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Biology 320 Invertebrate Zoology Fall 2005 Chapter 12 – Phylum Mollusca Part One Introduction Second largest phylum at 100,000 described spp. Chitons, snails, clams, and squids Rich fossil record, as calcareous shell preserves easily Seven classes Mostly marine, but some freshwater and terrestrial spp. Generalized Mollusc Also sometimes referred to as HAM (hypothetical ancestral mollusc) Seven classes share common features Amendments to general body plan Decent with modification General mollusc shape: Bilaterally symmetrical Dorsoventrally compressed Oval outline General mollusc structures: Poorly defined anterior head Dorsal visceral mass Ventral muscular foot Feeding apparatus known as radula Mantle Shell Foot Broad, flat, and muscular Located ventrally Functions in adhesion and locomotion Contains secretory mucus glands which facilitate locomotion Posses several pairs of pedal retractor muscles Connect foot to shell Contractions of theses muscles allow animal to pull shell over visceral mass and foot, or vice versa Mantle Dorsal covering of visceral mass Secretes shell Forms mantle cavity Dorsal pocket that seawater flows through Gills are housed here Important for many processes such as respiration, excretion, and feeding in some cases Shell Mantle epidermis secretes proteins and calcium salts that form the shell Three layers to the shell Periostracum – outermost and proteinaceous Ostracum – middle and calcareous Hypostracum – innermost and calcareous; may be nacreous Shell increases in size as animal grows Respiration Gills are termed ctenidia Several pairs in HAM One pair or one gill in modern molluscs Housed in mantle cavity Gills are attached to mantle via axis Axis houses branchial blood vessels Afferent – delivers deoxygenated blood from body to gills Efferent – delivers newly oxygenated blood from gills to heart; then on to body Leaf-like gill filaments radiate from axis Bipectinate if radiate from both sides of axis Monopectinate if only one row of gill filaments Interfilamentary water spaces separate individual gill filaments The position of the gills in the mantle cavity divides the cavity into inhalant and exhalant chambers Gill filaments have cilia that generate water currents Water enters inhalant chamber ventrally Passes between gill filaments Exits through the dorsal exhalant chamber This system is an example of countercurrent gas exchange, which is common in aquatic animals Water and blood flow opposite of each other Maintains diffusion gradients for O2 and CO2 Digestive System and Nutrition HAM is a browser; scrapes algae and small organisms off of substratum As a result of this feeding strategy, a great deal of substrate, minerals, and other inorganic particles are ingested Digestive system is adapted for browsing Ciliary sorting fields to separate food from non-food Foregut and hindgut are lined with cuticle to protect from abrasion Gut is less complicated in molluscs that feed on larger organic particles Foregut – mainly for ingestion Mouth Buccal cavity Radula housed here Salivary glands – particles are trapped in mucus and transported in strings by cilia Pharynx Radula Feeding apparatus Long chitinous structure consisting of many rows of curved teeth Supported by a connective tissue structure called the odontophore Protractor and retractor muscles control odontophore Food is scraped from substrate and pulled towards mouth Anterior radular teeth are oldest, and one to five new rows grow daily Midgut – sorting, digestion, and absorption Esophagus (considered to be part of foregut in Ch. 9) Stomach Located in visceral mass Site of extracellular digestion Possesses chitinous gastric shield Sorting fields that sort particles by type / size Digestive ceca Connect to stomach Produce enzymes and deliver to stomach Site of absorption, intracellular digestion, and nutrient storage Hindgut – elimination Intestine Anus Located in exhalant chamber Wastes are swept away during ventilation Coelom Coelomate Protostomes Small coelom Not used as hydrostat as most have a shell that serves as an exoskeleton Coelom houses heart, and gonads in HAM Spiral cleavage Schizocoely Blastopore becomes mouth Have not encountered pseudocoelomate animals yet We will discuss Phylum Nematoda later Hemal System Classified as an open system Blood is not always contained in vessels Hemocoelic tissues and organs are bathed in blood Hemal system