BIO 385 Invertebrate Zoology
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Invertebrate Diversity
Phylum Platyhelminthes
The Flatworms

(Click on any image for larger version)

Platyhelminthes Characteristics

  • Triploblastic, acoelomate
  • Bilaterally symmetric
  • Cephalization; with central nervous system
  • Elongate, dorso-ventrally flattened
  • Incomplete gut (gastrovascular cavity) or lack gut
  • Free-living and parasitic forms

Class Turbellaria Free-living Flatworms


  • Free-living; mostly aquatic
  • Epidermis ciliated
Planarian, Dugesia sp., carbon-fed whole mount
See also labeled photo.
Dugesia section
Planarian, Dugesia sp., stained cross-section (40x)
See also labeled photo.
Brown Planarian, Dugesia trigrina
Unidentified fresh-water flatworm; AZ
Freshwater flatworm, Mesostoma sp.
A primitive, freshwater flatworm, Stenostomum sp.
Fresh-water planarians, possibly Phagocata velata, KY
Brown Flatworm, Notoplana acticola, in tidepool, CA
LandP lanarian
Land Planarian, Bipalium kewense, a terrestrial flatworm native to SE Asia but widely introduced elsewhere; Phoenix, AZ

Class Trematoda Trematode Flukes


  • Internal parasites with often complex life cycles
  • Epidermis forms tegument
  • Attach to host via oral and ventral suckers
Sheep Fluke
Sheep Liver Fluke, Fasciola hepatica, a parasite of sheep livers.
See also labeled photo.
Sheep Fluke Larvae
Sheep Liver Fluke, Fasciola hepatica, Cercariae (left) and Miracidia (right) larval stages found in snails (stained whole mounts).
Schistosome, Schistosoma japonicum; fresh-water snails are intermediate host and various vertebrates are definitive host (stained whole mount).
Liver Fluke
Chinese Liver Fluke, Clonorchis sinensis; stained whole mount.
See also labeled photo.
Liver Fluke 2
Chinese Liver Fluke, Clonorchis sinensis; 2nd example.
See also labeled photo.

Class Monogenea Monogenetic Flukes


  • External parasites (or in open cavities) of aquatic organisms
  • Distinctive posterior attachment organ
  • Simple life cycle
  • No specimens available in lab
Probable unidentified monogenean parasite in the mantle cavity of a squid

Class Cestoda Tapeworms


  • Parasites of vertebrate guts
  • Epidermis forms tegument
  • Attach to host via anterior scolex
  • Trunk (strobila) greatly elongated, consists of numerous segmented proglottids
Dog Tapeworm Preserved
Dog Tapeworm, Taenia pisiformis, a common intestinal parasite of dogs and pigs
Dog Tapeworm
Dog Tapeworm,Taenia pisiformis, microscopic view of solex, proglottids.
See also labeled photo.
Sheep Tapeworm
Sheep Tapeworm,Moniezia expansa, stained slide of different body regions.
This page last updated 28 June 2015 by Udo M. Savalli ()
Images and text Udo M. Savalli. All rights reserved.