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Phylum Mollusca “soft bodied” animals Ch 12 Chordata Arthropoda Annelida Mollusca Protostomes † Triploblasts * Diploblasts Hemichordata Echinodermata Deuterostomes Coelomates Pseudocoelomates Acoelomates Bilateria† Radiata* Eumetazoa Mesozoa Metazoa Protozoa Monera (Bacteria) Cnidaria Ctenophora Parazoa Mollusca Characteristics Triploblastic Organ level of organization Bilateral Symmetry Cephalization Eucoelomate Protostome Eucoelomate Have a “true” body cavity that is completely surrounded by mesoderm ectoderm coelom mesoderm Gut endoderm Mollusca Characteristics Gas Exchange • mainly gills, however terrestrial species have evolved “lungs” Circulatory System • open circulatory system (in most classes) • heart and blood sinuses Excretion • most have kidneys (metanephridia) Nervous system • ventral nerve cords, sensory organs Mollusca Characteristics Digestive System • complete with regional specialization • Radula: a tongue-like organ bearing rows of teeth that is used in feeding (scraping, drilling, etc.) hole made by a moon snail radula Cross section of a Gastropod head Mollusca Characteristics Reproduction • monoecious and dioecious species exist • usually internal fertilization • indirect development with the presence of a trochophore larva (link to annelids), and in most cases a veliger larva trochophore veliger Mollusca Characteristics Body Plan • head-foot: contains sensory organs and muscles • visceral mass: contains digestive, reproductive, circulatory organs • mantle: secretes the shell (if there is one), cavity contains gills or lungs •Shell: in most groups. shell Visceral mass foot mantle mantle cavity (Hypothetical Ancestral Mollusc) gonads Mantle coelom Shell Nephridium Anus Digestive gland head Gill Mouth Radula foot Nerve collar Nerve cords Intestine Molluscan Radiation • There are approximately 93,000 living species in phylum Mollusca (70,000 are extinct) • The great morphological diversity is the result of elaboration on the basic body plan Molluscan Radiation Gastropoda torsion coiling of the shell Scaphopoda ventral shell fusion Polyplacophora Solenogastres Caudofoveata shell consists of 8 plates HAM Shell-less but scales or spicules present Bivalvia bivalved shell dorsal hinge Cephalopoda Monoplacophora Cap-like shell lobed foot highly developed head shell reduced or lost Phylum Mollusca Class Polyplacophora Class Gastropoda Class Cephalopoda Class Bivalvia Class Scaphopoda Class Monoplacophora Class Soenogastres (Aplacophora) Class Caudofoveata Class Polyplacophora the “chitons” Class Polyplacophora • have 8 rows of articulating plates • use radula to graze algae on substrate • mantle forms a girdle around plate edge • no veliger larvae girdle plates Live in Intertidal Zone All Marine More Anatomy of a Chiton! Cryptochiton sp. • The Gumboot Chiton The gumboot chiton • the largest species of chiton in the world, growing up to 30 cm in length • eight plates are covered up by thick, leathery, brick-red flesh so we cannot see them. Cryptochiton sp. Class Gastropoda snails and slugs Gastropoda Origin and Evolution 1. 2. 3. 4. Development of a head Dorsoventral elongation of the body Shell become complex retreat: coiling Torsion Torsion • Torsion is not the spiraling of the shell • The body behind the head was twisted 180 degrees counterclockwise • Organs reversed location • Gut twisted mouth mouth anus gills gills anus anus gills Coiling Planospiral shell Apex of shell extends out making the shell more compact Conispiral shell Shell shifts over body for better weight distribution Class Gastropoda Subclass Prosobranchia Subclass Opisthobranchia Subclass Pulmonata Class Gastropoda Subclass Prosobranchia • aquatic snails (marine and freshwater) • have undergone torsion and most have undergone shell coiling • have gills • all have shells • use radula for feeding; can be herbivores, carnivores, or detritivores In some, mantle extends over shell Class Gastropoda Subclass Prosobranchia • snails often lay eggs in protective capsules Naticidae Moon Snails Close-up of a moon snail drill hole. Gastropods in the subclass Prosobranchia all contain an Operculum on the shell. Class Gastropoda Subclass Opisthobranchia • sea hares and sea slugs (nudibranchs) • have undergone detorsion • have gills • sea hares have a reduced shell • sea slugs have no shell Nudibranchs Ecology • Molluscs have many different means of defense • many nudibranchs that feed on cnidarians collect and store the undischarged nematocysts in cnidosacs • they then can use these nematocysts for their own protection Class Gastropoda Subclass Opisthobranchia • sea hares usually crawl along the bottom but many can also swim by using their modified mantle as “wings” Class Gastropoda Subclass Opisthobranchia • all are monoecious • sea hares often form “mating chains” acts as male only act as both males and females simultaneously acts as female only Class Gastropoda Subclass Opisthobranchia • sea slugs often lay “ribbons” of eggs which stick to the substrate A Sea Hare Anaspidean A Sea Hare Anaspidean, Aplysia dactylomela A Sea Hare Anaspidean, Aplysia dactylomela A Sea Hare Anaspidean, Aplysia dactylomela Aplysia gigantea Internal Shell of Sea Hare Sea Hares Aplysia dactylomela
