus and micronucleus of Paramecium (5 marks)

Paramecium is a ciliate protozoan with two types of nuclei, an example of nuclear dimorphism.

Macronucleus

• A single, large, bean- or kidney-shaped nucleus near the centre of the cell.
• It is polyploid and contains much DNA.
• It controls vegetative functions: metabolism, growth and protein synthesis. It also regulates other normal day-to-day cell activities.
• It disintegrates during conjugation and is built again from the micronucleus.

Micronucleus

• One or more small, compact, spherical nuclei lying in a depression of the macronucleus (P. caudatum has one; P. aurelia has two).
• It is diploid.
• It controls reproductive functions: it divides by mitosis during binary fission and by meiosis during conjugation, and it is concerned with genetic exchange.
• It is the source of the new macronucleus.

Functional difference: the macronucleus is the "somatic" or vegetative nucleus, while the micronucleus is the "germinal" or generative nucleus.

Locomotion in Paramecium (5 marks)

Paramecium moves by ciliary movement. The whole body is covered with thousands of short, hair-like cilia arranged in longitudinal rows. Each cilium arises from a basal body (kinetosome) under the pellicle, and the basal bodies are joined by kinetodesmata.

Structure of a cilium: has a $9+2$ arrangement of microtubules (nine outer doublets and two central singlets).

Mechanism

1. Each cilium beats in two strokes: a quick effective (power) stroke in which it is held stiff and sweeps backwards, pushing against the water; and a slower recovery stroke in which it bends and returns forward.
2. The cilia beat in a coordinated metachronal rhythm: the beat passes in waves from the anterior to the posterior end, like wind passing over a field of grain, so neighbouring cilia are at different phases.
3. The backward effective stroke pushes the animal forward. Because the oral groove is on one side and the beat is oblique, the animal rotates on its long axis, moving in a spiral path.
4. To reverse (avoiding reaction), the cilia beat forward for a moment, and the animal moves backward, turns, and moves forward again.

The movement is controlled by the neuromotor system (motorium) in the cytoplasm.

Life cycle of Plasmodium in the mosquito (sexual cycle, sporogony) (8 marks)

The female Anopheles mosquito is the definitive host of Plasmodium, where the sexual phase occurs. The cycle in the mosquito is called sporogony.

1. Entry: when a female Anopheles sucks blood from a malaria patient, it takes in all stages of the parasite. Only the gametocytes survive in the mosquito's stomach; the other stages are digested.
2. Gamete formation (gametogony): in the stomach, the male microgametocyte forms 6 to 8 thread-like microgametes (flagellated, by exflagellation) and the female macrogametocyte forms one macrogamete.
3. Fertilisation: a microgamete fuses with the macrogamete to form a diploid zygote.
4. Ookinete: the zygote becomes elongated and motile, called the ookinete, which penetrates the stomach wall.
5. Oocyst: the ookinete comes to lie under the outer wall of the stomach and rounds off into an oocyst. Meiosis occurs, and the nucleus divides repeatedly.
6. Sporozoites: the oocyst enlarges and produces thousands of haploid, spindle-shaped sporozoites (the infective stage). The oocyst bursts and releases them into the haemocoel.
7. Migration: the sporozoites move to the salivary glands of the mosquito.
8. Transmission: when the infected mosquito bites a human, sporozoites are injected with its saliva, starting the cycle in man.

The sequence in the mosquito takes about 10 to 14 days, so the mosquito becomes infective only after this period.

Alimentary canal of the earthworm (Pheretima posthuma) (5 to 8 marks)

The alimentary canal is a straight tube running from the mouth (segment 1) to the anus (last segment). It has these regions.

