Life and Diversity of Animals – Nonchordates (Protozoa to Annelida) BZO1T01

Syllabus topics (RTMNU)

• 1.1 Kingdom Protista, Phylum - Protozoa: General characters of Protozoa.
• 1.2 Paramecium: Structure, locomotion and reproduction.
• 1.3 Plasmodium: Structure and life cycle.
• 1.4 Parasitic Protozoans of Man : Entamoeba, Trypanosoma, Giardia and Leishmania - Mode of infection, Damage and control.

1.1 Kingdom Protista, Phylum Protozoa: General characters

Protozoa (Gr. protos = first, zoon = animal) are microscopic, unicellular, eukaryotic animal-like organisms. In the five-kingdom system of R. H. Whittaker (1969) they are placed in Kingdom Protista. The single cell does all life functions, so protozoa show the protoplasmic (acellular) grade of organisation.

General characters

• Habitat: free-living in fresh water, sea water and damp soil; some are commensals or parasites (e.g. Plasmodium, Entamoeba).
• Body: single cell, naked or covered by a pellicle, test or shell. Shape may be fixed (Paramecium) or changing (Amoeba).
• Symmetry: asymmetrical, radial or bilateral.
• Nucleus: one or more; ciliates have two types – macronucleus (vegetative) and micronucleus (reproductive).
• Locomotion: by pseudopodia, flagella or cilia; some (Sporozoa) have no locomotory organelle.
• Nutrition: holozoic, holophytic (Euglena), saprozoic or parasitic. Digestion is intracellular in food vacuoles.
• Respiration and excretion: by diffusion through the body surface. Contractile vacuoles remove excess water (osmoregulation) in fresh-water forms.
• Reproduction: asexual by binary fission, multiple fission or budding; sexual by syngamy or conjugation.
• Encystment: many form cysts to survive bad conditions and for dispersal.

Outline classification (based on locomotory organelles)

1.2 Paramecium: Structure, locomotion and reproduction

Paramecium caudatum is a free-living ciliate of stagnant fresh water. It is called the slipper animalcule because of its slipper-like shape. Size about 170–290 µm.

Structure

• Pellicle: firm, elastic outer covering with hexagonal depressions; gives fixed shape.
• Cilia: short hair-like processes covering the whole body; longer cilia at the posterior end form the caudal tuft.
• Trichocysts: spindle-shaped bodies in the ectoplasm; discharged as threads for defence and attachment.
• Oral groove: a shallow depression on the ventral side leading to the vestibule, buccal cavity, cytostome (cell mouth) and cytopharynx.
• Food vacuoles: formed at the end of the cytopharynx; they circulate in the endoplasm (cyclosis) while food is digested. Undigested matter leaves through the cytopyge (cell anus).
• Contractile vacuoles: two, one at each end, each with 5–12 radiating canals; they pump out excess water.
• Nuclei: one large kidney-shaped macronucleus (controls metabolism) and one small micronucleus (controls reproduction).

Locomotion

• Ciliary locomotion: each cilium beats in two strokes – a fast, stiff effective stroke backwards and a slow, bent recovery stroke.
• Cilia beat one after another in a wave (metachronal rhythm), controlled by the neuromotor system.
• Because cilia beat obliquely, the animal moves forward in a spiral path, rotating on its long axis. It can reverse the beat to swim backwards (avoiding reaction).
• Body contortion: it can squeeze through narrow gaps by temporary change of shape.

Reproduction

• Transverse binary fission (asexual): micronucleus divides by mitosis, macronucleus divides by amitosis; a constriction divides the body into an anterior proter and posterior opisthe. Occurs 2–3 times a day in good conditions.
• Conjugation (sexual): two individuals of opposite mating types join at the oral surfaces. Macronucleus degenerates; micronucleus divides by meiosis to give four, three degenerate; the remaining one divides into a stationary (female) and a migratory (male) pronucleus. Migratory pronuclei are exchanged and fuse to form a synkaryon. Exconjugants separate and, after further divisions, each gives four daughter Paramecia.
• Other processes: autogamy (self-fertilisation in one individual), endomixis and cytogamy.
• Significance of conjugation: brings genetic recombination and rejuvenation (new macronucleus).

