Study Notes on Invertebrates

Invertebrates are animals that lack a backbone.
They account for over 95% of known animal species and occupy almost every habitat on Earth.
Morphological diversity in invertebrates ranges from microscopic species to organisms that grow up to 18 meters long.

Metabolic Characteristics
- Compare the metabolic characteristics of different phyla: Porifera, Cnidaria, Lophotrochozoa, Ecdysozoa, Deuterostomia.
Physical Characteristics
- Compare physical characteristics of different phyla: Porifera, Cnidaria, Lophotrochozoa, Ecdysozoa, Deuterostomia.
Comparison
- Analyze differences in physical and metabolic characteristics of each genus within the above groups.
Importance of Structures
- Assess roles and significance of various structures in these organisms.

Classifying invertebrate species aids in understanding their diversity and evolutionary relationships.
- Common ancestor of all animals, Eumetazoa:
- Includes Porifera, Cnidaria, Lophotrochozoa (great morphological diversity), and Deuterostomia (includes both vertebrates and invertebrates).
- Bilateria: Animals with two-sided (bilateral) symmetry and three tissue layers exist in Ecdysozoa and Deuterostomia.

Total Number of Known Species: Kingdom Animalia has approximately 1.3 million known species; estimates of total species range from 10 to 20 million.

Porifera (Sponges)
- Total species: 5,500
- Characteristics:
  - Sessile filter feeders that lack true tissues.
Cnidaria (Jellyfish, Corals, Hydras)
- Total species: 10,000
- Characteristics:
  - Radially symmetrical, diploblastic animals with a gastrovascular cavity.
Acoela (Flatworms)
- Total species: 400
- Characteristics:
  - Simple nervous system and sac-like gut.
Placozoa
- Total species: 1 (Trichoplax adhaerens)
- Characteristics:
  - Composed of a bilayer of a few thousand cells; reproduce by dividing or budding.
Ctenophora (Comb jellies)
- Total species: 100
- Characteristics:
  - Diploblastic, radially symmetrical with eight ciliated 'combs' for water propulsion.

Comprises phyla with a cuticle and includes more species than all other eukaryotic groups combined:
- Nematoda (Roundworms): 25,000 species, cylindrical bodies, many of which are parasitic.
- Arthropoda (Insects, crustaceans): 1,000,000 species; segmented body with jointed appendages and exoskeleton.

Includes both invertebrate (Echinodermata) and vertebrate species (Chordata).
- Echinodermata: 7,000 species, include sea stars and sea urchins; exhibit unique features like a water vascular system enabling movement and feeding.
- Hemichordata (Acorn worms): 85 species, share traits with chordates.
- Chordata: 60,000 species, includes vertebrates and invertebrate groups like lancets and tunicates.

Sponges are among the simplest animals, sometimes called basal animals.
A sponge's body resembles a sac perforated with pores – it draws water through its body to filter feed, utilizing a central cavity called the spongocoel.
Key cell types:
- Choanocytes: Flagellated cells that engulf food by phagocytosis.
- Amoebocytes: Totipotent cells that digest food, transport nutrients, and form skeletal fibers.
Morphology: Layers of cells separated by mesohyl (gelatinous matrix).
Most sponges are hermaphrodites, exhibiting sequential hermaphroditism. Zygotes develop into flagellated larvae that settle to form adult sponges.

Cnidarians include diverse forms, both sessile and motile, such as jellies and corals.
They possess a centralized gastrovascular cavity with a single opening acting as both mouth and anus.
Body Plan Variations:
- Polyp: Sessile, adheres to substrates.
- Medusa: Free-swimming, bell-shaped.
Feeding Mechanism: Predatory species use tentacles armed with cnidocytes for prey capture, which contain specialized organelles called nematocysts for stinging.
Movement and Responsiveness: Cnidarians have a simple nervous system known as a nerve net.

Medusozoa: Includes those producing medusa (e.g., jellies, box jellies).
Anthozoa: Includes only polyp forms (e.g., sea anemones, corals) and can have symbiotic relationships with algae.
- Coral reefs, vital marine ecosystems, currently face threats from pollution, overharvesting, and climate change.

Flatworms: Are characterized by their flat bodies and include free-living species like planarians and parasitic forms like tapeworms.
- Trematodes (flukes): Complex life cycles often requiring multiple hosts.
- Tapeworms: Parasites that attach to hosts using a scolex, absorbing nutrients from the host's digestive tract without a mouth.
Rotifers: Tiny multicellular animals with a crown of cilia for feeding.
Annelids: Distinguished by segmentation; includes earthworms and leeches, exhibiting varied lifestyles.

Defined by having a cuticle that must be molted for growth; significant groups include:
- Nematodes: Cylindrical shape, no segmentation, prevalent in various habitats including soil.
- Arthropoda: Highly diverse with segmented bodies and jointed appendages.
  - Crustaceans: Include a variety of marine species with specialized appendages for various functions (e.g., feeding, locomotion).
  - Insects: Highly adapted for terrestrial life, with complex behaviors and developmental cycles (both complete and incomplete metamorphosis).

Echinodermata: Exhibit unique features such as a water vascular system and the ability to regenerate lost body parts.
Chordata: Incorporates both vertebrates and some invertebrate species like tunicates and lancelets, characterized by unique structural traits during some stage of their life cycle.

In summary, invertebrates display significant diversity in morphology and life strategies, contributing to their successful adaptation in various environments worldwide.