1. Mouth: a small opening on the first segment under the prostomium, leading into the buccal cavity.
2. Buccal cavity: in segments 1 to 3; it is small and leads into the pharynx.
3. Pharynx: a muscular, bulb-like organ in segments 3 to 4, surrounded by pharyngeal (salivary) glands that secrete saliva containing mucus and proteolytic enzymes. It pumps in food by sucking.
4. Oesophagus: a narrow tube in segments 5 to 7, which passes the food on. It is a thin-walled tube that passes food backward to the gizzard.
5. Gizzard: a hard, thick-walled muscular organ in segments 8 to 9 which grinds food with the help of small stones.
6. Stomach: in segments 9 to 14; digestion continues here. Calciferous glands in segments 10 to 12 remove excess calcium from the food and regulate pH.
7. Intestine: a long tube from segment 15 to the last segment. Between segments 26 to 95 the dorsal wall is folded into the lumen as a typhlosole to increase the surface area for digestion and absorption. Intestinal glands secrete enzymes (amylase, lipase, protease, cellulase).
8. Rectum and anus: the last part of the intestine opens to the outside through the anus and passes out waste as castings.

Digestion: food (decaying matter and soil) is ground in the gizzard and digested in the intestine; undigested matter is expelled as worm castings.

Mouth -> Buccal cavity -> Pharynx -> Oesophagus -> Gizzard -> Stomach -> Intestine (with typhlosole) -> Anus

Alimentary canal and blood vascular system of the earthworm (Pheretima posthuma) (8 marks)

A. Alimentary canal (a straight tube from the mouth to the anus)

1. Mouth (segment 1) $\to$ buccal cavity (segments 1 to 3).
2. Pharynx (segments 3 to 4): muscular and sucking, with salivary glands.
3. Oesophagus (segments 5 to 7): narrow tube conducting food to the gizzard.
4. Gizzard (segments 8 to 9): thick, muscular, grinds the food using soil particles.
5. Stomach (segments 9 to 14): digestion continues; calciferous glands (segments 10 to 12) remove excess calcium and regulate pH.
6. Intestine (segment 15 to the last): long, with a typhlosole (dorsal fold in segments 26 to 95) that increases the area for digestion and absorption.
7. Anus: undigested waste is expelled as castings.

B. Blood vascular system

It is a closed system in which blood flows in vessels. The blood is red due to dissolved haemoglobin in the plasma, and has colourless amoeboid corpuscles.

1. Dorsal vessel: a large contractile vessel above the gut which carries blood forward.
2. Ventral vessel: below the gut, carries blood backward and distributes it.
3. Subneural vessel: lies below the nerve cord.
4. Hearts: four pairs of contractile hearts connect the dorsal and ventral vessels: lateral hearts in segments 7 and 9, and lateral-oesophageal hearts in segments 12 and 13. They have valves that keep the flow in one direction. The dorsal vessel is also pulsatile.
5. Capillary network: connects arteries and veins in the skin, gut and nephridia for oxygen and food supply.

Circulation: blood moves forward in the dorsal vessel and passes through the hearts to the ventral vessel, which sends it backwards to the capillaries of the body wall and gut.

External features and respiration of the frog (Rana cyanophlyctis) (2+3 marks)

External features (2 marks)

• Body is smooth, moist and streamlined, divided into head and trunk (no neck, no tail in the adult).
• Skin is slimy, with no scales; the dorsal side is olive green with dark patches, and the ventral side is pale.
• Head is triangular with two large bulging eyes having upper, lower and transparent nictitating eyelids, two nostrils at the tip, and a tympanum (ear drum) behind each eye. The mouth is wide.
• Forelimbs have four digits, and the hind limbs are long, muscular, with five webbed toes, adapted for swimming and jumping.
• Males have vocal sacs and nuptial pads.

Respiratory mechanism (3 marks)

A frog respires in three ways.

1. Cutaneous respiration: gas exchange through the moist, vascular skin; the main method in water and during hibernation.
2. Buccopharyngeal respiration: gas exchange across the moist lining of the mouth cavity, by raising and lowering the floor of the buccal cavity.
3. Pulmonary respiration: a pair of simple, sac-like lungs. With the mouth closed and nostrils open, the buccal floor is lowered, drawing air into the buccal cavity; the nostrils close and the floor is raised, forcing the air into the lungs. Exhalation occurs when the body-wall muscles contract and the lungs recoil.

