BIO 385 — Invertebrate Zoology
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Invertebrate Diversity
Basal Animals

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Phylum Placozoa

Characteristics

  • 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

Characteristics

  • 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 Xenacoelomorpha

Characteristics

  • Acoels and Xenoturbellids
  • Triploblastic, acoelomate
  • Bilaterally symmetric
  • Minimal cephalization; lack central nervous system
  • Small size (<2 mm for most); 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)
Brown Acoel
Brown Acoel, possibly Heterochaerus sp.; the brown structures are endosymbiotic dinoflagellates
Brown Acoel 2
Acoel Flatworm, possibly Amphiscolops sp.
Acoela3
Acoel flatworm, possibly Philocelis sp. Note statocyst.
This page last updated 3 May 2019 by Udo M. Savalli ()
Images and text © Udo M. Savalli. All rights reserved.