Turbellaria
Free-living flatworms, mostly predators, with ciliated skin (traditional grouping, now considered paraphyletic).
The Turbellaria are a traditional grouping within the phylum Platyhelminthes, or flatworms, that historically included all flatworm subgroups that were not entirely parasitic, though some parasitic groups (e.g., the Temnocephalida) were also included. Modern taxonomy has largely abandoned Turbellaria as a valid clade because it is paraphyletic. This group contains roughly 3,000 to 4,000 described species of free-living flatworms, with sizes ranging from 1 mm to more than 600 mm (about 23.6 inches) in length. Most are free-living predators or scavengers that live in water or damp environments on land. The name "Turbellaria" comes from the small whirlpools of particles that form near the skin of aquatic species when their cilia beat.
All flatworms are triploblastic bilaterians with no internal body cavity (acoelomate) and no dedicated circulatory or respiratory organs. Gas exchange happens through simple diffusion, which limits body thickness. As a result, these animals are either microscopic or flat and ribbon- or leaf-shaped, and they lose moisture easily. The body is filled with mesenchyme, a connective tissue that can repair injuries and allows for asexual reproduction. The nervous system is concentrated at the front end.
Turbellarians lack a cuticle. In a few species, the skin is a syncytium—a single shared membrane enclosing many nuclei—but in most species, the skin is a single layer of cells, each usually bearing multiple cilia, though some large species have no cilia on their upper surface. Microvilli sit between the cilia. Numerous glands are embedded in the muscle layers beneath the skin and open through pores to secrete mucus, adhesives, and other substances. Small aquatic species move using cilia, while larger ones rely on muscular contractions of the whole body or a specialized sole to creep or swim. Some can burrow, anchoring their rear end and extending the head upward to feed before pulling back. A few terrestrial species throw a mucus thread to climb between leaves. Some larger aquatic species have a ruffled or ringed appearance due to folds in the body wall, but no turbellarians have true spicular skeletons or segmentation.
Most turbellarians are carnivorous, preying on small invertebrates or protozoans, or scavenging dead animals.
- Classification
- Traditional sub-division of Platyhelminthes (paraphyletic; largely abandoned in modern taxonomy)
- Species count
- About 3,000 to 4,000 described free-living species
- Size range
- 1 mm to 600 mm (about 23.6 in)
- Habitat
- Water or moist terrestrial environments
- Reproduction
- Simultaneous hermaphrodites; sexual and asexual reproduction
- Key feature
- No cuticle; ciliated skin; eversible pharynx in many; no spicular skeletons or segmentation
Lore & Background
Turbellarians are triploblastic, acoelomate bilaterians lacking specialized circulatory and respiratory organs, relying on diffusion for internal transport. Their bodies are filled with mesenchyme, a connective tissue that can regenerate injured tissues and permits asexual reproduction. Most are black, brown, or gray, but some larger species are brightly colored. Small aquatic species use cilia for locomotion, while larger ones use muscular movements of the whole body or a specialized sole to creep or swim. Some terrestrial species throw a thread of mucus to climb from leaf to leaf.
Reader's Guide
The Turbellaria are significant as the paraphyletic group from which all exclusively parasitic Platyhelminthes (the Neodermata) are descended. This means that the traditional 'Turbellaria' do not form a natural monophyletic group, as they exclude the Neodermata, which evolved from a small sub-group within the free-living Platyhelminthes. The Acoelomorpha and Xenoturbella, formerly included in Turbellaria, are now regarded as not being Platyhelminthes at all, with Acoelomorpha considered the sister group to all other bilaterians. Thus, the concept of Turbellaria has been revised in modern taxonomy, but the group remains important for understanding the evolution of flatworms and the origins of parasitism.
Did You Know?
- Despite the name 'Turbellaria' meaning 'little whirlpools,' these flatworms are not all free-living—some groups historically included here, like Temnocephalida, are parasitic or commensal.
- No turbellarian has a true spicular skeleton or segmented body; any ringed appearance comes from folds in the body wall, not internal skeletons.
More in Worms And Worm-Like Invertebrates 1-23
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