External morphology of the frog (dorsal view) (5 marks)

The frog (Rana tigrina) has a streamlined, bilaterally symmetrical body of head and trunk; there is no neck and no tail.

1. Skin: smooth, moist and slimy, without scales, with mucous glands. The dorsal surface is olive green with irregular dark spots, which acts as camouflage; the ventral surface is pale yellow.
2. Head: triangular and flat, with a wide mouth.
   • Eyes: two large, bulging, on top of the head; each has an upper eyelid, a lower eyelid and a transparent nictitating membrane.
   • External nostrils: a pair at the snout tip.
   • Tympanum: a circular eardrum behind each eye.
3. Trunk: broad, ending in the cloacal aperture (vent) at the posterior end.
4. Forelimbs: short, with four digits (no webbing).
5. Hind limbs: long and strong with five webbed toes, for swimming and jumping.

   nostril  eye   tympanum
      \  /\ /\   /
       (head)---(trunk)---> forelimb at the side
                 |
        hind limb (webbed toes)  ... cloacal aperture

Internal structure of the frog's heart (5 marks)

The frog's heart is three-chambered and enclosed in a thin double-walled pericardium with pericardial fluid, lying in the thorax. It consists of four main parts.

1. Sinus venosus: a thin-walled, triangular sac on the dorsal side. It receives deoxygenated blood from the body through two precaval veins and one postcaval vein, and opens into the right atrium via the sinu-atrial aperture (with valves). It acts as a pacemaker.
2. Atria: two thin-walled chambers at the anterior, separated by an interatrial septum. The right atrium receives deoxygenated blood from the sinus venosus; the left atrium receives oxygenated blood from the lungs through the pulmonary veins. Both open into the ventricle through a single atrio-ventricular aperture guarded by valves.
3. Ventricle: a single, thick-walled, muscular, conical chamber at the posterior end. The inner wall has muscular ridges (columnae carneae) that reduce mixing of the oxygenated and deoxygenated blood.
4. Truncus (conus) arteriosus: a thick, muscular tube from the ventricle, with semilunar valves at its base to stop backflow. It divides into right and left aortic trunks, each giving carotid, systemic and pulmocutaneous arches. A spiral valve in the conus guides the flow.

Blood mixes only slightly in the ventricle.

Alimentary canal and digestive system of the frog (Rana tigrina) (8 marks)

The frog is carnivorous (it eats insects, worms, etc.). The digestive system consists of the alimentary canal and digestive glands.

A. Alimentary canal

1. Mouth: a wide opening bounded by upper and lower jaws. The upper jaw has small maxillary teeth and vomerine teeth on the palate (for holding prey, not chewing). The tongue is bifid, attached at the front and free behind, and sticky, used to catch insects.
2. Buccopharyngeal cavity: leads to the pharynx; it opens to the internal nares, Eustachian tubes, glottis and gullet.
3. Oesophagus: a short, wide tube, ciliated, that carries food into the stomach.
4. Stomach: a J-shaped muscular sac, with cardiac and pyloric parts; the gastric glands secrete HCl and pepsin, so protein digestion starts.
5. Small intestine: the duodenum (receives the common bile duct) and the long coiled ileum, where digestion ends and absorption occurs.
6. Large intestine (rectum): short and wide; it stores faeces and opens into the cloaca, then to the exterior through the cloacal aperture.

B. Digestive glands

• Liver: large, three-lobed, secretes bile (stored in the gall bladder), which emulsifies fats.
• Pancreas: a pinkish gland in the loop of the stomach and duodenum, secreting pancreatic juice (trypsin, amylase, lipase).
• Intestinal glands also secrete juice.

Digestion: food is swallowed whole; it is digested in the stomach and intestine, absorbed, and the waste is expelled through the cloaca.