invertebrates

• • Specific cell junctions (anchoring & tight junctions) are important for holding cells together and allowing communication between cells
  • Mode of nutrition:
  • Animals are heterotrophs
  • Many different modes of feeding are used including suspension feeding (filtering), bulk feeding, and fluid feeding
  • Movement:
  • Muscle and nerve tissues are unique to animals
  • Most animals are capable of some type of locomotion to acquire food or escape predators, and a nervous system coordinates movement
  • Genomes:
  • Most animals possess Hox genes that function in patterning the body axis
• Characteristics of animals:

  • Animals constitute the most species rich kingdom
  • Defining an “animal” is difficult because they are so diverse that there is an exception to nearly all characteristics
  • Broadly, animals are characterized by cell structure, mode of nutrition, movement, genomes, reproduction, and development
  • Cell structure:
  • Animals are multicellular, lack cell walls, and are flexible
  • Cells are supported by an extracellular matrix (ECM)
• • Animals also have very similar genes that encode the small ribosomal subunit rRNA
  • Reproduction and development:
  • Nearly all animals reproduce sexually using a small mobile sperm and a much larger egg
  • Fertilization can be internal (terrestrial species) or external (aquatic species)
  • Embryos go through various stages of development and some species undergo metamorphosis
• Animal classification:

  • Most biologists agree that the animal kingdom is monophyletic
  • There are currently ~35 recognized animal phyla
  • Animals evolved from a Choanoflagellate-like ancestor
  • Genome similarities indicate that the closest living relative of animals is a flagellated protist known as a choanoflagellate
  • Choanoflagellates are tiny, single-celled or colonial organisms, each with a single flagellum surrounded by a collar composed of microvilli
  • Receptor tyrosine kinases (RTKs) are found in all animals and in choanoflagellates, important in cell communication
• • Animal phyla have broad differences related to body plan, germ layers, and embryonic development
  • Biologists traditionally classified animal diversity based on type of body symmetry, number of germ layers, and specific features of embryonic development
  • Symmetry can be asymmetry, radial symmetry, or bilateral symmetry
  • Bilaterally symmetric animals are collectively known as the Bilateria, with specific body plan characteristics
• Ctenophores (comb jellies):

  • Ctenophores constitute the earliest-diverging animal lineage, with about 200 marine species
  • Have 8 rows of cilia on their surface resembling combs
  • Typically 1-10 cm in length, possibly the largest animals to use cilia for locomotion
  • Lack stinging cells, secrete a sticky substance to catch prey
  • Digestion occurs in a gastrovascular cavity
  • Hermaphroditic, possessing both ovaries and testes; gametes are released into the water
  • Nearly all exhibit bioluminescence
  • Contain muscle and nerve cells organized as a diffuse net, centralized at an elementary brain
• • Lack Hox genes and many other genes found in bilaterians
• Porifera (sponges):

  • Sponges lack true tissues but are multicellular with different cell types
  • Lack a nervous system but carry necessary genes
  • About 8,000 species identified, mostly marine and asymmetric
  • Choanocytes help circulate water for feeding
  • Sponges have mechanical and chemical defenses against predators
  • Reproduce sexually as hermaphrodites and asexually through fragmentation or budding
• Cnidaria (jellyfish and other radially symmetric animals):

