Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 33.3 Lophotrochozoans, a Clade Identified by Molecular Data, Have the Widest Range of Animal Body Forms
Lophotrochozoa is a major clade of bilaterally symmetrical animals identified through molecular data. It includes a wide variety of animal phyla—many of which exhibit highly diverse body plans—and features such as trochophore larvae or lophophores in some members. This group includes flatworms, rotifers, ectoprocts, brachiopods, molluscs, and annelids.
Flatworms (Phylum Platyhelminthes)
- Live in marine, freshwater, and damp terrestrial habitats
- Dorsoventrally flattened bodies increase diffusion surface
- Lack a body cavity and specialized gas exchange organs
- Have protonephridia for osmoregulation and excretion
- Gastrovascular cavity with one opening; no circulatory system
Major groups:
- Catenulida: Mostly asexual freshwater species ("chain worms")
- Rhabditophora: Diverse group with both free-living and parasitic members
Free-living rhabditophorans:
- Planarians (e.g., Dugesia): Have eye spots and chemical-detecting flaps
- Reproduce asexually by fission or sexually as hermaphrodites
Parasitic rhabditophorans:
- Trematodes: Complex life cycles (e.g., Schistosoma)
- Tapeworms: No digestive tract; absorb nutrients in host intestines
Rotifers and Acanthocephalans (Phylum Syndermata)
- Rotifers: Microscopic with complete alimentary canal
- Use cilia to draw in food
- Reproduce via parthenogenesis
- Bdelloids survive long-term without sex
- Acanthocephalans: Parasitic with spiny heads; manipulate host behavior
Ectoprocts and Brachiopods
- Use lophophores for suspension feeding
- Ectoprocts: Colonial with hard exoskeletons; reef contributors
- Brachiopods: Shells are dorsal and ventral, unlike clams; many extinct
Molluscs (Phylum Mollusca)
- Second most diverse animal phylum
- Body plan: muscular foot, visceral mass, mantle
- Most have a shell and use a radula (except bivalves)
- Open circulatory system (closed in cephalopods)
- Marine species have trochophore larvae
Major clades:
- Chitons: 8 dorsal plates; cling to rocks
- Gastropods: Snails/slugs; terrestrial and aquatic; use radula
- Bivalves: Clams, oysters; no radula; filter feeders
- Cephalopods: Squids, octopuses; closed circulatory system, beak, complex brain
Annelids (Phylum Annelida)
- Segmented worms with a true coelom
- Two main clades:
- Errantia: Marine, mobile; have parapodia
- Sedentaria: Includes earthworms and leeches
- Earthworms: Hermaphroditic, improve soil
- Leeches: Secrete anesthetics and anticoagulants like hirudin
In a Nutshell
Lophotrochozoa is the most diverse bilaterian clade, united by molecular evidence but highly variable in form. From parasitic tapeworms to intelligent squids and soil-enriching earthworms, this group illustrates the wide range of adaptations that have evolved across animal phyla.