BIO 121 — Diversity
of Invertebrates

1.1 What Is an Invertebrate?

The term invertebrate describes any animal that lacks a vertebral column (backbone). Unlike the phylum Chordata's vertebrate subphylum, "Invertebrata" is not a formal taxonomic group — it is a convenient umbrella term embracing roughly 35 animal phyla as diverse as a jellyfish, a tapeworm, a beetle, and a squid.

Invertebrates account for about 97% of all described animal species and occupy virtually every habitat on Earth: oceans, fresh water, soil, air, the bodies of other organisms, and even deep hydrothermal vents.

1.2 Why Study Invertebrates?

1.3 Phyla Covered in BIO 121

*Protozoa today are placed in the kingdom Protista under modern classification, but they are traditionally surveyed in zoology as the simplest "animal-like" eukaryotes.

1.4 Body-Plan Innovations

Key features used to group invertebrates include symmetry, number of germ layers, and presence/absence of a body cavity (coelom).

1.5 A Brief Tour Through This Course

2.1 What Are Protozoa?

The word Protozoa comes from the Greek protos (first) + zōon (animal) — "first animals". They are unicellular eukaryotic organisms that display animal-like characteristics such as heterotrophic nutrition and active locomotion.

Although traditionally placed within the animal kingdom as Phylum Protozoa, modern taxonomy assigns them to the kingdom Protista (or splits them across several protist phyla). For BIO 121 we retain the classical four-class scheme.

2.2 General Characteristics

2.3 The Four Traditional Classes

2.4 Modes of Nutrition

2.5 Reproduction

2.6 Economic and Medical Importance

3.1 Class Sarcodina — Characteristics

3.2 Amoeba proteus — Structure

Main cell components

3.3 Mode of Life

Amoeba proteus inhabits the bottom of freshwater ponds, ditches, and slow streams, feeding on diatoms, bacteria and other protozoans. It creeps along submerged surfaces by a sol-gel transformation of cytoplasm.

Locomotion — Amoeboid Movement

The fluid endoplasm streams forward into a newly-forming pseudopodium; on reaching the tip it converts to stiffer ectoplasm (a gel). At the rear, ectoplasm liquefies back to sol and is drawn forward. This cyclic sol ↔ gel transformation is controlled by the actin–myosin cytoskeleton.

Nutrition — Phagocytosis ("cell-eating")

1. Pseudopodia flow around a food particle.
2. The particle is enclosed in a food vacuole.
3. Lysosomal enzymes digest it intracellularly.
4. Soluble products diffuse into cytoplasm; residue is egested at the rear.

Respiration & Excretion

Gaseous exchange (O₂ in, CO₂ out) occurs by simple diffusion across the plasma membrane — the high surface-area-to-volume ratio of the small cell makes this sufficient.

Osmoregulation

Fresh water constantly enters by osmosis. The contractile vacuole accumulates this excess water and periodically contracts to expel it to the exterior, preventing cell lysis.

3.4 Life Cycle and Reproduction

3.5 Economic / Medical Importance

4.1 Class Mastigophora — Characteristics

4.2 Euglena viridis — Structure

4.3 Mode of Life

Habitat

Euglena viridis is abundant in still, sunlit freshwater ponds and ditches rich in organic matter. Dense populations turn the water green — a "bloom".

Locomotion

The anterior flagellum beats with a lashing motion creating a propeller-like effective stroke. The whole cell rotates about its long axis as it advances. An auxiliary stouter flagellum anchors the reservoir. When disturbed, Euglena also shows euglenoid movement — peristaltic body contractions.

Nutrition — Mixotrophy

Photoreception

The stigma (eyespot) is a red pigment patch that shades a light-sensitive swelling at the base of the flagellum. Together they act as a simple photoreceptor — causing the cell to swim toward optimum light intensity (positive phototaxis).

4.4 Life Cycle and Reproduction

4.5 Other Important Mastigophorans

5.1 Class Sporozoa — Characteristics

5.2 Species Causing Human Malaria

Vector: female Anopheles mosquito (only the female bites — it needs a blood meal to develop eggs).