1.3 Plasmodium: Structure and life cycle

Plasmodium is a sporozoan parasite that causes malaria in man. It is digenetic: the primary (definitive) host is the female Anopheles mosquito (sexual phase) and the secondary (intermediate) host is man (asexual phase). Species: P. vivax (benign tertian), P. falciparum (malignant tertian, most dangerous), P. malariae (quartan), P. ovale (mild tertian).

Structure of sporozoite

Sickle-shaped, about 11–15 µm long, with a pellicle, a central nucleus and an apical complex at the anterior end that helps it enter liver cells. It has no locomotory organelle; it moves by gliding.

Life cycle in man (asexual – schizogony)

1. Infection: an infected female Anopheles bites and injects sporozoites with saliva into the blood.
2. Pre-erythrocytic cycle (in liver cells): sporozoites become schizonts, divide by multiple fission and release thousands of merozoites. In P. vivax some stay dormant as hypnozoites and cause relapse.
3. Exo-erythrocytic cycle: some merozoites infect fresh liver cells and repeat the cycle.
4. Erythrocytic cycle (in RBCs): merozoite → signet-ring stage (trophozoite with a vacuole) → amoeboid trophozoite → schizont → 12–24 merozoites. The RBC bursts and releases merozoites with toxic haemozoin. This takes 48 hours in P. vivax; the release causes the chill and fever of malaria.
5. Gametocyte formation: some merozoites form male microgametocytes and female macrogametocytes, which wait in the blood for the mosquito.

Life cycle in mosquito (sexual – gamogony and sporogony)

1. The female mosquito sucks blood with gametocytes.
2. In the stomach, the microgametocyte forms 4–8 flagellated microgametes (exflagellation); the macrogametocyte forms one macrogamete.
3. Fertilisation gives a zygote which becomes the motile ookinete.
4. The ookinete enters the stomach wall and forms an oocyst.
5. Sporogony: the oocyst nucleus divides many times; thousands of sporozoites are formed. The oocyst bursts and sporozoites reach the salivary glands, ready to infect man. The cycle in the mosquito takes about 10–14 days.

1.4 Parasitic Protozoans of Man

Note: Leishmania has two forms – amastigote (no free flagellum, in man) and promastigote (with flagellum, in sand fly). Giardia is bilaterally symmetrical with two nuclei and four pairs of flagella (“face-like” look).

Likely exam questions

1. (Long, 8 marks) Describe the structure of Paramecium with a labelled diagram and explain conjugation.
2. (Long, 8 marks) Describe the life cycle of Plasmodium vivax in man and mosquito.
3. (Short, 4 marks) Give the general characters of Protozoa.
4. (Short, 4 marks) Write the mode of infection, damage and control of Entamoeba histolytica.
5. (Short, 4 marks) Write a note on Leishmania donovani or Trypanosoma.

Syllabus topics (RTMNU)

• 2.1 Kingdom- Animalia: Major phyla of Animalia, general characters and classification upto classes of Phylum Porifera.
• 2.2 Sycon: Structure, reproduction and development, Canal system in sponges.
• 2.3 Coelenterata : General characters and classification up to classes.
• 2.4 Obelia : Structure and life cycle, corals and coral reef formation.

2.1 Kingdom Animalia and Phylum Porifera

Kingdom Animalia includes multicellular, eukaryotic, heterotrophic organisms whose cells have no cell wall. Most are motile, digest food inside the body and develop from a blastula stage.

Major phyla of Animalia (non-chordates to chordates)

Porifera → Coelenterata (Cnidaria) → Ctenophora → Platyhelminthes → Aschelminthes (Nematoda) → Annelida → Arthropoda → Mollusca → Echinodermata → Hemichordata → Chordata.

Phylum Porifera – general characters

• Porifera (L. porus = pore, ferre = to bear) are the sponges; mostly marine, a few fresh-water (Spongilla). All are sessile and attached to rocks.
• Cellular grade of organisation: cells are loosely arranged, true tissues and organs are absent.
• Body asymmetrical or radially symmetrical, diploblastic: outer pinacoderm and inner choanoderm with gelatinous mesenchyme between them.
• Body wall has many small pores, the ostia (inhalant), and one or more large openings, the osculum (exhalant).
• A unique canal system (water current system) helps in feeding, respiration and excretion.
• Choanocytes (collar cells) line the spongocoel or chambers; their flagella create the water current and they trap food.
• Skeleton of calcareous or siliceous spicules or spongin fibres.
• Digestion intracellular; no mouth, no digestive, nervous, respiratory or excretory organs.
• Reproduction asexual by budding, fragmentation and gemmules (fresh-water sponges); sexual by ova and sperms. Most are hermaphrodite. Development indirect with amphiblastula or parenchymula larva.
• High power of regeneration.