  • Phylum Cnidaria includes hydra, jellyfish, sea anemones, and coral, mostly found in marine environments
  • Diploblastic with radial symmetry
  • Exist in two body forms: polyp and medusa
  • Specialized stinging cells called cnidocytes for defense or prey capture
  • Cnidarians have a single opening for both mouth and anus
  • Nematocysts in cnidocytes can inject toxins, some of which can be harmful to humans
• Lophotrochozoa is a diverse group that includes taxa possessing a lophophore, a trochophore, or share molecular similarities with other members of the Lophotrochozoa
• Bryozoans, Brachiopods, and Rotifers have a lophophore (or lophophore-like structure) which is a horseshoe-shaped crown of ciliated tentacles used for feeding
• Mollusks, Annelids, and Platyhelminthes have a trochophore larval stage (or trochophore-like larvae) characterized by a band of cilia around the middle used for swimming
• Platyhelminthes were among the first animals to develop an active predatory lifestyle with bilateral symmetry, cephalization, and mesoderm
• Flatworms have well-developed muscles derived from mesoderm, lack specialized respiratory or circulatory systems, and have an incomplete digestive system with one opening serving as both mouth and anus
• Flatworms have a pair of cerebral ganglia, lateral nerve cords, and a nerve net on the ventral surface; they can reproduce sexually or asexually
• Flatworms are organized into four classes, some are free-living predators, while many invade other animals as parasites with complex lifecycles
• Rotifers have a pseudocoelom and a ciliated crown (corona) for feeding, with about 2,200 mostly microscopic species feeding on planktonic organisms and decomposing organic material
• Bryozoans and Brachiopods possess a lophophore for feeding and gas exchange, with Bryozoans being small colonial animals found encrusted on rocks and Brachiopods being bottom-dwelling marine organisms with two shell halves
• Mollusks have over 100,000 living species with a soft body protected by a shell, a basic body plan consisting of a muscular foot, visceral mass, and mantle, and most have separate sexes with external or internal fertilization
• Mollusks have a diverse body plan related to environmental diversity, with a mantle cavity housing gills, an open circulatory system, and some classes like cephalopods being fast-swimming marine predators with a closed circulatory system
• Annelids have about 18,000 described species including earthworms, marine worms, tube worms, and leeches, with segmentation providing advantages like repetition of body components, effective locomotion, and specialized segments
• Annelids have a double transport system with a closed circulatory system, respiration through the skin, a complete digestive system, and sexual reproduction involving internal fertilization or asexual reproduction by fission
• Recent studies suggest two major groups of annelids: Errantia are active free-ranging worms, while Sedentaria are burrowers or parasites
• Arthropod body plan:

  • Open circulatory system where hemolymph is pumped from a tube-like heart into short vessels, then into sinuses where gases and nutrients diffuse into tissues
  • Hemolymph flows back to the heart through pores called ostia
  • Aquatic arthropods have gills
  • Terrestrial species have a tracheal system
  • Spiracles are pores on the surface of the body that open to a series of finely branched air tubes; the tracheal system delivers oxygen directly to tissues
  • The cuticle blocks diffusion of gases
• Arthropod classification:

  • Recent study of mitochondrial DNA suggests a phylogeny with 5 main subphyla: Trilobita, Chelicerata, Myriapoda, Hexapoda, and Crustacea
• Subphylum Chelicerata (spiders and relatives):

  • Body with 2 tagmata: cephalothorax (fused head and thorax) and abdomen
  • 6 pairs of appendages: chelicerae (fangs), pedipalps, four pairs of walking legs
  • Spiders and scorpions are predatory, mites may be free-living scavengers or parasites, and all ticks are parasites
• Subphylum Myriapoda (millipedes and centipedes):

  • Millipedes have two pairs of legs per segment, slow-moving herbivores
  • Centipedes have one pair of walking legs per segment, fast-moving carnivores
  • Many species have glands that can secrete a toxic repellant substance for protection
• Subphylum Hexapoda (insects and relatives):

  • Have three tagmata (head, thorax, and abdomen) and three pairs of walking legs
  • Insects live in all terrestrial habitats
  • Wings developed as an outgrowth of the body wall cuticle
  • Class Insecta has more species than all other species of animal life combined
• Insect reproduction and development:

  • All insects have separate sexes and use internal fertilization
  • Most insects (~85%) undergo complete metamorphosis: egg, larva, pupa, adult
  • Remaining insects undergo incomplete metamorphosis: egg, nymph, adult
  • Nymphs look like miniature adults, but without wings
• Subphylum Crustacea (crabs, lobsters, barnacles, and shrimp):

  • Commonly inhabit marine environments
  • Possess two pairs of antenna and have multiple sensory and feeding appendages
  • Must molt to grow
  • Many crustaceans are predators, some are scavengers, some are filter feeders