Lab 6: Annelida

Fig. 8. Schematic diagram of a pogonophoran (from Brusca X2 Invertebrates).Echiura (several families; formerly a phylum)Echiurans are unsegmented, coelomate worms, with a fairly simple morphology. They are largely infaunal in softsediments (extending to the deepest ocean depths, where they can be common), although several species nestle increvices. Recent evidence suggests that echiurans are highly derived polychaetes that have lost segmentation, butwhich retain a couple of chaetae in most species. They are selective deposit feeders that use their proboscis togather surface sediments. In some echiurans (family Echiuridae) the proboscis is chunky and short and both sexesare large. In the Bonellidae the proboscis is spaghetti-like, with a T-shaped end, and extends to many times thelength of the body. Bonellids have dwarf, degenerate males, which are not much more than bags of testes, and livewithin the female nephridium-gonoduct complex. Sex determination in bonellids is environmental: larvae that settleon females become males; those settling freely develop into females. Study the worms provided, noting theproboscis and body; find the chaetae in echiurids.Fig. 9. Three genera of echiurans: A. Echiurus, B. Listriolobus, C. Bonellia (left), echiuran anatomy (right) (fromBrusca X2 Invertebrates).CLITELLATA – OligochaetesOligochaetes are characterized by segments that have but two small clumps of chaetae and lack parapodia; they alsolack the various head appendages found in most polychaetes. They differ substantially in their reproductive biology