Classification up to classes (based on skeleton)

2.2 Sycon: Structure, reproduction, development and canal system

Sycon (Scypha) is a small, marine, calcareous sponge, found attached to rocks in shallow sea water. It is vase-shaped, solitary or in small colonies, 2.5–7.5 cm long.

Structure

• Body cylindrical with a terminal osculum surrounded by a fringe of long monaxon spicules (oscular fringe).
• Surface bears polygonal elevations (ends of radial canals) with small ostia (dermal pores) between them.
• Central cavity is the spongocoel (paragaster), lined by pinacocytes.
• Body wall is folded: incurrent canals (open outside through ostia) alternate with radial canals (lined by choanocytes, blind at the outer end).
• Incurrent and radial canals are connected by minute pores, the prosopyles; radial canals open into the spongocoel by apopyles.
• Mesenchyme contains amoebocytes, archaeocytes (totipotent), scleroblasts (form spicules) and germ cells.

Canal system in sponges

The canal system is a network of pores, canals and chambers through which water flows. Water brings food and oxygen, and carries away wastes and gametes.

In the leucon type the spongocoel is reduced and choanocytes are only in small chambers, so water flow is fastest and most efficient. (Some books add a fourth, rhagon type, seen in young demosponges.)

Reproduction

• Asexual: by external budding and regeneration.
• Sexual: Sycon is hermaphrodite but protogynous (ova mature first), so cross-fertilisation occurs. Gametes arise from archaeocytes/choanocytes. Sperms enter with the water current and reach the ovum through a carrier (nurse) cell – fertilisation is internal.

Development

1. Zygote undergoes holoblastic cleavage → 16–64 celled stomoblastula with small flagellated micromeres and large non-flagellated macromeres.
2. Inversion: the embryo turns inside out so flagella face outside → amphiblastula larva (free-swimming; anterior half flagellated micromeres, posterior half macromeres).
3. Larva swims out through the osculum, settles, and gastrulation occurs by invagination of micromeres.
4. It attaches by the blastopore end, develops into a young olynthus (ascon-like) stage and then into an adult Sycon.

2.3 Coelenterata: General characters and classification

• Aquatic, mostly marine; solitary or colonial; sessile or free-swimming.
• Tissue grade of organisation; radially symmetrical; diploblastic with ectoderm and endoderm separated by non-cellular mesogloea.
• A single body cavity, the coelenteron (gastrovascular cavity), with one opening, the mouth (hypostome) surrounded by tentacles.
• Cnidoblasts with stinging nematocysts for offence, defence and food capture – hence the name Cnidaria.
• Two body forms – polyp (sessile, cylindrical) and medusa (free-swimming, umbrella-like). Polymorphism is common.
• Digestion both extracellular and intracellular. Nervous system a diffuse nerve net.
• Reproduction asexual by budding; sexual by gametes. Many show metagenesis (alternation of polyp and medusa generations). Larva is planula.

2.4 Obelia: Structure, life cycle; corals and coral reefs

Obelia (sea fur) is a marine, sedentary, colonial hydrozoan growing on rocks and seaweeds. It shows trimorphism: polyps, blastostyles and medusae.

Structure of colony

• Hydrorhiza: horizontal root-like stolon attaching the colony.
• Hydrocaulus: upright branched stem; covered by a chitinous perisarc; living tissue inside is the coenosarc.
• Polyp (hydranth / gastrozooid): feeding zooid in a cup-shaped hydrotheca; has a hypostome with mouth and 20–30 solid tentacles.
• Blastostyle (gonozooid): reproductive zooid in a gonotheca; produces medusae by budding.
• Medusa: free-swimming sexual zooid; saucer-shaped with a short manubrium, four radial canals, a ring canal, 16 or more marginal tentacles, 8 statocysts (adradial) and 4 gonads on the radial canals. Velum is rudimentary.

Life cycle (metagenesis)

1. Asexual polyp colony → blastostyles bud off medusae.
2. Medusae are dioecious; they release ova and sperms into sea water; fertilisation is external.
3. Zygote → blastula → solid ciliated planula larva.
4. Planula settles and develops into a new polyp, which buds to form a new colony.

