Biol120 - Lecture 6: Worms and Body Cavities

Understanding the difference types of body cavities found in animals.
Understanding the diversity, adaptations and life cycles of parasitic worms
Understanding the key characteristics and diversity of parasitic Platyhelminthes.
Understanding the key characteristics and diversity of Nematoda.
Understanding the key characteristics and diversity of Annelida.
Understanding the role and function of metameric segmentation in Annelida.

Acoelomate: no body cavity; space between gut & body wall filled with mesodermal tissue.
Pseudocoelomate: “false” cavity; cavity lined by mesoderm externally only.
Coelomate: true coelom fully enclosed within mesoderm; permits complex organs.
Functional advantages of fluid-filled cavities:
• Hydrostatic skeleton (muscles work against fluid)
• Transport medium for nutrients, gases, wastes
• Shock absorption & organ protection

Body: dorsoventrally flattened, no body cavity.
Tegument (parasitic forms): protective, absorptive, derived mainly from mesoderm.
Major groups
• Class Trematoda (flukes)
• Class Cestoda (tapeworms)

$\approx 18\,000$ species; endoparasitic.
Two suckers: oral (feeding & attach) + ventral (attach).
Complex life cycle (2–3 hosts); 1st host usually snail.
Example Fasciola (liver fluke)
• Hermaphroditic adults in vertebrate → eggs → miracidium infects snail → asexual stages produce cercariae → encyst as metacercariae on plants → ingested by vertebrate.

$\approx 6\,000$ species; intestinal endoparasites; no digestive tract.
Body divided into:
• Scolex (attachment)
• Strobila = chain of proglottids (immature → mature → gravid with eggs).
Tegument (“Neodermis”): secretory, absorptive, continually renewed.
Life cycle usually $3{-}4$ hosts; vertebrate definitive host.
Selected examples
• Dipylidium caninum (flea tapeworm): flea intermediate, dogs/humans final.
• Echinococcus granulosus (hydatid): dog final, ungulate intermediate; hydatid cysts with massive larval multiplication.

Bilateral, unsegmented, covered by cuticle; only longitudinal muscles.
Pseudocoel = hydrostatic skeleton & transport system.
Diversity: $\ge 12\,000$ described (est. $\le 5\times10^5$ ).
Habitats: nearly all; parasitism evolved $\ge 15$ times.
Medically/veterinary important taxa
• Ascaris spp. – intestinal; $\sim1.2$ billion infections.
• Dracunculus medinensis – Guinea worm; copepod vector.
• Filarial worms (Wuchereria, Brugia, Dirofilaria) → elephantiasis, dog heartworm; mosquito vector.

Traits: bilateral, triploblastic, true coelom, protostome, metameric segmentation, setae (lost in leeches).
Benefits of segmentation:
• Redundancy; localized damage not fatal.
• Enhanced locomotion; segments operate semi-independently.
Coelom enables closed circulatory system & muscular gut (efficient digestion).

Class Polychaeta
• Marine, many setae/parapodia.
• Subclass Errantia – active swimmers/crawlers.
• Subclass Sedentaria – tube dwellers, filter feeders.
Class Euclitellata (with clitellum, no parapodia)
• Subclass Branchiobdellida – ectosymbionts of crayfish.
• Subclass Oligochaeta – earthworms; few setae; soil processors.
• Subclass Hirudinea – leeches; no setae, anterior & posterior suckers; many free-living predators/parasites.

Trematodes & cestodes: hermaphroditic adults, complex multihost cycles.
Earthworms: hermaphrodites, reciprocal sperm exchange; clitellum secretes cocoon for external fertilisation.

Platyhelminthes: acoelomate; many parasitic; tegument for protection.
Nematoda: pseudocoelomate; cylindrical; cuticle; huge diversity; numerous human/animal diseases.
Annelida: coelomate; segmented; closed circulation; diverse lifestyles (polychaetes, earthworms, leeches).