Characteristics of Invertebrates with examples

Habitat

• They are found in seas, freshwater, air, land from snow to desert.
• 80% are found in terrestrial habitat.
• Most successful invertebrates of the lands are Arthropods.
• Protozoans are free-living, parasites or commensals.
• Sponges and Coelenterates are aquatic animals.

Numerical Strength

• Among 1.25 million animal species, 95% (1.2 million) are invertebrates.
• Among 1.2 million invertebrates, 1 million are Arthropods.

Shape

• Varied shape.
• Amoeba are irregular every changing bodies.
• Sponges and Coelenterates are plant-like.
• Flatworms are leaf-like and ribbon-shaped.
• Annelids, Nemerteans, and Nematodes are vermiform.
• Starfishes are star-shaped.

Size

• Great variation in size.
• Ranges from microscopic protozoans to large-sized cephalopods.
• Malaria parasites (Plasmodium) are the smallest one (one-fifth of the human RBC) while the largest one is giant squids, Architeuthis with a body length of 16.5 meters including tentacles.

Symmetry

• All types of symmetry.
• Protozoans are bilateral or radial or asymmetrical.
• Sponges are asymmetrical or radially symmetrical.
• Coelenterates are radially symmetrical.
• Ctenophores are biradial symmetry.
• Some are spherical symmetry (Heliozoan and Radiolaria).

Grade of Organisation

• All grades of the organization.
• Protoplasmic grade- Protozoa.
• Cellular grade- Sponges.
• Cell-tissue grade- Coelenterates.
• Tissue-organ grade- Flatworms.

Germ Layers

• Absent in Protozoans.
• Some are Diploblastic (derived from 2 germ layers), while others are triploblastic (3 germ layers).
• Diploblastic- Sponges, Coelenterates.
• Triploblastic- Other Invertebrates than Sponges & Coelenterates.

Simple Integument

• Body covering is simple.
• Protozoa- Plasma membrane
• Other posses an outer protective layer called the epidermis.
• Some have non-cellular cuticle or chitinous covering secreted by the epidermis.

Locomotion

• Sessile- Sponges, Corals.
• Pseudopodia, Cilia, Flagella- Protozoans.
• Tentacular movements- Coelenterates, Molluscs.
• Setae, Parapodia, Suckers- Annelids.
• Jointed Legs- Arthropods.
• Arms- Echinoderms.

Segmentation

• Flatworms exhibit pseudo-segmentation.
• True segmentation is found in Annelida and Arthropoda.

Living Endoskeleton

• Do not possess a rigid internal skeleton.
• Some like arthropods and molluscs possess hard exoskeleton for supporting and protecting the body.

Coelom

• Sponges and Coelenterates- Body is a double-layered sac surrounding a single cavity (Acoelomate- No Coelom).
• Pseudocoelom- possess a cavity in between body wall and the gut (Nematodes).
• Some possess true coelom.

Dorsal gut

• The alimentary canal is either absent or partially formed or complete.
• If present, lies dorsal to the nerve cord, runs anterior terminal mouth up to the posterior terminal anus.
• Gill-slits are never formed in the pharyngeal wall.

Digestive System

• Digestion takes place within the cell (intracellular digestion)- Protozoans, Sponges.
• Digestion also takes place outside the cell (extracellular digestion).
• Coelenterates exhibits both intracellular and extracellular digestion.

Circulatory System

• A blood vascular system is well developed.
• Open or lacunar circulatory system- Arthropods, Molluscs.
• Closed circulatory system are also present.
• The heart is dorsal to the gut.
• The hepatic portal system is absent.

Respiratory System

• Protozoans, Sponges, Coelenterates and many worms have a direct diffusion of gases.
• Annelids exchange gases through moist skin.
• Gills are present in higher invertebrates.
• Echinoderms possess branchiae and tube feet for respiration.
• In insects, the tracheal system is adapted for aerial respiration.

Excretory Mechanisms

• Direct diffusion through cell membranes- Protozoans, Sponges, Coelenterates.
• Flame cells- Flatworms.
• True nephridia- Annelids and Molluscs.
• Malpighian tubules- Insects.
• Amoeboid Cells- Echinoderms.

Nervous System

• Coelenterates (radially symmetry)- Head is absent, CNS is represented by a ring of nerve-tissue encircling the body.
• In bilaterally symmetrical invertebrates- CNS is represented by a pair of nerve cord running along the mid-ventral line of the body.
• In higher invertebrates- head ganglia form the brain.
• Solid nerves, not hollow within.

Sense Organs

• Protozoans- protoplast acts as a receptor.
• Flagellates- stigma or eyespot acts as a photoreceptor.
• Coelenterates- long sensory cells.
• Flatworms- Eyespot, chemoreceptors.
• Annelids- simple eyes.
• Arthropods- compound eyes.
• Arthropods and Molluscs- Statocyst (equilibrium), Tactile receptors, Chemoreceptors.

Reproduction

• Asexual binary fission.
• Sexual reproduction- Coelenterates, Platyhelminthes, Annelids, Crustaceans.
• Fertilization may be internal as well as external.
• Development is direct or indirect.
• Parthenogenesis- Rotifers, Bees.

Cold-blooded animals

• All invertebrates are cold-blooded.

References

1. Kotpal RL. 2017. Modern Text Book of Zoology- Invertebrates. 11th Edition. Rastogi Publications.
2. Jordan EL and Verma PS. 2018. Invertebrate Zoology. 14th Edition. S Chand Publishing.