Thus the asexual polyp generation alternates with the sexual medusa generation – alternation of generations (metagenesis). Both generations are diploid.

Corals and coral reef formation

• Corals are mainly anthozoan polyps that secrete a hard skeleton of calcium carbonate (stony corals, e.g. Madrepora, Fungia). Other corals: horny (Gorgonia), organ-pipe (Tubipora), precious red coral (Corallium).
• Reef-building corals live with symbiotic algae, the zooxanthellae, and need warm (about 20–28 °C), clear, shallow (less than ~50 m), salty water.
• Over thousands of years skeletons of dead polyps accumulate and form coral reefs.
• Types (Darwin): (i) Fringing reef – close to the shore; (ii) Barrier reef – separated from the shore by a lagoon (e.g. Great Barrier Reef, Australia); (iii) Atoll – a ring-shaped reef enclosing a lagoon (e.g. Lakshadweep).
• Theories of formation: Darwin–Dana subsidence theory (fringing → barrier → atoll as the island sinks); Daly’s glacial control theory.
• Importance: rich biodiversity, protect coasts from waves, source of lime, tourism. Threat: coral bleaching due to rise in sea temperature.

Likely exam questions

1. (Long, 8 marks) Describe the canal system in sponges with suitable diagrams.
2. (Long, 8 marks) Describe the structure and life cycle of Obelia. Explain metagenesis.
3. (Short, 4 marks) Classify Phylum Porifera up to classes with examples.
4. (Short, 4 marks) Write a note on development of Sycon (amphiblastula larva).
5. (Short, 4 marks) What are coral reefs? Describe their types.

Syllabus topics (RTMNU)

• 3.1 Platyhelminthes: General characters and classification up to class.
• 3.2 Taenia solium: Structure and life cycle, parasitic adaptations in Taenia solium.
• 3.3 Aschelminthes: General characters and classification up to classes.
• 3.4 Ascaris : External morphology, reproductive system and life cycle.

3.1 Platyhelminthes: General characters and classification

Platyhelminthes (Gr. platys = flat, helminth = worm) are the flatworms. They are the first triploblastic, bilaterally symmetrical animals with the organ grade of organisation.

General characters

• Free-living (fresh water, sea, moist soil) or parasitic (endo- and ectoparasites).
• Body soft, dorso-ventrally flattened, leaf-like or ribbon-like; unsegmented or pseudo-segmented (tapeworms).
• Acoelomate: space between body wall and gut is filled with parenchyma (mesenchyme).
• Parasitic forms have suckers and hooks for attachment and a thick tegument.
• Digestive system incomplete (mouth, no anus) or absent (tapeworms).
• No respiratory or circulatory system; respiration through the body surface or anaerobic in parasites.
• Excretion and osmoregulation by flame cells (protonephridia).
• Nervous system: a pair of cerebral ganglia and longitudinal nerve cords connected by transverse commissures (ladder-like).
• Mostly hermaphrodite with complex reproductive organs; fertilisation internal. Life cycles complex with one or more intermediate hosts and larval stages.
• Free-living forms show high regeneration (Planaria).

Classification up to classes

3.2 Taenia solium: Structure, life cycle and parasitic adaptations

Taenia solium is the pork tapeworm. The adult lives in the small intestine of man (primary/definitive host); the larva lives in the muscles of pig (secondary/intermediate host). Disease: taeniasis; larval infection in man is cysticercosis.

Structure

• Body long (2–3 m or more), ribbon-like, white; divided into scolex, neck and strobila.
• Scolex (head): pin-head sized, with 4 cup-shaped suckers and a projecting rostellum bearing two rows of curved hooks (22–32) for attachment to the intestinal wall.
• Neck: short, unsegmented, the zone of budding (forms new proglottids).
• Strobila: 800–1000 proglottids of three types – (a) immature (near neck, no developed sex organs), (b) mature (middle, each with a complete set of male and female organs), (c) gravid (posterior, longer than broad; uterus with 7–13 lateral branches full of eggs/onchospheres).
• Body covered by a thick, absorptive tegument with microvilli (microtriches).

