Eucoelomates most invertebrate coelomates are protostomes animals having a true coelom are referred to as being in deuterostomes, the coelom appears as eucoelomates outpocketings of the archenteron eucoelomate animals have a body cavity that is echinoderms and chordates and a few minor phyla are deuterostomes completely lined with mesoderm the mesoderm can develop into muscle layers & internal skeletal elements ! a large fluid filled coelom surrounded by muscle layers makes a more effective hydrostatic skeleton in many worms ! mesodermal layers lead to development of arteries and veins, ie circulatory systems; better blood supply to all internal organs ! since mesoderm lines the digestive tract, this allows for the development of much more elaborate digestive organs eg. compare the digestive tract of earthworms to Ascaris. ! mesenteries to support internal organs there are 2 main ways that a true coelom can develop in an embryo: in protostomes the coelom appears as a split in the mesoderm layer of the embryo Animals: Phylum Mollusca; Ziser Lecture Notes, 2012.10 1 Animals: Phylum Mollusca; Ziser Lecture Notes, 2012.10 2 Animals – Molluscs the polar seas 110,000 living; 70,000 fossils occur from the bottom of the oceans to 7000 M above sea level second largest phylum of animals in terms of number of known species typical fauna of all parts of the ocean, ponds, lakes, streams and rivers, mudflats, intertidal most versatile body plan of all animals and terrestrial habitats range from fairly simple organisms to some of the bottom feeders, planktonic, burrowers, borers, most complex and specialized of invertebrates pelagic forms includes: snails, limpets, clams, mussels, chitons, one group, cephalopods, are considered the most octopus, squid, oysters, slugs, nautilus, tooth intelligent of all invertebrates shells the phylum originated in the sea and most of them good fossil record; since most secrete a shell remain there all living classes were well established early in the ! only bivalves and snails moved to brackish fossil record and freshwaters but did not become dominant until the ! only snails invaded land brachiopods mostly died out in the Permian Extinction (~250 MY ago) mollusks are closely related to segmented worms microscopic to 20 M (50-60’ = giant squid) and up to 900 kg (1980 lbs; ~ 1 ton) ! same larval form = trochophore eg. Tridacna 1.5 M and 250 kg (500 lbs) Body Plan but most (80%) less than 10 cm (~4”) well developed bilateral symmetry molluscs are mostly aquatic; found from the tropics to most are unsegmented Animals: Phylum Mollusca; Ziser Lecture Notes, 2012.10 3 Animals: Phylum Mollusca; Ziser Lecture Notes, 2012.10 4 but is greatly reduced to a small space around a few primitive forms seem to show some segmentation. the heart most molluscs (except bivalves) have a well developed head it has become part of an open circulatory system bears mouth and various sense organs in some mollusks the mantle hangs down to create in spite of wide diversity of group most share mantle cavity around the internal organs basic features of body plan: houses respiratory organs; gills mantle - secretes shell or becomes outer body covering itself sometimes the mantle itself serves as a shell - (internal or external) radula – specialized feeding organ in mouth respiratory organ foot - usually used for locomotion products from the digestive, excretory & 1. Mantle reproductive systems empty into mantle cavity before release the body wall of mollusks consists of an outer layer of epidermis that extends over most of a continuous current of water is created by the animal as a mantle cilia to bring in food and oxygen and to remove wastes and gametes often contains various sense organs some molluscs are able to retract head or foot into eg. ocelli, sensory papillae mantle cavity for protection glands in epidermis secretes mucous, cement and sometimes a shell 2. Shell mantle is underlain by complex layers of muscle in most molluscs the mantle secretes a shell and connective tissues which serves as a protective exoskeleton the body cavity of molluscs is a true coelom great variation in shell form and structure Animals: Phylum Mollusca; Ziser Lecture Notes, 2012.10 5 Animals: Phylum Mollusca; Ziser Lecture Notes, 2012.10 6 shell made of calcium carbonate in some such as squid and octopus the shell is internal and greatly reduced calcium extracted from water, soil or food the mantle serves as the animals outer shell is composed of 3 layers: covering periostracum = outer 3. Radula especially thick in freshwater molluscs ! protects from acidity common in fw in mouth is tongue-like rasping organ = radula in many marine forms this layer is thin or absent found in all groups except bivalves and aplacophora (solenogastres) prismatic