Introduction to Invertebrate Zoology

• In invertebrates, cell number can vary:

  • Acellular structures contain only genetic material
  • Multi-celled organisms developed from a single-celled ancestor, leading to an explosion of organisms during the Cambrian Period known as the Cambrian explosion
• Bilateral symmetry in invertebrates is strongly associated with cephalization, which is the differentiation of a head and the concentration of nervous and sensory tissues and organs at one end of an animal
• Biradial symmetry in invertebrates means what is found on the left side of the organism is mirrored on the right side
• Symmetry in organisms typically progresses from asymmetrical to radially symmetrical to bilaterally symmetrical, with bilaterally symmetrical organisms developing germ layers during embryonic stages that eventually give rise to tissues and organs
• Embryogenesis in invertebrates involves cleavage patterns that differ between protostome and deuterostome embryos:

  • Protostome embryos have spiral cleavage
  • Deuterostome embryos have radial cleavage
  • Deuterostomes exhibit indeterminate cleavage, where each separated cell has the potential to develop into an entire organism on its own
• Gastrulation in invertebrates involves the rearrangement of blastula cells to form an embryo with 2 or 3 germinal layers, leading to the development of ectoderm, endoderm, and mesoderm layers
• In invertebrates, mouth formation differs between protostomes and deuterostomes:

  • Protostomes form the mouth from the blastopore
  • Deuterostomes do not form the mouth from the blastopore
• Taxonomy, developed by Carolus Linnaeus, is a system of classifying organisms hierarchically and naming them using the binomial system of nomenclature
• Acellular organisms have no cells and are just genetic material
• Bilateral symmetry is strongly associated with cephalization, the differentiation of a head
• Biradial symmetry means what is found on the left side of the organism is mirrored on the right side
• Cleavage in protostome embryos results in spiral cleavage, while in deuterostome embryos, it leads to radial cleavage
• In deuterostomes, each separated cell has the potential to develop into an entire organism on its own
• Gastrulation is the rearrangement of the cells of a blasula to form an embryo with 3 germinal layers:

  1. Ectoderm

  2. Endoderm

  3. Mesoderm
• Diploblastic organisms have:

  • a. ectoderm only
  • b. endoderm only
  • c. mesoderm only
  • d. ectoderm and endoderm
  • e. all germ layers
• Coelom Formation:

  • Acoelomates have the gut surrounded by mesoderm
  • Pseudocoelomates have a body cavity partially lined by mesoderm and endoderm
  • Coelomates have a true body cavity completely lined by mesoderm
• Protostome development (e.g., molluscs, annelids) differs from deuterostome development (e.g., echinoderms, chordates) in:

  • Coelom development
  • Cleavage pattern: spiral, determinate for protostomes; radial, indeterminate for deuterostomes
  • Mouth origin: from blastopore for protostomes, not from blastopore for deuterostomes
  • Larval ciliary bands: compound cilia from multi-ciliated cells; downstream particle capture for protostomes, simple cilia, 1 cilium per cell; upstream particle capture for deuterostomes
• Polar lobe formation is a conspicuous bulge of cytoplasm that forms prior to cell division and contains no nuclear material:

  • Found only in protostomes
  • Functional significance not known
• Upstream larvae are often large, with large surface areas, swim slowly, and have surprisingly bizarre shapes
• Upstream larvae are usually transparent and rather delicate
• Rapid growth to a comparatively large size, using minimal amounts of tissue, confers a significant adaptive advantage to upstream larvae
• Upstream larvae make excellent microscopical subjects and are more prominent than downstream larvae in research on the mechanics of development
• Phylogenetic trees constructed based on larval type align well with those based on other criteria, such as molecular sequences
• Two major, well-defined groups emerge from phylogenetic trees: protostomes with downstream larvae and deuterostomes with upstream larvae
• Larval morphology provides additional support for the premise that the protostome/deuterostome distinction is a natural one of long evolutionary standing
• Classification in terms of Evolutionary Relationship:

  • Taxonomy developed by Carolus Linnaeus
  • Shows evolutionary relationship taxon - any group of organisms distinct enough to be assigned to a category
• Hierarchical Organization:

  • Kingdom: Animalia
  • Phylum: Arthropoda
  • Class: Insecta
  • Order: Diptera
  • Family: Muscidae
  • Genus: Musca
  • Species: M. domestica
• Binomial System of Nomenclature:

  • All organisms have a unique binomial name
  • Rules for naming organisms
• Phylogeny is the hypothesis of evolutionary relationships, represented by phylogenetic trees
• Phylogenetic Terms:

  • Monophyletic Group
  • Polyphyletic Group
  • Paraphyletic Group
• The group Reptiles does not form a clade, so the name Reptilia cannot be used as a valid phylogenetic name
• Example of a valid clade and identifying the synapomorphic feature of shaded groups
• Classification in terms of Habitat and Lifestyle:

  • Habitat: terrestrial, aquatic (freshwater, marine, intertidal, subtidal, open sea)
  • Mobility: mobile, sessile, sedentary, planktonic
• Trophic categories: autotrophic, heterotrophic, mixotrophic
• Feeding habits: herbivores, carnivores, suspension feeders, deposit feeders
• Sponges are suspension feeders that filter small organic particles and plankton from the water using specialized cells called choanocytes
• Symbiosis types: ectosymbionts, endosymbionts, ectoparasites, endoparasites