Life cycle

1. Self-fertilisation (or cross-fertilisation between proglottids). The zygote develops into a six-hooked embryo, the hexacanth (onchosphere), inside the egg.
2. Gravid proglottids break off (apolysis) and pass out with human faeces.
3. Pig eats food contaminated with human faeces. In the pig’s intestine the onchosphere hatches, bores the intestinal wall, enters blood and reaches the striated muscles (tongue, heart, shoulder).
4. In muscle it develops into a bladder worm, the cysticercus (Cysticercus cellulosae), with an invaginated scolex. Infected pork is called measly pork.
5. Man eats raw or undercooked measly pork. In the intestine the scolex evaginates, attaches to the wall and grows into an adult worm in about 2–3 months.

Note: if man swallows eggs (auto-infection or contaminated food), cysticerci form in the human brain, eyes and muscles – dangerous neurocysticercosis (can cause epilepsy). Control: cook pork well, sanitary latrines, meat inspection; drug praziquantel/niclosamide.

Parasitic adaptations

• Attachment: hooks and suckers on the scolex prevent removal by peristalsis.
• Protection: thick tegument resistant to digestive enzymes; secretion of anti-enzymes.
• Nutrition: no mouth or gut; absorbs digested food through the tegument (microtriches increase surface).
• Respiration: anaerobic (glycolysis), suitable for the oxygen-poor intestine.
• Reduced organs: no sense organs, simple nervous system, no locomotory organs.
• Reproduction: hermaphrodite, self-fertilisation possible, enormous egg production (each gravid proglottid has 30,000–50,000 eggs).
• Two hosts and resistant egg shell ensure transmission.

3.3 Aschelminthes: General characters and classification

Aschelminthes (Nemathelminthes) are the roundworms. Many modern books treat Nematoda as a separate phylum; the RTMNU syllabus uses the older name Aschelminthes.

• Free-living (soil, water) or parasitic in plants and animals.
• Body elongated, cylindrical, unsegmented, tapering at both ends; bilaterally symmetrical, triploblastic.
• Pseudocoelomate: body cavity not lined by mesoderm on the gut side (remnant of blastocoel).
• Body covered by a tough, non-living cuticle, moulted during growth.
• Body wall has only longitudinal muscles, so movement is whip-like.
• Digestive tract complete (mouth and anus), with a muscular pharynx.
• No circulatory or respiratory organs. Excretion by renette cells or an H-shaped canal system; no flame cells.
• Sexes separate (dioecious); sexual dimorphism – male smaller with a curved tail. Fertilisation internal; development direct or with larval moults.

Classification (common Indian textbook scheme)

(Some books split Nematoda into Aphasmidia and Phasmidia; give the above scheme unless your teacher follows the other.)

3.4 Ascaris: External morphology, reproductive system and life cycle

Ascaris lumbricoides is the common round worm of man, living in the small intestine. It is monogenetic (only one host). Disease: ascariasis.

External morphology

• Body long, cylindrical, pinkish-white, pointed at both ends. Female 20–40 cm, male 15–30 cm.
• Mouth at the anterior end, surrounded by three lips – one mid-dorsal and two ventro-lateral – bearing sensory papillae.
• Four longitudinal lines: lateral lines (with excretory canals), dorsal and ventral lines.
• Excretory pore mid-ventral, about 2 mm behind the mouth.
• Female: straight posterior end; female genital pore (vulva) mid-ventral at about one-third body length from the anterior end; separate anus near the tip.
• Male: posterior end curved ventrally like a hook; a cloacal aperture with two penial setae (spicules) and pre- and post-anal papillae.

Reproductive system

• Male: single, thread-like coiled tube – testis → vas deferens → wide seminal vesicle → muscular ejaculatory duct opening into the cloaca. Penial setae help in copulation. Sperms are amoeboid (no tail).
• Female: paired (Y-shaped) – each limb has ovary → oviduct → wide uterus; the two uteri unite into a short muscular vagina opening at the vulva. A female lays about 2 lakh eggs per day.

Life cycle

1. Copulation and internal fertilisation in the host intestine. Fertilised eggs have a thick, rough, mammillated albuminous coat and pass out with faeces.
2. In moist soil, in about 2–3 weeks, a rhabditiform larva develops inside the egg and moults once to become infective (second-stage larva).
3. Man swallows embryonated eggs with contaminated food, vegetables or water.
4. Larvae hatch in the small intestine, bore through the wall and enter blood → liver → heart → lungs (hepato-cardio-pulmonary migration). In the lungs they moult twice and grow.
5. Larvae climb up the trachea to the pharynx, are swallowed again, reach the intestine, moult the last time and become adults in about 8–10 weeks.

