Chapter 20

Giant Squid and Animal Characteristics

  • Giant Squid Sighting

    • First photographed in its natural environment in 2004.

    • Prior to this, a live specimen in its natural habitat had eluded scientists.

Key Features of Animals

  • Multicellular: Animals are composed of many cells that form complex structures.

  • Heterotrophic: Animals obtain their nutrition by consuming other organisms, either plants or other animals.

  • Reproduction: Typically reproduce sexually, though asexual reproduction can also occur.

  • Cell Structure: Animal cells lack a cell wall, which is characteristic of plant cells.

  • Motility: Most animals are capable of movement at some point in their life cycle.

  • Response to Stimuli: Animals can respond rapidly to external stimuli, enabling them to adapt to changes in their environment.

Evolutionary Tree and Anatomical Features

  • Branch Points in Animal Evolution:

    • Animals are categorized based on tissue development:

    • No tissue: Simple organisms without differentiated tissues.

    • Tissue: Animals with specialized tissues, classified into:

      • Radial symmetry: Body parts arranged around a central axis.

      • Bilateral symmetry: Distinct left and right sides; organisms can be divided into mirror images along one plane.

  • Deuterostome vs. Protostome Development:

    • Deuterostome: The mouth develops second, with the anus forming first during embryonic development.

    • Protostome: The mouth forms first during embryonic development.

Additional Anatomical Features Impacting Evolution

  • Symmetry Types:

    • Radially Symmetrical Animals (Diploblastic)

    • Have two embryonic tissue layers:

      • Endoderm: Inner layer that forms the digestive system.

      • Ectoderm: Outer layer that forms skin and nervous tissue.

    • Bilateral Symmetrical Animals (Triploblastic)

    • Have three embryonic tissue layers:

      • Endoderm

      • Ectoderm

      • Mesoderm: Middle layer that forms muscles and many internal organs.

      • Notable characteristic: only bilateral animals have a distinct head (cephalization).

Body Symmetry and Cephalization

  • Radial Symmetry:

    • Lacks a well-defined head; any plane through the central axis results in mirror-image halves.

  • Bilateral Symmetry:

    • Posses an anterior (head) end and a posterior (tail) end; only one plane (midline) divides the body into two mirror-image halves.

Advanced Features in Animals

  • Segmentation:

    • The body is divided into a series of repeating segments (metameres), with each segment containing a set of repeated internal organs.

  • Coelom: A fluid-filled body cavity that provides a hydrostatic skeleton.

  • Closed Circulatory System: Blood remains enclosed within vessels.

  • Excretory Organs: Specialized structures for waste elimination.

  • Tubular Gut: Digestive system organized as a tube, allowing for more efficient processing of food.

  • Three Main Subgroups of Annelids:

    • Polychaeta (e.g., bristleworms)

    • Oligochaeta (e.g., earthworms)

    • Hirudinea (e.g., leeches)

Diversity of Annelids

  • Habitat: Annelids can be found in various habitats including freshwater, marine, and terrestrial environments.

  • Size Range: From microscopic to approximately 22 feet long.

Body Cavity Types in Animals

  • Coelomate:

    • Example: Annelids possess a true coelom, providing a body cavity.

  • Pseudocoelomates:

    • Example: Roundworms (Nematoda) have a body cavity that is not fully lined by mesoderm.

  • Acoelomates:

    • Example: Flatworms (Platyhelminthes) have no body cavity between their body wall and digestive tract.

Development Mechanisms in Animals

  • Protostome Development:

    • Body cavity forms within the space between body wall and digestive cavity; includes annelids, arthropods, and mollusks.

  • Deuterostome Development:

    • Body cavity forms as an outgrowth of the digestive cavity; includes echinoderms and chordates.

Invertebrate Parasites

  • Definition of a Parasite:

    • Symbionts that derive benefits from their host while harming the host in the process.

  • Three Types of Parasites:

    • Ectoparasites: Live on the surface of the host.

    • Endoparasites: Live inside the host's body.

    • Mesoparasites: Live in a host's cavity that opens to the outside (e.g., nose, mouth, anus).

  • Life Cycle of Parasites: Fertilized eggs are typically released outside the host's body.

Examples of Mesoparasites

  • Cymothoa exigua: A parasitic crustacean known as the tongue-eating louse, which enters through the gills of fish and attaches to their tongue.

