Ch 26 - Invertebrates - concepts & terms

Chapter 26 – Invertebrates: The Vast Array of Animal Life Without a Backbone

1. Characteristics of Animals

• Animals constitute the most species-rich kingdom.

• Defining an "animal" is complex due to diversity, with exceptions to many characteristics.

Key Features That Characterize Animals:

Cell Structure

• Multicellular, lack cell walls, are flexible.

• Supported by an extracellular matrix (ECM).

• Specific cell junctions like anchoring & tight junctions are essential for cellular integrity and communication.

Mode of Nutrition

• Animals are heterotrophs, utilizing various feeding methods:

  • Suspension feeding (filtering).

  • Bulk feeding.

  • Fluid feeding.

Movement

• Unique muscle and nerve tissues enable movement.

• Most animals exhibit some form of locomotion; movement is coordinated by the nervous system.

Genomes

• Possess Hox genes crucial for body axis patterning.

• Similar genes encode small ribosomal subunit rRNA across diverse species.

Reproduction and Development

• Predominantly sexual reproduction with small mobile sperm and larger eggs.

• Fertilization can occur internally (terrestrial species) or externally (aquatic species).

• Embryonic development includes various stages, with some species undergoing metamorphosis.

2. Animal Classification

• Majority of biologists consider the animal kingdom monophyletic.

• Approximately 35 recognized phyla; 13 will be explored in detail.

3. Evolutionary Relationships

• Choanoflagellates are believed to be the closest living relatives of animals, sharing significant genomic similarities.

4. Symmetry and Body Plans

Types of Symmetry

• Asymmetry: No symmetrical plane.

• Radial symmetry: Divisible through multiple longitudinal planes.

• Bilateral symmetry: Divisible into equal halves through a single vertical plane; correlated with cephalization.

Body Plans and Germ Layers

• Animal classification also considers body plan, which includes:

  • Types of body cavities (coelom, pseudocoelom, acoelom).

  • Presence of different germ layers (endoderm, mesoderm, ectoderm).

5. Ctenophores: The Earliest Animals

• Ctenophores represent the earliest-diverging animal lineage, exhibiting unique features:

  • 200 marine species.

  • 8 rows of cilia resembling combs; utilize them for locomotion.

  • Have tentacles that secrete sticky substances for prey capture (no stinging cells).

  • Digestion occurs in a gastrovascular cavity.

  • Hermaphroditic, releasing gametes into water for external fertilization.

  • Exhibit bioluminescence and have a diffuse nervous system without Hox genes.

6. Porifera: The Sponges

• Sponges lack true tissues, but are multicellular and perform various functions:

  • Have diverse cell types but no nervous system.

  • Over 8,000 recognized species; most are marine and asymmetric.

  • Adults are sessile, while larvae are motile.

Choanocytes and Water Circulation

• Choanocytes circulate water via flagella, drawing it in through pores and out through the osculum.

  • They trap and digest small particles of food; amoebocytes distribute absorbed nutrients.

  • Utilize intracellular digestion, a unique characteristic among animals.

Defense Mechanisms

• Mechanical defenses (e.g., tough spicules) limit predation.

• Some produce toxic chemicals for defense.

Reproduction

• Sponges reproduce both sexually (hermaphroditic) and asexually (fragmentation).

7. Cnidaria: Jellyfish and Other Radially Symmetric Animals

• Cnidarians include hydras, jellyfish, and anemones; mostly marine animals:

  • Diploblastic with two germ layers (ectoderm and endoderm).

  • Exhibit radial symmetry with tentacles for prey capture.

  • Gastrovascular cavity facilitates extracellular digestion.

Body Forms

• Exist in two forms: polyp (sessile) and medusa (motile).

Specialized Cells

• Cnidocytes, containing nematocysts, used for defense and prey capture.

  • Most nematocysts are not harmful to humans, except in larger jellyfish species.

8. Lophotrochozoa: Diverse Worms and Mollusks

• Include flatworms, rotifers, brachiopods, mollusks, and annelids.

Platyhelminthes (Flatworms)

• Acoelomate with bilateral symmetry; predatory lifestyle.

• Reproduction is sexual or asexual; many are hermaphroditic.

Rotifera

• Possess a pseudocoelom and a ciliated crown for feeding.

• Primarily freshwater, with many species being microscopic.

Mollusca

• A large, diverse phylum with various classes (cephalopods, bivalves, etc.).

  • Body plan includes muscular foot, visceral mass, and mantle.

  • Reproduction typically involves external fertilization; some snails exhibit internal fertilization.

9. Ecdysozoa: Nematodes and Arthropods

• Ecdysozoans are characterized by a tough exoskeleton and molting (ecdysis).

Nematoda (Roundworms)

• Small, pseudocoelomate worms found in diverse habitats, including as parasites.

  • Covering of tough collagen cuticle; sexual reproduction is common.

Arthropoda (Insects and Allies)

• Largest phylum; body plan includes jointed appendages and segmented bodies.

  • Complex classification; includes subphyla such as Chelicerata, Myriapoda, Hexapoda, and Crustacea.

10. Deuterostomia: Echinoderms and Chordates

• Share developmental similarities and molecular evidence.

Echinodermata

• Exhibits modified radial symmetry; includes sea stars and urchins.

  • Possess a water vascular system for movement and feeding.

Chordata

• Distinguished by notochord, dorsal hollow nerve cord, pharyngeal slits, and postanal tail.

  • Contains subphyla Cephalochordata, Urochordata, and Vertebrata.