5.3 Life Cycle of Plasmodium

The parasite lives in two hosts: humans (intermediate host — asexual multiplication) and the female Anopheles mosquito (definitive host — sexual reproduction).

In the Human Host — Asexual (Schizogony)

1. Sporozoites injected with mosquito saliva reach the liver in ~30 min.
2. Exo-erythrocytic schizogony — each sporozoite multiplies inside a liver cell producing thousands of merozoites.
3. Merozoites burst out, invade red blood cells (RBCs).
4. Erythrocytic schizogony — inside the RBC, the parasite passes through ring → trophozoite → schizont stages, producing more merozoites.
5. Synchronised rupture of RBCs releases merozoites + toxins — causing the characteristic fever, chills, sweating cycle (every 48 or 72 h).
6. Some merozoites differentiate into male & female gametocytes, waiting to be taken up by another mosquito.

In the Mosquito — Sexual (Gametogony → Sporogony)

1. Gametocytes sucked up by a biting Anopheles mature in her gut: micro- (♂) and macro- (♀) gametes.
2. Fertilisation yields a motile ookinete that burrows through the gut wall.
3. On the gut wall it becomes an oocyst; inside, sporogony produces thousands of sporozoites.
4. Sporozoites migrate to the salivary glands — ready to infect the next human bitten.

5.4 Pathology, Prevention & Control

6.1 Class Ciliata — Characteristics

6.2 Paramecium caudatum — Structure

6.3 Mode of Life

Habitat

Common in freshwater ponds, ditches and hay infusions rich in decaying organic matter and bacteria.

Locomotion

Thousands of cilia beat in coordinated metachronal waves, rotating the body about its long axis while propelling it forward. An encounter with an obstacle triggers the avoiding reaction — cilia reverse direction, the animal backs off, turns, and tries a new path.

Nutrition

1. Cilia of the oral groove waft bacteria & debris into the cytostome.
2. A food vacuole forms at the end of the cytopharynx and detaches.
3. Vacuoles circulate through cytoplasm (cyclosis) while lysosomal enzymes digest their contents.
4. Indigestible residue is expelled through a fixed cytopyge (cell-anus).

Osmoregulation

Two contractile vacuoles (one anterior, one posterior), each with radiating canals, alternately fill and expel excess water.

6.4 Reproduction

6.5 Importance of Paramecium

7.1 General Characteristics

7.2 Body Plan & Cell Types

Major cell types

7.3 Canal Systems

7.4 Reproduction

Asexual

• Budding — small outgrowth detaches to form new sponge.
• Fragmentation — a broken piece develops into a whole animal (basis of sponge farming).
• Gemmules — dormant, resistant internal buds formed in freshwater sponges before winter; germinate in spring.

Sexual

1. Most are hermaphrodite but rarely self-fertilise.
2. Sperm released into water are drawn in by neighbour sponges and captured by choanocytes.
3. An amoebocyte carries sperm to an egg in the mesohyl — internal fertilisation.
4. A free-swimming amphiblastula or parenchymula larva develops, settles, and metamorphoses.

7.5 Economic Importance

8.1 General Characteristics

8.2 Major Classes

8.3 Hydra — the Freshwater Polyp

Mode of Life

Attaches by its basal disc to aquatic plants in ponds. Captures crustaceans, insect larvae and small worms that blunder into its tentacles. Cnidocytes on the tentacles discharge nematocysts — coiled thread-tubes that either penetrate the prey and inject toxin (penetrant) or wrap around it (volvent).

Life Cycle

8.4 Obelia — a Colonial Marine Hydroid

Obelia demonstrates textbook alternation of generations — a polyp colony that reproduces asexually to release free-swimming medusae, which reproduce sexually.

8.5 Economic Importance

• Coral reefs — built by colonial anthozoans, support 25% of marine biodiversity and attract multi-billion-dollar tourism.
• Reef stones used as building material and in the lime industry.
• Red coral (Corallium) and precious coral — jewellery.
• Dangerous medusae — box jellyfish, Portuguese man-of-war — cause severe stings and occasional fatalities.
• Research model Hydra — regeneration, stem-cell biology, senescence studies.

