Coelomate Worms
Coelomate Worms: Sipunculans, Pogonophorans, Echiurans and Vestimenifera Ectoderm Mesoderm Dt Endoderm Coelom Often dismissed as minor or lesser Comprise less than 600 species total Never abundant, often in extreme environments Not ecologically important as some worms Sipunculans, echiurans = protostomes Pogonophorans, vestimentifera = protostome/deuterostome characteristics
Sipunculids = little tube Peanut worm Sausage shaped Retractible introvert Range between 1-50cm Benthic Exclusively marine Burrowers, inhabit other shells, tubes etc. Intertidal - 5000 m In Vietnam occasionally consumed as food
Sipunculid Bauplan Built similar to annelids No segmentation Intestine = twisted U- shaped tube Introvert = retractor muscle Mouth with tentacles (60-150) Large, fluid filled coelom Hemerythrocytes No circulatory/ respiratory system
Sipunculid Physiology Movement with help of hydrostatic skeleton! Peristalsis (annelids)! circular/longitudinal muscles Feed on particulates in the sediment!introvert and tentacles Tentacles/oral for gas exchange Dioecious, no sexual dimorphism Gametes do not grow or ripen in gonads = small at all times Trochophore larva
Sipunculid Taxonomic Conundrum Sometimes grouped with annelids However: No segmentation No setae (bristles) Shared characteristics are general Also been linked to Molluscs: Early development Trochozoa, which could include annelids
Echiurans = sausage or spoon worms All marine Live in U-shaped tubes Bilateral symmetry Molecular phylogeny places them with annelids Lack metamerism 150 described species Few mm - 50cm Diagnostic proboscis = may be 2 m long i.e. Bonellia viridis Males either live on female or in groups of 20
Echiuran Bauplan Divided into proboscis and trunk Proboscis = large & flattened! homologous with prostomium in annelids! Not retractible Few setae = often just 2 ventral setae Complete digestive tract Nerve ring around gut connecting to cord
Echiuran Physiology Simple closed circulation = dorsal and ventral longitudinal bv = hemoglobin and erythrin Feed on epibenthic detritus Mucus coating on proboscis Metanephridia (1-100 pairs) Cloaca and hindgut for gas exchange Bonellin
Echiuran Reproduction Dioecious Some reproductive at 7mm others at 2m (Japanese) Most extreme sexual dimorphism Males often live on or in female = parasitic Males with large genital setae Sex determination environmentally determined = settlement near female => male Trochophore larva Asexual reproduction unknown = no regeneration
Pogonophora and Vestimenifera Pogon = beard, phor = to bear Vestimentum = garment, ferre = to bear 145 species Strange, vermiform marine creatures = beard worms Live in thin tubes buried in substrate, at depths from 200-10,000m Most with a long thin body, no segmentation Less than 1mm diameter, 10-75 cm long Ristia (Vestimenifera) reaches 3 m in length
Characteristics: Shared and Separate Bilateral, coelomate Hind part appears metameric Lack digestive system in adults Closed circulation Marine, benthic tube dwellers Pogonophora Anterior cavity developed Three nerve cords bearing ganglia Serially arranged coeloms Apparently without gut at all times Vestimenifera Anterior cavity not developed Single nerve cord Coeloms serially arranged and paired Transitory gut in juveniles
Body divided into 4 regions: Anatomy 1. Prosoma = cephalic lobe, 1-200 Tentacles with pinnules 2. Mesosoma = forepart 3. Metasoma = trunk, longest body region, bears annuli, papillae and ciliary tracts 4. Opisthosoma = last part to be discovered prosoma = cephalic lobes forepart trunk Vestimeniferan anatomy under debate 1. Obturaculum 2. Tentacle bearing 3. Vestimentum = wings
Some Comments on Phylogeny Evolutionary relationships = difficult to evaluate - almost no fossil record - no agreed upon ancestor Embryonic: S & E = protostomes! treated as members of annelids, early polychaete P = tripartite structure = early deuterostome?