BIO 385 Invertebrate Zoology
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Invertebrate Diversity
Minor Phyla of Uncertain Relationship

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Phylum Gastrotricha


  • Minute (<1 mm) unsegmented animals
  • Body moderately elongated, somewhat flattened
  • Epidermis with plates or spines
  • Cilia on ventral surface
  • Sensory bristles on head
  • Typically have "bowling pin" shape: rounded head with slight neck constriction
  • Many have a pair of posterior projections (caudal furca)
  • In fresh water and marine sediments
  • Phylogenetic relationships uncertain
Lepidodermella Gastrotrich
Fresh-water gastrotrich, Lepidodermella sp.
Chaetonotus Gastrotrich
Fresh-water gastrotrich, Chaetonotus sp.
Macrodasys Gastrotrich
Marine gastrotrich, Macrodasys sp.

Phylum Chaetognatha Arrow Worms


  • Elongate, streamlined bodies
  • Lateral & caudal fins supported by rays
  • Mouth surrounded by grasping spines
  • Complete gut
  • Longitudinal but not circular muscles
  • Pelagic marine predators
Arrow Worm
Sagitta sp., stained whole mount
Arrow Worm
Sagitta sp., closeup of head; stained whole mount
Arrow Worm
Spadella cephaloptera (Encyclopedia Britannica 1911)

Phylum Entoprocta (Kamptozoa)


  • Solitary or colonial, often stalked
  • Tiny size (usually <2 mm)
  • Ring of ciliated tentacles surround mouth and anus
  • U-shaped gut
  • This phylum may not be represented in lab

Solitary Entoproct, possibly Loxosoma sp., with bud

Solitary Entoproct, possibly Loxosoma sp., tentacles retracted

Nodding Head Entoproct, possibly Barentsia sp.; CA
This page last updated 19 March 2017 by Udo M. Savalli ()
Images and text Udo M. Savalli. All rights reserved.