9.1 General Characteristics

9.2 The Four Classes

*"Aphasmidia" is the term used in the BIO 121 syllabus; modern texts use Monogenea.

9.3 Class Turbellaria — Planaria

Free-living flatworm of clean streams and ponds. Arrow-shaped head with two pigmented eyespots (ocelli) and lateral auricles (chemoreceptors). Glides on a ciliated ventral surface over a mucus trail.

Feeding

Carnivorous. Protrudes a muscular pharynx from mid-ventral surface, sucks out tissue juices of prey into a highly branched gastrovascular cavity. Undigested remains ejected through the same mouth.

Regeneration

Cut into several pieces, each fragment regenerates a complete worm — a classic demonstration of neoblast (adult stem cell) biology.

9.4 Class Trematoda — Fasciola hepatica (Liver Fluke)

Endoparasite of the bile ducts of sheep, cattle (definitive host) and occasionally humans. Causes fascioliasis (liver rot) — major veterinary disease.

9.5 Class Cestoda — Taenia solium (Pork Tapeworm)

Human intestinal parasite; pig is the intermediate host (cysticercus stage in pork muscle).

9.6 Economic / Medical Importance

10.1 General Characteristics

10.2 Ascaris lumbricoides — the Giant Human Roundworm

Long cylindrical worm (♂ ~15 cm, ♀ ~30 cm). Inhabits the small intestine of humans, where it swims against the flow of chyme. Infection — ascariasis — affects approximately 800 million people, mostly children.

10.3 Life Cycle of Ascaris

10.4 Economic Importance of Nematodes

11.1 General Characteristics

11.2 The Three Classes

11.3 Class Oligochaeta — the Earthworm

Mode of Life

Burrows in damp soil, feeding by swallowing soil and organic debris. Peristaltic contractions of circular & longitudinal muscles, anchored by chaetae, drive locomotion. Respires through the moist, vascularised skin.

Reproduction

Earthworms are hermaphrodite but cross-fertilise. During copulation, two worms align head-to-tail with clitella exchanging sperm. The clitellum later secretes a cocoon that slides off the anterior end collecting eggs and sperm; fertilised eggs develop inside the cocoon and hatch as miniature worms.

11.4 Class Polychaeta — Bristleworms

Marine, free-swimming or burrowing. Each segment bears a pair of lateral parapodia carrying many bristles — used for locomotion & gas exchange.

• Nereis (ragworm / sandworm) — active predator on sandy / muddy shores; common bait for sport fishing.
• Arenicola (lugworm) — lives in a U-shaped burrow in estuarine mud; valuable prey for fish and shorebirds.
• Tube-builders — Sabella (feather duster worm) — sessile, secrete leathery / calcareous tubes; filter-feed with plumose tentacles.

11.5 Class Hirudinea — Leeches

Dorso-ventrally flattened, with a fixed number of segments and two suckers (anterior + posterior). Ectoparasitic on vertebrates, feeding on blood.

Their saliva contains hirudin, a powerful anticoagulant which prevents the host's blood from clotting. Hirudin and related molecules are now produced as pharmaceutical anticoagulants.

11.6 Economic Importance of Annelids

12.1 General Characteristics of Phylum Arthropoda

12.2 Classes of Arthropoda at a Glance

Myriapoda is sometimes used as an umbrella class containing Chilopoda and Diplopoda; BIO 121 treats them as separate classes.

12.3 Class Crustacea — the Crayfish / Prawn

Mode of life

• Mostly aquatic (marine or fresh water); breathe by gills.
• Scavengers and predators — use chelipeds to grasp food, pass it forward through maxillipeds to the mouth.
• Locomotion — walking on 4 pairs of thoracic legs; can escape backwards by rapid flexion of the abdomen (caridoid escape reaction).

Examples: prawn, shrimp, crab, lobster, crayfish, barnacle, Daphnia (water flea), woodlouse.