Pathogenicity: larvae in lungs cause pneumonia-like symptoms (Loeffler’s syndrome); adults cause abdominal pain, malnutrition, intestinal blockage. Control: sanitary latrines, washing vegetables and hands, no use of human faeces as manure; drugs albendazole, mebendazole.

Likely exam questions

1. (Long, 8 marks) Describe the life cycle of Taenia solium and add a note on its parasitic adaptations.
2. (Long, 8 marks) Describe the external morphology and life cycle of Ascaris lumbricoides.
3. (Short, 4 marks) Give the general characters of Platyhelminthes and classify them up to classes.
4. (Short, 4 marks) Distinguish between male and female Ascaris.
5. (Short, 4 marks) Draw a labelled diagram of a mature proglottid of Taenia solium.

Syllabus topics (RTMNU)

• 4.1 Annelida: General characters and classification up to classes.
• 4.2 Leech: Morphology, digestive and urinogenital system.
• 4.3 Annelidan larvae: Trochophore larva and its significance.
• 4.4 Economic importance of Annelids: Harmful Annelids, Beneficial Annelids, Therapeutic value of Annelids.

4.1 Annelida: General characters and classification

Annelida (L. annulus = little ring) are the segmented worms. They are the first animals with a true coelom and metameric segmentation.

• Aquatic (marine, fresh water) or terrestrial (moist soil); free-living, some ectoparasitic (leeches).
• Body elongated, soft, bilaterally symmetrical, triploblastic, with metameric segmentation – body divided externally and internally into similar segments (metameres).
• Schizocoelous true coelom, divided by septa.
• Locomotion by chitinous setae (chaetae), parapodia or suckers.
• Body covered by a thin, moist cuticle secreted by the epidermis.
• Digestive tract complete and straight.
• Closed circulatory system; blood red due to haemoglobin dissolved in plasma.
• Respiration through moist skin or gills (parapodial gills).
• Excretion by segmentally arranged nephridia.
• Nervous system: a pair of cerebral ganglia (brain), circum-pharyngeal connectives and a double, ganglionated ventral nerve cord.
• Sexes separate (polychaetes) or hermaphrodite (earthworm, leech). Development direct or with a trochophore larva.

Classification up to classes

4.2 Leech: Morphology, digestive and urinogenital system

Hirudinaria granulosa is the common Indian cattle leech, found in ponds, tanks and slow streams. It is a sanguivorous (blood-sucking) ectoparasite of cattle, fish and man.

External morphology

• Body elongated, soft, dorso-ventrally flattened, 10–15 cm (can stretch to about 30 cm); olive-green dorsally with orange-yellow stripes.
• Body has 33 segments; each segment is externally divided into rings called annuli (typically 5 per segment in the middle region).
• Anterior sucker (oral sucker) – ventral, formed by the first few segments, with mouth at its centre.
• Posterior sucker – large, ventral, formed by the last 7 segments; used for attachment and locomotion.
• Five pairs of eyes on the dorsal side of the first five segments.
• Clitellum (temporary, in breeding season) on segments 9–11.
• Male genital pore mid-ventral between 2nd and 3rd annulus of segment 10; female genital pore similarly on segment 11.
• Nephridiopores: 17 pairs, ventro-lateral, on segments 6–22.
• Anus: small, mid-dorsal at the base of the posterior sucker (segment 26).
• Sensory annulus receptors and segmental receptors on the surface.

Digestive system

• Mouth → buccal chamber with three jaws (one mid-dorsal, two ventro-lateral) arranged in a triradiate manner, each with a row of fine teeth; they make a Y-shaped (tri-radiate) wound.
• Pharynx: muscular, sucks blood; surrounded by salivary glands that secrete hirudin, an anticoagulant that prevents clotting of blood.
• Oesophagus: short.
• Crop: large, divided into 10 chambers with paired lateral caeca (diverticula); the last pair is the longest and runs far back alongside the intestine. The crop stores blood – a leech can take blood several times its weight and live for months on one meal.
• Stomach (small, with a sphincter) → intestine (straight tube; digestion and absorption) → rectum → anus.
• Digestion is slow; it is aided by symbiotic bacteria in the gut.

Excretory system

17 pairs of nephridia in segments 6–22. Each nephridium has a main lobe, an initial lobe, a ciliated organ (in the testis-sac region it lies in the coelomic sinus), a vesicle and a duct opening at a nephridiopore. They remove nitrogenous waste (mainly ammonia) and help in osmoregulation.

