Chapter 33: An Introduction to Invertebrates – Study Notes

Introduction to Invertebrates

Invertebrates are animals that lack a backbone and represent over 95% of known animal species. They are morphologically diverse and inhabit nearly every environment on Earth. Understanding their diversity requires examining their evolutionary relationships and major body plans.

Major Animal Clades

Overview of Animal Phylogeny

Animal diversity is organized into several major clades based on evolutionary relationships. The main groups include Porifera, Cnidaria, Lophotrochozoa, Ecdysozoa, and Deuterostomia. Eumetazoa is a clade that includes all animals with true tissues except sponges.

Phylum Platyhelminthes: Flatworms

Tapeworms

Tapeworms are parasitic flatworms that lack a digestive system and absorb nutrients directly from their host's intestine. Their body consists of a scolex (with hooks and suckers for attachment) and a series of proglottids containing reproductive organs. Tapeworms reproduce by releasing eggs in the host's feces, which can infect new hosts through contaminated food or water.

• Scolex: The anterior end with hooks and suckers for attachment.

• Proglottids: Repetitive segments containing reproductive structures.

• Transmission: Eggs are released in feces and ingested by intermediate hosts (e.g., pigs, cattle), then infect humans who eat undercooked meat.

Phylum Syndermata: Rotifers and Acanthocephalans

Phylum Ectoprocta: Bryozoans (Ectoprocts)

Ectoprocts, also known as bryozoans, are sessile colonial animals that superficially resemble moss. They possess a lophophore, a crown of ciliated tentacles used for feeding. Most species are marine and some contribute to reef building.

• Lophophore: Specialized feeding structure with ciliated tentacles.

• Colony Structure: Many species form colonies encased in a hard exoskeleton.

Phylum Mollusca: Bivalves

Bivalves

Bivalves are aquatic molluscs including clams, oysters, mussels, and scallops. They have a shell divided into two hinged halves and lack a distinct head or radula. Bivalves are mostly sedentary filter feeders, using gills for both feeding and gas exchange.

• Shell: Two hinged halves joined by adductor muscles.

• Feeding: Water enters through an incurrent siphon, passes over gills for filter feeding, and exits through an excurrent siphon.

• Mobility: Most are sedentary, but some (e.g., scallops) can move by clapping their shells.

Phylum Arthropoda

Myriapods: Millipedes and Centipedes

Myriapods are terrestrial arthropods that include millipedes and centipedes. They are characterized by elongated bodies with many segments and numerous legs.

• Millipedes: Herbivorous, two pairs of legs per segment, feed on decaying plant matter.

• Centipedes: Carnivorous, one pair of legs per segment, possess poison claws for capturing prey.

Crustaceans: Isopods

Isopods are a diverse group of crustaceans that include both aquatic and terrestrial species. Pill bugs are common terrestrial isopods found under moist logs and leaves. They have a segmented exoskeleton and play important roles in decomposition.

• Habitat: Terrestrial, freshwater, and marine environments.

• Role: Decomposers, breaking down organic matter.

Summary Table: Major Invertebrate Clades

Clade	Key Features	Examples
Porifera	No true tissues, filter feeders	Sponges
Cnidaria	Diploblastic, radial symmetry, stinging cells	Jellyfish, corals, hydras
Lophotrochozoa	Bilateral symmetry, diverse body plans	Flatworms, molluscs, annelids
Ecdysozoa	Molting of cuticle (ecdysis)	Nematodes, arthropods
Deuterostomia	Deuterostome development	Echinoderms, chordates