Phylogeny (briefly) Closely related to Platyhelminthes? Post Platyhelminthes? Big question: Do they have a coelom?
Diversity Mostly free-living Primarily predators Habitats Benthic marine Ex. 1: Mussel beds Ex. 2: Antarctic (big!) Some in freshwater and terrestrial A few are parasitic One type in Dungeness crabs
Bauplan basics Similar to Platyhelminthes Triploblastic (distinct mesoderm muscles) Bilateral symmetry & cephalization Share similar features of protostome development Coelom?
Feeding/Digestion Eversible proboscis: Class Anopla Stored inverted within the rhynchocoel Rhynchocoel from coelom? Separate from digestive system Associated muscles Location/Functions Surface glands secrete Adhesives Toxins KNOW how proboscis everts & captures prey! Food intake? Diagram: Pearse/Buchsbaum Living Invertebrates
Eversible proboscis: Class Anopla
Feeding/Digestion Eversible proboscis: Class Enopla Differences from Anopla Common opening with digestive system = mouth Proboscis eversion Eversion of foregut. Why? Stylet at end of proboscis Additional neurotoxins Central channel; toxins ejected next to stylet Reserve stylet sac! Diagram: Pearse/Buchsbaum Living Invertebrates
Focus: reserve stylet sac
Feeding/digestion Digestion Complete digestive system Some specialization of regions Extra- and intracellular digestion Eversible foregut extracorporeal Movement through tract Ciliary Peristalsis of body wall NOTE: No muscle surrounds digestive tract!
Circulation Closed circulatory system Partly coelomic in origin? Blood vessels and lacunae Circulation primarily via peristalsis of body wall. Blood cells with respiratory pigments (including Hb) Functions Transport of nutrients, gases, hormones, waste; Additional metabolism Hydrostatic skeleton
Gas exchange Outer body surface Shape favorable S/V ratio Some larger forms: gas exchange via digestive tract Irrigation of foregut
Osmoregulation/excretion Protonephridia Most important in which habitats? Absent in deep-sea, pelagic forms Role in excretion? Associated with blood vessels Behavioral osmoregulation Mucus covering Burrowing
Excretion Ammonia loss primarily via…?
Nervous system Organization (briefly) Somewhat similar to Platyhelminthes Two lateral nerve cords Often an additional dorsal nerve cord Cerebral ganglia more developed Extensive innervation from anterior sensory structures
Sensory structures Adapted for active, predatory lifestyle Sensory structures concentrated at “head” Additional sensory structures on entire body
Sensory structures Chemoreceptors Locate prey & mates Cerebral organ Function of ciliated canal? Other functions of organ?
Sensory structures Chemoreceptors Frontal sense organ Chemosensory? Ocelli Mostly pigment-cup 1 pair to many Some with lenses Negatively phototaxic Nemerteans usually active nocturnally
Movement Mesoderm-derived muscles Primarily longitudinal & circular Peristalsis, etc… Fluid-infiltrated mesoderm & circulatory system allow for hydrostatic skeleton Cilia-mucus
Reproduction Asexual reproduction Transverse fission is common Small fragments new individuals
Reproduction Sexual reproduction Gonads develop within mesenchyme Along body length Mass mating Mostly external fertilization May occur in mucus sacs Spawning via temporary pores or body wall rupture Sometimes internal fertilization Males with claspers or penis
Reproduction Sexual reproduction Early development Holoblastic Spiral cleavage Determinate cell fate Mesoderm usually from 4D cell Indirect or direct development Pilidium larvae