BIO 385 — Invertebrate Zoology
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Invertebrate Diversity
Basal Animals

(Click on any image for larger version)

Phylum Placozoa


  • Flattened, asymmetric; can change shape
  • 2 cell layers separated by gel-like mesenchyme
  • Small, <2 mm diameter
  • Glides (using flagella) over substrates in shallow marine waters
  • Only 1 described species
Trichoplax adherens
Placozoan, Trichoplax adhaerens
Trichoplax adherens
Placozoan, Trichoplax adhaerens

Phylum Ctenophora


  • Diploblastic; endoderm & ectoderm separated by mesenchyme
  • Biradial symmetry
  • Gastrovascular cavity with anal pores
  • No alternation of generations; no sessile stage
  • 8 combs (rows of ciliary plates)
  • Most with pair of long tentacles
Sea Gooseberry
Sea Gooseberry, Pleurobranchia sp.
See also labeled photo.
Lobed Comb Jelly
Sea Walnut, Mnemiopsis leidyi
Lobed Comb Jelly
Beröe's Comb Jelly, Beroe cucumis

Phylum Acoelomorpha (or Acoela)


  • Triploblastic, acoelomate
  • Bilaterally symmetric
  • Minimal cephalization; lack central nervous system
  • Small size (<2 mm); dorso-ventrally flattened
  • Lack digestive cavity: feed via mass of mid-ventral digestive cells
  • Free-living in marine environments
  • Formerly considered part of Phylum Platyhelminthes (specifically Turbellarians)
  • Phylogenetic position debated: either basal bilaterians or basal deuterostomes
Acoel flatworm
Acoel flatworm, possibly Amphiscolops sp.?; the brown structures are endosymbiotic dinoflagellates
Acoel flatworm, possibly Amphiscolops sp.?
Unidentified Acoel flatworm. Note statocyst.
This page last updated 15 December 2014 by Udo M. Savalli ()
Images and text © Udo M. Savalli. All rights reserved.