Protostomes Overview

Protostomes Overview

  • Protostomes: A major group of animals characterized by the development of the mouth from the blastopore during embryonic development. They are coelomate, meaning they possess a body cavity derived from mesoderm.
    • Clades:
    • Lophotrochozoa:
      • Characteristics include: embryos develop using spiral cleavage.
      • Most members live in aquatic environments.
      • Movement is facilitated by cilia or body muscle contractions.
      • Defined by the presence of:
      • Trochophore: A free-living larval stage.
      • Lophophore: A horseshoe-shaped crown of ciliated tentacles surrounding the mouth, used for filter feeding.
    • Ecdysozoa:
      • Contains animals that undergo ecdysis, or molting.
      • Characterized by an exoskeleton, which serves as a hard external skeleton.
      • Major groups include arthropods and nematodes.
    • Micrognathozoa
    • Chaetognatha
    • Platyhelminthes
    • Cycliophora
    • Mollusca (e.g., snails, clams, squids)
    • Annelida (e.g., earthworms, sea worms)
    • Bryozoa
    • Brachiopoda
    • Nemertea
    • Loricifera
    • Nematoda
    • Kinorhyncha
    • Arthropoda (e.g., insects, spiders, centipedes)
    • Onychophora
    • Tardigrada
    • Echinodermata
    • Chordata

Mollusca

  • Phylum Mollusca
    • Diversity: Second in diversity only to arthropods.
    • Examples: Include gastropods (snails, slugs), cephalopods (octopuses, squids), and bivalves (clams, oysters).
    • Shell: Some have a shell, some do not.

Features of the Phylum Mollusca

  • Size Range: Varies from microscopic forms to giant clams weighing up to 270 kg.
  • Habitat: Most groups evolved in oceanic environments and continue to inhabit them.
  • Human Relevance: Important food source and economically significant. Pearls produced in oysters and mother-of-pearl created by abalones.
  • Pests: Zebra mussel (Dreissena polymorpha).

Mollusk Body Plan

  • Mantle:
    • A thick epidermal sheet that bounds the mantle cavity and secretes the shell if present.
  • Foot:
    • Serves as the primary means of locomotion for many mollusks and can be divided into arms or tentacles in cephalopods.

Internal Organs

  • The coelom is highly reduced and is limited to small spaces adjacent to the excretory organs, heart, and part of the intestine.
  • Visceral Mass: Concentration of digestive, excretory, and reproductive organs.
  • Ctenidia: Gills in aquatic mollusks, which also function in food filtration in most bivalves.

Mollusk Shell Characteristics

  • Protects against predators and harsh environments, though not essential as many species have reduced or lost their shells.
  • The typical shell structure consists of two layers of calcium carbonate with an internal layer possibly being nacre (mother-of-pearl).
  • Formation of Pearls: Occurs via coating a foreign object with nacre to alleviate irritation.

Radula

  • A rasping, tongue-like structure employed for feeding, found in most mollusks (except bivalves).
  • Functions include scraping algae in herbivorous mollusks and drilling in predatory gastropods, modified into a harpoon with venom glands in Cone snails.

Circulatory and Excretory Systems

  • Nitrogenous Waste Removal: Achieved via nephridia, consisting of cilia-lined openings called nephrostomes that lead to the excretory pore.
  • Circulatory System: Most mollusks have an open circulatory system where hemolymph circulates within a hemocoel; cephalopods possess a closed circulatory system.

Reproduction and Development

  • Most mollusks are gonochoric (individuals are either male or female) while a few, like some oysters, are hermaphroditic.
  • External fertilization is common, though gastropods typically exhibit internal fertilization.
  • Development includes a zygote undergoing spiral cleavage, with two larval stages:
    • Trochophore: A free-swimming larval stage.
    • Veliger: The second free-swimming larval stage, unique to bivalves and most marine snails.

Classes of Mollusks

  • There are 7 or 8 recognized classes, focusing on four:
    • Polyplacophora (Chitons): Marine mollusks with a shell composed of 8 overlapping plates and are primarily herbivorous grazers.
    • Gastropoda: Includes limpets, snails, and slugs, with most having a single shell and notable for unique coiling and torsion in development.
    • Bivalvia (Bivalves): Comprising clams, oysters, and mussels, characterized by two hinged shell valves without a radula.
    • Cephalopoda: Includes squids, octopuses, and chambered nautilus, known for their highly developed nervous systems and closed circulatory systems.

Annelida

  • Annelids: Segmented worms whose body structure comprises repeat units that allow for specialization.

Annelid Features

  • Body Plan: Includes a well-developed cerebral ganglion, a ventral nerve cord, internal segmentation with septa, and a closed circulatory system.
  • Includes specialized body parts for digestion, locomotion via coelomic fluid, and chaetae (chitin bristles).

Annelid Clades

  • Grouped into two clades:
    • Errantia: Characterized by active movement and paired parapodia for locomotion and gas exchange.
    • Sedentaria: Generally less mobile, live in tubes, and may utilize tentacles for feeding.

Earthworms and Leeches

  • Earthworms: Comprising 100-175 segments, they lack eyes but have senses for light and touch, are hermaphroditic, and engage in cross-fertilization.
  • Leeches: Mostly freshwater species, flattened dorsoventrally, with a reduced coelom without segmented division, using suckers for attachment and feeding.

Arthropoda

  • Characteristics: Most successful group of animals with over 1,200,000 species. Comprised of:
    • Chelicerata: Spiders, scorpions, ticks, with an anterior pair of chelicerae.
    • Crustacea: Crabs, shrimps, with biramous appendages and two pairs of antennae.
    • Hexapoda: Insects, characterized by three body regions; has over half of all named species.
    • Myriapoda: Millipedes and centipedes, with numerous segments and paired appendages.

Arthropod Success Factors

  1. Segmentation: Allows for specialization of body regions.
  2. Exoskeleton: Protects against water loss and must be shed during molting (ecdysis).
  3. Jointed Appendages: Adaptable for various functions (antennae, mouthparts, wings).

Circulatory and Nervous Systems

  • Generally, an open circulatory system with a double chain of segmented ganglia controlling various activities.

Eyes and Sensory Systems

  • Compound eyes made of ommatidia or simple eyes (ocelli) that help in light detection and orientation.

Respiratory Systems

  • Gas Exchange in Marine Arthropods: Often via gills.
  • Terrestrial Arthropods: Use tracheae branching into tracheoles, with spiracles for airflow control, along with book lungs in certain species.

Excretory Systems

  • Aquatic arthropods use diffusion across gills while terrestrial insects excrete nitrogenous wastes efficiently via Malpighian tubules.

Classifications of Arthropods

  • Each class has specific traits and contains many grouped taxa, e.g., different types of insects (beetles, moths, flies, etc.) with respective life stages and reproductive strategies.