Bilateria

Classification of the Syndermata

  • Phylum Syndermata (sin-der-mah′tah)

    • Characteristics:

    • Syncytial epidermis

    • Intracellular skeletal lamina within the epidermis

    • Basal body structure comprised of microtubule doublets rather than triplets

    • Two major subphyla:

    • Rotifers:

      • Habitat: Freshwater and marine environments

      • Species Count: Approximately 2,000 species

    • Acanthocephalans:

      • Symbiotic relationships with vertebrates (definitive hosts) and crustaceans or insects (intermediate hosts)

      • Species Count: Approximately 1,400 species

Classes within Phylum Syndermata

  • Class Seisonidea (sy″son-id′e-ah)

    • Description:

    • Single genus of marine rotifers

    • Commensals of crustaceans

    • Body: Large and elongate with reduced corona

    • Example: Seison (only two species)

  • Class Acanthocephala (a-kan′′tho-sef′a-lah)

    • Description:

    • Elongate and wormlike structure

    • Eversible proboscis armed with recurved spines

    • Digestive system: Absent

    • Intestinal symbionts of vertebrates (definitive host) and crustaceans or insects (intermediate hosts)

    • Example: Macracanthorhynchus (approximately 1,400 species)

  • Class Bdelloidea (del-oid′e-ah)

    • Description:

    • Anterior end: Retractable with two trochal disks

    • Mastax (muscular structure): Adapted for grinding

    • Body shape: Cylindrical

    • Reproductive structure: Paired ovaries

    • Males: Absent

    • Genera: Adineta, Philodina, Rotaria (approximately 590 species)

  • Class Monogononta (mon″o-go-non′tah)

    • Description:

    • Rotifers with one ovary

    • Mastax: Not adapted for grinding

    • Egg types: Produce mictic (haploid) and amictic (diploid) eggs

    • Males: Appear sporadically

    • Genera: Conochilus, Collotheca, Notommata (approximately 1,400 species)

    • Note: The taxonomic rankings reflect traditional class names. The formal ranking for Acanthocephala has not been established.

Characteristics of Rotifers

  • Naming: Derived from the ciliated corona surrounding lobes on their heads

  • Habitat: Commonly found in freshwater environments

Common Characteristics:

  • Triploblastic: Possess three germ layers

  • Body plan: Bilateral symmetry, unsegmented, pseudocoelomate

  • Digestive system: Complete and regionally specialized

  • Anatomy:

    • Corona: Located at the anterior end

    • Toes with adhesive glands: Located at the posterior end

    • Cuticle: Well-developed

    • Excretory system: Protonephridia with flame cells

  • Reproduction:

    • Parthenogenesis common; males are rare or absent

    • Asexual reproduction generates genetic diversity without sexual input

    • Environmental influences can lead to the production of haploid (male) offspring

Life Cycle of Monogonont Rotifers

  • Terms:

    • Mictic: Refers to haploid eggs produced under certain conditions

    • Amictic: Refers to diploid female eggs produced under stable conditions

Acanthocephala

  • Habitat: Endoparasites primarily residing in the intestinal tracts of vertebrates, particularly fish

  • Life Cycle: Requires two hosts for completion

    • Juveniles: Parasitize crustaceans and insects

    • Adults: Parasites within the digestive tracts of vertebrates

  • Adaptation: Living syncytial tegument covers the body wall, adapted for parasitism

  • Sexual Characteristics:

    • Separate sexes; males possess a protrusible penis

    • Fertilization is internal

    • Eggs develop within the pseudocoelom

    • Notable for high biotic potential

Phylum Micrognathozoa

  • Discovery: Found in interstitial sediments of fresh water springs

  • Physical Characteristics:

    • Size: One of the smallest animals, averaging 1/10th of a mm long

    • Species: Limnognathia Maerski

    • Head Structure: Two-part head with a complex jaw made up of 15 parts

    • Nervous System: Large ganglion and paired nerve cords along the lower body

    • Body Features: Head, thorax, and abdomen carry stiff bristles with 1 to 3 cilia

    • Gut: Simple complete gut with anus opening periodically to the outside

    • Excretory System: Two pairs of protonephridia

  • Reproductive Mode: Likely parthenogenetic as no males have been observed

Phylum Gnathostomulida

  • Overview: Comprises over 100 species across 18 genera

    • Also called “jaw worms” due to unique jawed pharyngeal apparatus

    • Habitat: Found interstitially in marine sands, typically in low oxygen and sulfide-rich environments

  • Body Structure:

    • Shape: Minute, slender to thread-like

    • Body parts: Consists of head, trunk, and narrow tail regions

  • Physiology:

    • Nervous system includes sensory cilia

    • Digestive tract: Incomplete, likely uses diffusion for circulation and gas exchange

  • Reproductive Traits: Monoecious with cross-fertilization; protandry may be present

Phylum of Uncertain Affinity: Chaetognatha

  • Examples: Sagitta, Spadella; approximately 12 species

  • Description:

    • Dart-shaped marine worms, major components of plankton worldwide

    • All chaetognaths are carnivorous, preying on planktonic animals

    • Body: Coelomate, 2 to 120 mm long, covered in cuticle, distinct head, trunk, and tail

    • Features include hooked, prey-grasping spines and a mouth armed with tiny teeth

    • Body bears one or two pairs of lateral fins and a caudal fin; complex eyes and nervous system are present

    • Lack respiratory and circulatory systems; all species are hermaphroditic

  • Phylogenetic Relationships:

    • Fossil records indicate some chaetognaths date back to the Cambrian period

    • Historically classified among deuterostomes, ecdysozoan protostomes, and spiralians

    • Recent molecular studies classify chaetognaths within the Gnathifera as a sister group to the Syndermata

Evolutionary Connections

  • Learning Outcomes:

    • Justify the division of Spiralia into two clades: Lophotrochozoa and Gnathifera

    • Assess the validity of traditional platyhelminth classes

Phylogeny of the Spiralia

  • Figure Reference: Phylogeny based on molecular data from multiple studies

  • Notable Exclusions: Turbellarian orders omitted

  • Phylogenetic Status: Acoels and Xenoturbellida's status remains unsettled; some combine them into a single phylum, Xenacoelomorpha

  • Acceptance: The separation of Spiralia into Lophotrochozoa and Gnathifera is widely accepted

  • Platyhelminth Findings: Trematodes do not form a monophyletic lineage; relationships are being revised