12.4 Class Arachnida — Spiders & Scorpions

Key arachnid features

• Four pairs of walking legs — the easiest way to tell a spider from an insect.
• Body divided into cephalothorax (prosoma) and abdomen (opisthosoma).
• First pair of appendages: chelicerae; second pair: pedipalps.
• No antennae; respire with book lungs or tracheae.

12.5 Myriapoda — Chilopoda vs. Diplopoda

13.1 Characteristics of Class Insecta

13.2 Metamorphosis

13.3 The Four Focus Orders

① Order Diptera — the True Flies (housefly, mosquito, tsetse)

• One pair of membranous fore-wings; hind-wings reduced to club-like halteres for balance.
• Mouthparts sucking / sponging / piercing; adults liquid-feeders.
• Life history holometabolous: egg → larva (maggot) → pupa → adult.
• Medical significance — vectors of malaria (Anopheles), yellow fever (Aedes), sleeping sickness (Glossina), onchocerciasis (Simulium); house fly mechanically transmits gut pathogens.

② Order Lepidoptera — Butterflies & Moths

• Two pairs of membranous wings covered with overlapping scales (hence "Lepidos" = scale).
• Mouthparts a coiled siphoning proboscis — sips nectar.
• Holometabolous — larvae are caterpillars; pupa = chrysalis (butterfly) or cocoon (moth).
• Importance — pollinators; caterpillars can be agricultural pests (armyworm, cotton bollworm). Silkworm (Bombyx mori) provides commercial silk.

③ Order Hemiptera — "True Bugs" (aphids, bedbugs, cicadas)

• Piercing-sucking rostrum / beak held under head.
• Fore-wings often hardened at base, membranous at tip (hemelytra).
• Hemimetabolous (nymphs resemble adults).
• Importance — many are crop pests (aphids, stink bugs, scale insects); some are disease vectors (Triatoma → Chagas disease). Cochineal scale insects yield red dye.

④ Order Hymenoptera — Bees, Wasps & Ants

• Two pairs of membranous wings linked by tiny hooks (hamuli).
• Chewing-lapping mouthparts; females often bear a sting (a modified ovipositor).
• Holometabolous; many are social with strict castes (queen, workers, drones).
• Importance — pollinators (honey bee = most economically valuable insect); biological-control parasitoids; ants are major ecosystem engineers. Honey, beeswax, royal jelly — major agricultural products.

13.4 Comparative Summary Table

14.1 General Characteristics

14.2 Classes of Mollusca

14.3 The Generalised Molluscan Body Plan

14.4 A Closer Look at Each Class

① Gastropoda — Snails & Slugs

The largest molluscan class. Characterised by torsion — a 180° twisting of the visceral mass that places the anus above the head during development. Giant African snail (Achatina fulva) is a significant agricultural pest in Nigeria.

② Bivalvia — Clams, Oysters, Mussels

Two lateral shells hinged dorsally. No head; no radula. Filter-feed using enlarged ctenidia that trap particles and convey them to the labial palps. Anodonta (freshwater mussel) uses a specialised glochidium larva that parasitises fish gills briefly before dropping off.

③ Cephalopoda — Squid, Octopus, Cuttlefish

Most advanced invertebrates — closed circulatory system, camera-type eyes comparable to ours, complex brain, and remarkable learning ability. Foot modified into arms/tentacles bearing suckers. Jet propulsion by forcefully expelling water through the siphon.

14.5 Economic Importance

15.1 General Characteristics

15.2 The Five Classes

15.3 The Water Vascular System

15.4 The Invertebrate–Chordate Link

Despite their pentaradial adult form, echinoderms share deep developmental features with chordates, placing both in the super-phylum Deuterostomia:

Moreover, phylum Hemichordata (acorn worms) bridges the two groups with:

• A larva (tornaria) almost indistinguishable from an echinoderm larva.
• Pharyngeal gill slits — a defining chordate character.
• A primitive dorsal nerve cord.

15.5 Course Synthesis — Comparing All 10 Phyla

15.6 Final Revision Checklist