Invertebrates

Invertebrates are animals without a backbone; they make up more than 95% of animal species (an approximate share). They range from microscopic protozoa to giant squid. Studying invertebrates from the simplest phyla to the most complex reveals the main steps of animal evolution: multicellularity, germ layers, symmetry, a body cavity, and circulatory and nervous systems. Exams often ask you to compare the features of phyla, describe parasite life cycles and say “in which group an organ first appeared”.

§1. Single-celled animals (protozoa)

The body is one cell, but that cell carries out every function of the organism: movement, feeding, respiration, excretion, reproduction and response to stimuli. They reproduce mainly by binary fission.

• Amoeba — lives in fresh water; moves and engulfs food with pseudopodia (phagocytosis); digestion takes place in a food vacuole; the contractile vacuole removes excess water and wastes; in bad conditions it forms a thick-walled cyst.
• Euglena — moves with a flagellum and has chromatophores (chlorophyll) and a light-sensitive red eyespot. In light it photosynthesises (autotroph); in darkness it absorbs organic matter from the water (heterotroph) — this is mixotrophic nutrition.
• Paramecium — covered with cilia and has a fixed shape; it has two nuclei (the macronucleus controls everyday life, the micronucleus takes part in the sexual process), a cell mouth and gullet, and two contractile vacuoles. Its sexual process is conjugation.
• The malaria parasite (Plasmodium) — passed to humans by the bite of a female Anopheles mosquito; it multiplies in liver cells and then in red blood cells, destroying them — causing attacks of fever.

The dysentery amoeba and Giardia are also human parasites.

§2. Cnidarians (coelenterates)

Examples: Hydra, jellyfish, sea anemones, coral polyps.

• The body has two layers of cells: the outer ectoderm and the inner endoderm, with a non-cellular layer between them.
• Radial symmetry — several planes of symmetry pass through the body; this suits a sessile or drifting life.
• The gut cavity opens to the outside by one opening — the mouth — through which undigested remains also leave.
• The ectoderm has stinging cells for paralysing prey and for defence.
• A diffuse (net-like) nervous system — nerve cells appear for the first time.
• Hydra reproduces asexually by budding and sexually in autumn; it has great powers of regeneration (thanks to interstitial cells).

Coral polyps build coral reefs and islands in tropical seas.

§3. Worms

Flatworms (planarian, liver fluke, pork and beef tapeworms, Echinococcus):

• three-layered — the middle layer, mesoderm, appears for the first time; muscles and other organs develop from it;
• bilateral symmetry, with distinct front and back ends;
• no body cavity — the space between organs is filled with parenchyma;
• excretory organs are protonephridia; the gut is blind (no anus), and tapeworms have no gut at all — they absorb food over the whole body surface;
• most are hermaphrodites.

Parasites change hosts: the liver fluke’s main hosts are cattle and sheep, its intermediate host a small pond snail; the beef tapeworm’s intermediate host is cattle and its main host is humans (infection via undercooked meat).

Roundworms (human roundworm Ascaris, pinworm, guinea worm):

• a primary body cavity (fluid-filled) appears for the first time;
• the gut is a tube: a hind gut and anus appear for the first time, so food moves one way;
• separate sexes; the body is covered with cuticle.

Ascaris eggs are swallowed with unwashed vegetables and from dirty hands; the larva travels in the blood to the lungs and then returns to the intestine.

Annelids (segmented worms) (earthworm, leech, Nereis):

• a secondary body cavity (coelom) appears for the first time;
• the body is divided into segments;
• a closed circulatory system appears for the first time (dorsal and ventral vessels, “hearts” — ring vessels);
• the nervous system is a cerebral ganglion and a ventral nerve cord; excretion by metanephridia (a pair in each segment).

The earthworm breathes through its skin and is a hermaphrodite but cross-fertilises; it loosens soil, enriches it with humus and raises fertility (C. Darwin wrote about this). The medicinal leech releases hirudin, which stops blood clotting, while it feeds.

§4. Molluscs

• The body is not segmented: a head, a visceral mass and a muscular foot.
• A fold of skin — the mantle — surrounds the body and secretes a chalky shell.
• The circulatory system is open, with a heart; the body cavity is reduced.

Classes:

• Gastropods (slugs, Roman snail, great pond snail) — one spirally coiled shell (slugs have none); land species breathe with a lung; a rasping tongue — the radula.
• Bivalves (freshwater mussel, mussel, oyster) — a shell of two valves, no head; they breathe with gills and feed by filtering water, cleaning water bodies; some make pearls.
• Cephalopods (squid, octopus, cuttlefish) — the foot has become tentacles, the shell is internal or absent, there is an ink sac, and they move by jet propulsion; the brain and eyes are highly developed, and circulation is almost closed.

§5. Arthropods

The largest phylum (most of all animal species).

• The body is covered with a chitinous exoskeleton (cuticle); it cannot grow, so the animal grows by moulting.
• The limbs are jointed; the body is divided into regions.
• The body cavity is mixed, the circulation is open, the heart is dorsal.
• The nervous system is a cerebral ganglion and a ventral nerve cord; compound eyes.

Ticks spread dangerous diseases (such as encephalitis); the black widow spider and scorpions are venomous.

§6. Insects and the evolution of invertebrates

Insect development:

• complete metamorphosis: egg → larva → pupa → adult (butterflies, beetles, flies, bees, ants). The larva is quite unlike the adult (caterpillar and butterfly);
• incomplete metamorphosis: egg → nymph → adult, with no pupa; the nymph resembles the adult (locust, dragonfly, bug, cockroach).

Useful insects: honeybee (honey, wax, pollination), silkworm (silk — raised in Uzbekistan since ancient times), ladybird (eats aphids). Pests: locusts, cotton bollworm, Colorado beetle; disease carriers: Anopheles mosquito, housefly.

Worked example

In which group did these first appear: 1) mesoderm; 2) hind gut; 3) circulatory system?

1. Mesoderm (three layers) — in flatworms.
2. Hind gut and anus — in roundworms.
3. A (closed) circulatory system — in annelids.
4. Answer: 1 — flatworms, 2 — roundworms, 3 — annelids.

Worked example

An animal has 8 walking legs, no antennae, and a body of cephalothorax and abdomen. Which class does it belong to?

1. The limbs are jointed — phylum Arthropoda.
2. 8 legs = 4 pairs; no antennae.
3. Two body regions — cephalothorax and abdomen.
4. Answer: class Arachnida (for example, a spider or a tick).

Worked example

Why must meat be cooked thoroughly to avoid beef tapeworm?

1. The worm’s larvae (cysts) live in cattle muscle — cattle are the intermediate host.
2. When a person eats raw or undercooked meat, the cysts reach the intestine.
3. In the intestine they grow into adult worms — humans are the main host.
4. Answer: thorough cooking kills the cysts and breaks the chain of infection.

Check yourself

1. Which organism shows mixotrophic nutrition?

2. What kind of nervous system does Hydra have?

3. What is the intermediate host of the liver fluke?

4. How many walking legs does a spider have?

5. Give an example of an insect with incomplete metamorphosis.

6. What kind of circulatory system do molluscs have?

7. In which group did a closed circulatory system first appear?

8. Which insect passes the malaria parasite to humans?

Common mistakes

• Calling a spider an insect — it is an arachnid.
• Writing that a circulatory system first appeared in molluscs — it appeared in annelids.
• Calling cnidarians three-layered — they have two layers.
• Mixing up complete and incomplete metamorphosis — only complete metamorphosis has a pupa.
• Writing that bivalve molluscs have a head.
• Confusing main and intermediate hosts — the adult parasite lives in the main host.