Bilateralia(also referred to as "Bilateria"): Monophyletic group containing most animals.

Synapomorphies:

• Origin of an antero-posterior axis (over an oral-aboral axis)
• Presence of a flow-through gut with distinct mouth and anus.
• Triboblastic: I.e. a third germ layer, mesoderm is present. Consider the basic steps by which we get a proper embryo with a top, bottom, front, and back from a zygote:

  Polarity: In all ova yolky matrial tends to concentrate at one end, yeilding:

  • "Animal" pole, the non-yolky side
  • "Vegetal" pole, the yolky part.
  Cleavage Phase of rapid cell division with little overall growth. The zygote transforms into a hollow sphere of cells, the blastula. The space in the middle is the blastocoel
  

  The blastula has three cell types:
  

  • Smaller cells nearer the "animal" pole.
    
  • Larger, yolky cells nearer the "vegetal" pole.
    
  • A third type of cell girdling the large cells of the "vegetal" pole.

  Gastrulation:

  
  • Cells of the "vegetal" hemisphere form a flat plate then invaginate inward, yielding a two layered hemisphere.
  • The rim of this concavity qickly closes into a small opening, the blastopore. While this is occurring, cells from the third cell type stream in through the blastopore and move forward along the inner surface. These become mesoderm. Depending on the taxon, blastopore will become either mouth or the anus.
  The embryo is now a gastrula.

• Coelomate:
  
  • Presence of a coelom, or internal body cavity, surrounded by mesoderm in which the internal organs are suspended. Allows for:
    • increased gut expansion,
    • hydrostatic structure against which muscles may act, facilitating purposeful locomotion
    • use of coelomic fluid in gas and nutrient transport, breaking dependence on simple diffusion and allowing larger body size.
  • Developed as:
    • Pseudocoelomate: In which the coelom develops between the endoderm and mesoderm as a remnant of the blastocoel and is properly called the pseudocoel.
    • Eucoelomate: In which a proper coelom develops within the mesoderm. (See above illustration.)
    • Acoelomate: In some critters, including platyhelminth flatworms, the coelom is drastically reduced or absent.
  • Coelom formation in eucoelomates can occur in two fashions:
    • Schizocoely (typical of "protostomes") in which coelom forms in cavities that develop in existing mesoderm
    • Enterocoely (typical of "deuterostomes") where the coelom forms by the outpouching of mesoderm during gastrulation.
  • Specialized organs The presence of coelom and mesoderm seems to provide the developmental flexibility for the development of specialized organs. In bilateralians we usuall see organs dedicated to:
    • Circulation
    • Excretion of nitrogenous wastes
    • Nerve ganglia for coordination of behavior
    .

Bilaterian Phylogeny

Eek. This is truly a bucket of worms. No consensus, but among the many bilaterian groups, three larger clades are recognized by many authors:

• Deuterostomia: Includes vertebrates and echinoderms plus minor groups. Distinct deuterostomous pattern of development.
  • Blastopore becomes anus; mouth comes from secondary opening
  • Enterocoelomate
  • Indeterminate development: i.e. the fate of a given cell is fulid and based on inductive relationships with neighboring cells and tissues.
  • , Radial cleavage of embryonic cells (ability to twin). I
  • Nerve chord dorsal to digestive tract in adults.

• Typical bilateralians have protostomous development in which:
  • Blastopore becomes mouth
  • Schizocoelomate
  • Determinate development, in which cell fates are absolute.
  • Spiral cleavage (all mesoderm from 4d cell)
  • Nerve chord ventral to digestive tract

• Ecdysozoa: Includes arthropods, nematodes, onychophorans, and other critters that regularly shed an external cuticle.

• Lophotrochozoa: United by molecular characters. Further divided into:
  • Trochozoa
    • Trochophore larva
    • Annelida and Mollusca, only members with substantial fossil record
  • "Lophophorata"
    • Dubious monophyly on molecular work, but some morphological traits in common

Lophophorates share:

• Lophophores
  • Hollow extensions of second (of three) coeloms
  • Surround mouth, anus outside
  • Cilliated
  • Function in collection of food and in gas exchange
• Essentially headless
• U-shaped guts
• Two major groups: Bryozoa and (Phoronida + Brachiopoda)

Brachiopoda & Phoronida: Larger bodied. DNA strongly suggests two form a clade.