Reproductive system (hermaphrodite, protandrous)

• Male: 11 pairs of testis sacs (segments 12–22) → vasa efferentia → paired vas deferens → coiled epididymis → ejaculatory duct → atrium with prostate glands and protrusible penis; opens at the male pore (segment 10). Sperms are passed in spermatophores.
• Female: a pair of ovisacs with thread-like ovaries (segment 11) → oviducts → common oviduct (with albumin glands) → vagina → female pore (segment 11).
• Cross-fertilisation; fertilised eggs are laid in a cocoon secreted by the clitellum, kept in moist soil. Development is direct.

4.3 Annelidan larvae: Trochophore larva and its significance

The trochophore (Gr. trochos = wheel, phoros = bearing) is the free-swimming, ciliated larva of marine polychaetes (e.g. Nereis, Polygordius). A similar larva occurs in many molluscs.

Structure

• Small (about 1 mm), transparent, top-shaped (two cones joined at their bases).
• Apical plate at the upper pole with a tuft of long cilia and a pair of eyes; acts as the nerve centre.
• Prototroch: pre-oral girdle of cilia just above the mouth – main locomotory band.
• Metatroch: post-oral ring of cilia; telotroch: ring around the anus.
• Digestive tract: curved, complete – mouth, oesophagus, stomach, intestine and anus.
• A pair of protonephridia (with solenocytes) for excretion; blastocoel acts as body cavity; mesoderm bands from a pair of mesoblast (teloblast) cells near the anus.

Metamorphosis: the region between mouth and anus elongates; the mesoderm bands form segments one after another, setae and parapodia develop, the ciliary bands disappear and the larva becomes a young worm.

Significance

• Helps dispersal of sedentary or slow adults.
• Occurs in Annelida and Mollusca (also similar larvae in some other groups) – shows their common ancestry; Annelida and Mollusca are both Trochozoa (protostomes with spiral cleavage).
• Its resemblance to the rotifer body led to the trochophore theory – that a trochophore-like animal was the ancestor of these groups.
• Some workers compare it with the tornaria larva of Balanoglossus, but tornaria is a deuterostome larva and the resemblance is superficial.

4.4 Economic importance of Annelids

Beneficial annelids

• Soil fertility: earthworms (“farmer’s friend”, Darwin) burrow, aerate and mix soil, increase water holding; their castings are rich in N, P and K.
• Vermicomposting: Eisenia fetida, Eudrilus eugeniae, Perionyx excavatus convert organic waste into vermicompost and vermiwash (organic manure).
• Food and bait: earthworms and polychaetes are used as fish bait and fish/poultry feed; the palolo worm (Eunice viridis) is eaten in Samoa and Fiji.
• Research: earthworm and leech are used in teaching and neurobiology research.
• Pollution indicators: Tubifex indicates organic pollution of water.

Harmful annelids

• Leeches suck blood of cattle, man and fish, cause blood loss and secondary infection; aquatic leeches may enter the nose, pharynx or vagina of cattle and man.
• Earthworms may damage young seedlings and lawns, and some act as intermediate hosts of parasites of poultry and pigs.
• Some boring polychaetes (e.g. Polydora) damage oyster shells in oyster farms.

Therapeutic value of annelids

• Leech therapy (hirudotherapy): medicinal leech Hirudo medicinalis is used after plastic and reconstructive surgery and re-attachment of fingers/ears to remove pooled blood and restore circulation. Ayurveda calls it Jalaukavacharana.
• Hirudin: a powerful anticoagulant from leech saliva; recombinant hirudin (lepirudin) is used to prevent blood clots. Leech saliva also contains anaesthetic and vasodilator substances.
• Earthworm extracts: lumbrokinase (fibrinolytic enzyme) from earthworms is studied for dissolving blood clots; earthworms are used in traditional medicine for fever and joint pain.

Likely exam questions

1. (Long, 8 marks) Describe the digestive system of leech with a labelled diagram. Add a note on its feeding.
2. (Long, 8 marks) Give the general characters of Annelida and classify it up to classes with examples.
3. (Short, 4 marks) Describe the trochophore larva and give its significance.
4. (Short, 4 marks) Describe the urinogenital (reproductive and excretory) system of leech.
5. (Short, 4 marks) Write a note on the therapeutic value of annelids.