components Heart Vessels Hemocoel sinuses Head Foot Visceral mass Hemolymph containing hemocyanin Excretion Pair of metanephridia in close proximity to pericardial cavity (coelom) Hemolymph filtration occurs in visceral sinus of hemocoel Urine is dumped into exhalant chamber of mantle cavity via nephridiopores Nervous System CNS Esophageal nerve ring with pairs of ganglia Two pedal and two visceral nerve cords Additional pairs of ganglia Cerebral ganglia – brain that receives sensory input from eyes, tentacles, and statocysts Buccal ganglia – controls odontophore Pedal ganglia – controls muscular foot Pleural ganglia – controls mantle Sensory organs Cephalic tentacles – located on head Ocelli – located on head Statocysts – located on foot Osphradia – located in inhalant chamber. Monitor incoming water for chemicals and sediment. Ciliary beating cessates if conditions are unfavorable Reproduction Gonochoric External fertilization Gonads attached to coelom (pericardial cavity) Gametes released into coelom, enter nephridia, and exit nephridiopores into exhalant chamber Most produce a trochophore larva Top shaped Girdle of cilia called prototroch Apical tuft of cilia Planktotrophic Complete gut Some have veliger larvae or direct development Class Polyplacophora Chitons Marine; many live intertidally Physically challenging habitat In some ways resemble HAM Shell composed of 8 overlapping plates Allows flexibility when conforming to shape of substrata Indistinct head lacking eyes and tentacles 800 spp. 3 mm to 40 cm (gumshoe, Cryptochiton) Can be red, brown, yellow, or green in color Mantle Mantle covers entire dorsal surface, including (partially or entirely) the valves Thick and stiff Has lateral overhangs around mantle cavity Referred to as the girdle Shell Eight overlapping valves Name Polyplacophora means “bearer of many plates” Lateral insertion plates of valves are embedded in mantle tissue Variation in the amount of valve exposed Pair of pedal retractor muscles for each valve Four layers to each valve Second outermost layer possesses sense organs called esthetes (more on this later) Locomotion Similar to HAM Negatively phototactic, so creep away from light Found in crevices; under ledges and rocks If dislodged from substrate (by a wave, predator, etc.), chitons can contract their longitudinal enrollment muscles, and roll into a ball During adhesion, mantle and foot make contact with substrate Great deal of suction Almost impossible to remove animal without harming it Respiration Chitons have two lateral mantle cavities, as opposed to HAM’s one dorsal cavity Located in groove between the foot and mantle 6 to 88 bipectinate gills are located on each side When animal lifts anterior girdle, two inhalant apertures form Gill cilia beat to draw water posteriorly Water exits via one medial exhalant aperture Nutrition Most are browsers Up to 75% of gut contents may be sediment Some feed on seaweeds Some are carnivorous and may use mantle to trap small animals Chitons posses very long radulas Teeth may be capped with an iron-containing mineral called magnetite Digestive system is similar to HAM, with exceptions Subradular organ Located in buccal cavity Chemosensory organ that is extended like a tongue If food is detected, odontophore is extended Two salivary glands Also associated with buccal cavity Secretes mucus into which particles are trapped Strings of mucus with particles are moved to esophagus Esophagus Two esophageal glands dump amylase into esophagus Circulation and Excretion Hemal system is similar to HAM Posterior coelom (pericardial cavity) Two huge nephridia in lateral hemocoel Nervous System Unlike HAM, chitons lack ganglia Posses an anterior nerve ring that surrounds the anterior gut Four longitudinal nerve cords Two pedal - ventral Two visceral - lateral Commissures give nervous system a ladder-like appearance Sensory organs Subradular organ One osphradium in each mantle cavity Esthetes Unique to chitons Found on valves Consist of many canals that traverse the layers of the valves High density of canals: up to 1750 / mm2 Function is disputed Reproduction Gonochoric Single, large, median gonad Located in dorsal hemocoel, just anterior to coelom Two gonoducts that empty directly into exhalant chamber Gametes do not pass through coelom or nephridia Mostly external fertilization, but some internal fertilization occurs in mantle cavity of female Trochophore larva or direct development