LSN35: Animal Diversity II: Invertebrates

Animal Diversity II: Invertebrates

Overview

This section explores the diversity, classification, and evolutionary adaptations of invertebrate animals, focusing on the three largest lophotrochozoan phyla, the major arthropod groups, and the characteristics of metamorphosis in insects.

Major Lophotrochozoan Phyla

Key Characteristics

• Phylum Platyhelminthes (Flatworms): Bilaterally symmetrical, acoelomate, unsegmented; includes planarians, tapeworms, and flukes.

• Phylum Mollusca (Mollusks): Soft-bodied, usually with a hard shell; includes snails, clams, squids, and octopuses.

• Phylum Annelida (Segmented Worms): Segmented bodies, true coelom, includes earthworms, leeches, and polychaetes.

Evolutionary Adaptations: These phyla exhibit adaptations such as cephalization, specialized organ systems, and diverse reproductive strategies.

Ecdysozoa: Nematodes and Arthropods

Common Characteristics

• Nematoda (Roundworms): Unsegmented, pseudocoelomate, covered by a tough cuticle, undergo ecdysis (molting).

• Arthropoda: Segmented bodies, jointed appendages, exoskeleton made of chitin, undergo ecdysis.

Evolutionary Significance: The development of the exoskeleton and jointed appendages in arthropods allowed for greater mobility and protection, contributing to their evolutionary success.

Major Arthropod Groups

Key Groups and Features

• Chelicerates: Spiders, scorpions, ticks; possess chelicerae (fang-like mouthparts).

• Myriapods: Centipedes and millipedes; elongated bodies with many segments.

• Pancrustaceans: Crustaceans (crabs, lobsters, shrimp) and insects; diverse feeding and locomotion adaptations.

Insect Metamorphosis

Types of Metamorphosis

• Complete Metamorphosis: Four distinct stages: egg, larva, pupa, adult. Example: butterflies.

• Incomplete Metamorphosis: Three stages: egg, nymph, adult. Nymphs resemble small adults. Example: grasshoppers.

Significance: Metamorphosis allows insects to occupy different ecological niches during their life cycle, reducing competition between life stages.

Key Terms

Term	Definition
Planarians	Free-living flatworms, often used in regeneration studies.
Alimentary canal	Digestive tract with two openings (mouth and anus).
Pharynx	Muscular tube for food intake in some invertebrates.
Flame bulb	Excretory structure in flatworms for osmoregulation.
Parasitic	Organisms that live on or in a host, causing harm.
Hermaphroditic	Having both male and female reproductive organs.
Tapeworms	Parasitic flatworms with segmented bodies.
Trematodes	Parasitic flatworms, also known as flukes.
Gastropods	Class of mollusks including snails and slugs.
Bivalves	Mollusks with two shells, such as clams and oysters.
Cephalopods	Active predatory mollusks like squids and octopuses.
Errantia/Sedentaria	Major groups of annelids; errant (mobile), sedentary (sessile).
Molting (Ecdysis)	Shedding of the exoskeleton in ecdysozoans.
Open circulatory system	Blood is not always contained within vessels.
Chelicerates	Arthropods with chelicerae, such as spiders.
Myriapods	Centipedes and millipedes.
Pancrustaceans	Group including crustaceans and insects.
Complete metamorphosis	Life cycle with distinct larval, pupal, and adult stages.
Incomplete metamorphosis	Life cycle with nymphs resembling adults.

Summary Table: Complete vs. Incomplete Metamorphosis

Type	Stages	Example
Complete Metamorphosis	Egg → Larva → Pupa → Adult	Butterfly
Incomplete Metamorphosis	Egg → Nymph → Adult	Grasshopper

Additional Info

• Invertebrates make up over 95% of all animal species.

• Arthropods are the most diverse animal phylum, with insects being the most numerous group.