Phoronida: (No record) Represent what we might imagine to be the ideal ancestral lophophorate:

• Always solitary
• Macroscopic
• Infaunal benthic marine
• Possesses
  • Nephridia for waste excretion
  • Closed circulatory system with blood cells and hemoglobin (no distinct heart)

Brachiopoda: (Cambrian - Rec.)

• Always solitary
• Always encased in shell
• Always benthic marine
• Possesses
  • Nephridia for waste excretion
  • Closed circulatory system with a distinct heart but which doesn't seem to be involved in gas transport.
  • Coelomic fluid with the oxygen-binding protein hemerethryn.
  • "Mantle tissue" that secretes the valves. (Not homologous to mollusc mantle.)
  • Refer to lab for details of valve morphology.

Major clades:

• Lingulata (sometimes called "Linguliformes" or "Linguliformea"): (Cambrian - Rec.)
  • Chitinophosphatic valves, lacking hinge articulation
  • Muscular pedicle
  • Burrowing infaunal habit
• Inarticulata proper (sometimes called "Craniiformea" or "Craniata" (not to be confused with the TRUE Craniata) (Cambrian - Rec)
  • Calcareous valves, lacking hinge articulation
  • Pedicle reduced or absent
  • Epifaunal
• Articulata (sometimes "Rhynchonelliformea") (Late Cambrian - Rec.)
  • Greatest diversity of brachiopods, living or extinct
  • Calcareous valves with complex hinge articulation
  • In many groups, the lophophore is supported by a brachidium.
  • Pedicle variously developed, but dead horny tissue
  • Anus lacking. Gut tube is blind ended, with waste preiodically being expelled through mouth. (In stark contrast to lingulids, which have a proper gut tube.)
  • Epifaunal, with a wide variety of habits

• Systematics: Six classically recognized groups:
  • Strophomenida: (Ord. Jur) Strophic with D-shaped profile. Often lacking pedicle and colonizing soft substrates.
  • Orthida: (Cam. Per.) Morphologically the most ancestral of articulates. Possibly paraphyletic.
  • Pentamerida: (Cam. Dev.) Astrophic with biconvex valves. Large cruralium and spondylium divided mantle cavity into five chambers.
  • Rhynchonellida: (Ord - Rec.) Astrophic with pointed beak and biconvex valves. Strongly plicate with deep fold and sulcus.
  • Spiriferida: (Ord - Jur.) Paraphyletic? Brachidium developed into complex spiralium.
  • Terbratulida: (Dev. - Rec.) Astrophic, Smooth valves, looping brachidium.

• Ecology:
  • Shallow marine benthic
  • Lingulids can tolerate brackish water.
  • Typical articulates with pedicle required hard substrate. Those without (E.G. concavo-convex Strophomena) lay on soft substrate. Some were cemented to the substrate.
• Evolutionary trends:
  • Strophic, D-shaped types seem to predominate in Early Paleozoic.
  • In Late Paleozoic, proliferation of odd-ball strophomenids with specializations for sementation to hard substrate (E.g. Prorichthofenia, Leptodus). Productids developed morphology for support in soft sediment.
  • General trends toward increasing complexity of hinge articulations and brachidia.

• Exclusively colonial
• Non-living skeleton (zooecium)
• Each "individual" is a zooid
  

• Colony (zoarium) begins with an initial zooid, the ancestrula, which buds off clones to form the remainder of the colony. (But a soft tissue connection persists.)
• Individual zooids connected by funiculus and stolon through which nutrients are shared.
• Lophophore is protracted when it "inflated" by coelomic fluid when circumferential muscles of body wall contract. It is retracted by retractor muscles.
• Small enough to handle gas-exchange by simple diffusion.
• Lack nephridia (organs for excretion of insoluble wastes) These, thus, accumulate in tissues.
• Great powers of regeneration. Occasionally, all cells of the zooid except for the body wall degenerate into a brown body that is ultimately ejected from the zooecium. The zooid then regenerates from the body wall. Possible functions:
  • Resting stage during stressful intervals
  • A radical means of ridding the body of insoluble wastes.
• Reproduction of both zooids and colonies is often by budding. Individuals reproduce sexually, as well. Most zooids are hermaphroditic, although ovaries and testes are usually not active simultaneously.
  • Ova are retained in the zooecium
  • free-swimming sperm are snared by the lophophore and conveyed to the ovum.
  • The zygote escapes as it transforms into a planktonic larva.