layer = middle hardened file-like, up to 250,000 tiny teeth dense prisms of calcium carbonate crystals in protein matrix the numbers and pattern of teeth are used to identify certain species nacreous layer = inner new rows of teeth are continually added to the gloss film of calcium carbonate sheets back of the radula ad the front teeth are secreted continuously through live worn down thickens the radula is supported by a cartilage-like rod produces irridescent “mother of pearl” (=odontophore) shell appears in embryo used to scrape, pierce, tear or cut food grows throughout life ! often growth lines radula also acts as a conveyor belt to move food usually used for protection toward the digestive tract 4. Foot but some bivalves use shell to rasp into wood or rock all molluscs have a thick muscular foot Animals: Phylum Mollusca; Ziser Lecture Notes, 2012.10 7 Animals: Phylum Mollusca; Ziser Lecture Notes, 2012.10 8 variously modified for locomotion: in mouth is tonguelike rasping organ = radula creeping movements - produced by waves of muscular contractions that move through foot most internal organs, including the digestive system or by creating a slime trail from mucous glands in the are embedded in solid tissue called the visceral foot mass, instead of being free inside a body cavity eg. snails ! gliding movement (coelom) or digging into sediment complete digestive tract divided into discrete, being extended from body into sediment functionally specialized regions: hydraulically by engorging with blood to anchor then draw rest of body into sediment foregut eg. bivalves eg. scaphopoda buccal cavity mouth, radula, salivary glands, esophagus or for attachment: receives and prepares food may secrete mucus to adhere to solid substrate midgut eg. limpets, chitons, land snails stomach and associated digestive glands sorting region, crushing region the foot of many bivalves contain a gland that produces crystalline style in some to grind food byssal threads (sea silk) for attachment some digestion takes place in digestive gland silky filaments of keratin and other proteins hindgut attached to substrate by extremely effective cement intestine usually long and coiled absorption of nutrients in some molluscs a portion of the foot is modified formation of feces (undigested residue) into a long tubular proboscis for feeding Respiration Feeding and Digestion most mollusks have folded, ciliated gills (=ctenidia) detritus feeders, burrowers, borers, grazers, carnivores, filter feeders, etc !thin feathery sheets of tissue covered with cilia Animals: Phylum Mollusca; Ziser Lecture Notes, 2012.10 9 Animals: Phylum Mollusca; Ziser Lecture Notes, 2012.10 10 oxygen carrying cells with pigments to improve also used for feeding in bivalves efficiency of oxygen transport: some mollusks breath through their skin haemocyanin (Cu) - most - blue some with hemoglobin (Fe) - red many terresrial snails lack gills (=pulmonates) some white blood cells !mantle is modified into a saclike “lung” for no clotting agents or thrombocytes breathing air cephalopods have a closed circulatory system Circulation Nervous & Endocrine Systems open circulatory system in most CNS is a ring of ganglia in head area with paired nerves and ganglia extending to other parts of the ! blood not entirely contained within vessels body works good for slow moving animals eg. pedal ganglia ! control foot ! the most active molluscs, the cephalopods, have a closed also well developed endocrine system circulatory system simple heart with a few vessels ! controls egg laying and growth heart with 2 auricles and 1 ventricle cephalopods show some intelligence extending from heart is aorta are active predators reduced coelom becomes a haemocoel surrounding ! can recognize prey with sharp eyes heart ! can learn by watching blood contains several kinds of cells: Excretion Animals: Phylum Mollusca; Ziser Lecture Notes, 2012.10 11 Animals: Phylum Mollusca; Ziser Lecture Notes, 2012.10 12 usually 1 pair of nephridia (=metanephridia) some freshwater bivalves produce a parasitic larva = glochidium often called kidneys (not really true kidneys) it attaches to gills of host fish and (metanephridium consist of a tube that opens into body cavity feeds on blood at one end and drains to the outside) in many species the nephridial tubule also acts as development is direct, ie. no larval stage: gonoduct in cephalopods, many freshwater snails and some bivalves some molluscs have several pairs of nephridia some excretion occurs through body wall and gills Reproduction & Development most molluscs are dioecious; some are monoecious many marine forms produce characteristic larva = trochophore same larval form is found in some flatworms, annelids and a few other minor phyla in some gastropods and bivalves