Overview of Platyhelminthes (Flatworms)

  • Anatomy:

    • Nerve cord, excretory pore, brain, excretory canal, digestive system, eyespots, gastrovascular cavity, pharynx.

  • Characteristics:

    • Exhibit bilateral symmetry and cephalization.

    • Respiration occurs through diffusion between body cells and the environment.

    • ~20,000 species, some are free-living and some are parasitic.

    • Reproduction can be either asexual or sexual.

  • Note on Regeneration:

    • Nearly all mesodermal cells in Planaria (a type of flatworm) are stem cells, contributing to significant regenerative abilities.

Lifestyle of Platyhelminthes

  • Free-living:

    • Survive in an aerobic environment, reproduce outside the host, and attempt not to impact the environment significantly.

  • Parasitic:

    • Reproduce in definitive hosts, endure much work to travel between hosts, survive in anaerobic environments, and attempt not to kill the host (short-term).

The Life Cycle of the Human Pork Tapeworm (Taenia solium)

  • Structure: Each reproductive unit (proglottid) contains both male and female sex organs.

  • Questions:

    • How many hosts are involved in the life cycle? One or two?

    • Why have tapeworms evolved a long, flat shape?

  • Class Cestoda: Tapeworms are specialized for endoparasitism, with a scolex containing hooks and suckers.

Morphology of Tapeworms

  • Body Structure:

    • Long flat bodies, harboring no digestive system or sensory organs.

    • Proglottids are categorized into:

    • Immature (developing), mature (contains male & female organs), gravid (contains eggs).

    • Strobila: A chain of proglottids.

    • Caudal Migration: Movement of proglottids towards the end of the tapeworm body.

Lifecycle of Taenia

  • Process of Infection:

    • Proglottids containing shelled larvae/eggs are expelled with human feces.

    • Eggs are ingested by cattle, migrate to intestines, and hatch as onchospheres.

    • Onchospheres penetrate intestinal walls, migrate to muscles, and encyst forming cysts.

    • Humans become infected by eating undercooked meat containing the scolex which will mature and then expel proglottids into the environment.

Ectocommensal Symbionts in Platyhelminthes

  • Bdelloura candida: An ectocommensal species of flatworm that exists on the American horseshoe crab (Limulus polyphemus).

Overview of Nematoda (Roundworms)

  • Characteristics:

    • Tubular gut and pseudocoelom.

    • Exhibit molting during development.

    • Many nematodes are parasites and they are found in diverse ecosystems: freshwater, marine, and terrestrial.

    • Estimated to comprise ~27,000 species, but likely represent millions.

Parasitic Nematodes Examples

  • Trichinella: An encysted larva in muscle tissue of pigs, potentially living for up to 20 years.

  • Heartworms: Adult heartworms live in the hearts of infected dogs; juveniles enter the bloodstream and can be transferred to other dogs via mosquitoes.

Phylum Arthropoda

  • Subphylum Chelicerata: Includes ticks and mites.

  • Class Arachnida (Arachnids):

    • More than 110,000 species identified.

    • Body Structure:

    • Fused cephalothorax and abdomen, lack external segmentation.

    • Capitulum: The anterior region that houses the mouthparts.

  • Ticks:

    • Larger than mites; attach to vertebrates, suck blood, then detach.

    • Associated with agricultural dermatitic conditions.

    • Ixodes: Ticks that transmit Lyme disease, such as Ixodes pacificus.

Overview of Chordates

  • Key Features:

    • Notochord: A flexible rod providing skeletal support.

    • Dorsal, hollow nerve cord: Forms the central nervous system.

    • Pharyngeal gill slits: Openings in the pharynx region.

    • Post-anal tail: Extension of the body past the anal opening.

    • Endostyle: A gland necessary for feeding and metabolism.

Chordata: Vertebrates

  • Lampreys:

    • Class inhabited by approximately 40 species.

    • Juveniles feed on small particles in freshwater; adults attach to hosts.

    • Characterized by a flexible rod of cartilage instead of a true backbone.

  • Sensory Organs: Well-developed eyes and nostrils contribute to feeding and environmental interactions.

Lamprey Feeding Mechanism

  • Attachment to Host: By suction, creating wounds and secreting anticoagulants through oral glands.

  • Mouth Structure:

    • Fleshy funnel with a horny interior lined by conical teeth.

    • Protrusible tongue also has teeth for rasping.

  • Lampreys do not kill their hosts outright, but can make them weak due to blood loss.