• Phylactolaemata (schematic): (No record)
  • Chitinous (i.e.soft) zoarium
  • All zooids similar
  • Zooecial walls incomplete, allowing zooids freely to share coelomic fluid.
  • Freshwater
  • Can propagate through statoblasts - little capsules of cells enclosed in calcareous capsules that form along the funiculus. In times of stress, when the zooid dies, these disperse. When proper conditions return, they open and a new ancestrula regenerates.

• Gymnolaemata: Ordovican onward, probably paraphyletic, box-like zooids. Characters:
  • Orifice protected by operculum
  • Zooecia box-like with nearly complete walls perforated only by pore plates thus...
  • Lophophore protraction requires deformation of zooecium. Achieved differently in different taxa.
  • Colonies include specialized non-feeding zooids, including
    • Vibracia
    • Avicularia.

  • Two major groups:
    • Ctenostomata: (Ord. - Rec.)
      • excellent fossil record
      • Membranous zooecia.
      • Lophophore protraction similar to that in phylactolaemates.
    • Cheilostomata: (Jurassic - Rec.) (Schematic)
      • excellent fossil record
      • encrusters with largely calcareous zoaria.
      • Lophophore protraction through action of protractor muscles deforming:
        • flexible frontal wall
        • Flexible

• Stenolaemata: (Ord - Rec).
  • Zooids and zooecia tall and cylindrical
  • Calcareous zooecia
  • Dominant bryozoans of Paleozoic.

  • Major groups:

    • Trepostomata: (Ord. - Tri.)
      • Three morphs of zooecia:
        • Autopores: Largest - presumed inhabited by typical zooids
        • Mesopores: Smaller - presumed inhabited by specialized zooids
        • Acanthopores: Very small, at the tops of small cones - surmounted by spine
      • Autopores and mesopores display diaphragms - sequential floors of zooecium, analogous to cniderian tabulae.
      • Zoarium surface has regular mounds - monticules.

    • Cyclostomata: (Ord. - Rec.) (Living)
      • Mural pores perforate zooecium

    • Cryptostomata: (Ord. - Per.)
      • True aperture of zooecium is hidden behind a vestibule :
      • Hemiseptum: separates vestibule and proper zooecium

    • Fenestrata: (Ord. - Per.)
      • Upright, lacy zoaria perforated by fenestrules allowing flow of water.
      • Zooecia occupy obverse side only.

    • Cystoporata: (Ord. - Tri.)
      • Zooecia separated by curved partitions known as cystiphagms.
      • Lunaria - crescentic projections surrounding aperture.

  Diversity trends: Gymnolaemata, while present through most of the Phanerozoic, display no strong trends and Phylactolaemata have no record. Among Stenolaemata, however, patterns are evident.

  • Early Paleozoic: Trepostomatids and Cryptostomatids dominant in Ordovician, but sharply curtailed by Ordovician extinction, never regaining dominance and suffering again in Devonian and Permian events, with only some trepostomatids straggleing into the Triassic.
  • Late Paleozoic: The age of Fenestrata, some of which are Mississippian index fossils. These are extinguished by Permian event.
  • Late Mesozoic: The rise of Cheilostomata.

  Evolutionary trends: Gymnolaemata, while present through most of the Phanerozoic, display no strong trends and Phylactolaemata have no record. Among Stenolaemata, however, patterns are evident.

  • Increased differentiation of zooids.
  • Increased calcification of zooecia. In Cheilostomata this is seen by the evolution of several inqenious mechanisms for calcifying the frontal wall without compromising the ability to protract the lophophore.
  • Increased frequency of solid upright zoria in deep-water bryozoans.

To Syllabus.

Last modified: 22 August 2008