Lecture 10 - Mollusks I Flashcards
Phylum Mollusca:
Free-living invertebrates with an unsegmented, soft body consisting of a visceral mass, foot, mantle, gills, and usually a hard, calcareous, univalved or bivalved shell.
Three Important Classes of Phylum Mollusca:
Class Gastropoda
Class Pelecypoda
Class Cephalopoda
Class Gastropoda:
Snails, Slugs, & Pteropods
Class Gastropoda Geologic Range
Upper Cambrian to Recent
Class Pelecypoda:
Clams, Oysters, Scallops, & Mussels
Class Pelecypoda Geologic Range
Lower Cambrian to Recent
Class Cephalopoda:
Octopus, Squid, Nautiloids, & Ammonoids
Class Cephalopoda Geologic Range
Upper Cambrian to Recent
Visceral Mass:
Main part of the body containing the vital internal organs
Foot:
Muscular organ extending from the visceral mass; it is modified in various molluscan groups as a structure used in swimming, crawling, and/or burrowing.
Gills:
Sheets or filaments of blood-filled tissue that serve as respiratory organs.
Mantle:
Sheet of tissue that secretes the shell and lines the inside of the shell.
Siphon:
Fold in the mantle forming a tubular structure for bringing water into the mantle cavity.
Radula:
Rasping, tongue-like structure in the mouth of some mollusks (found in gastropods, cephalopods, & polyplacophores, but not in pelecypods); it is lined with tiny, sharp, tooth-like hard parts for scraping and/or drilling.
Molluscan Shell Composition:
Conchiolin (organic material composing the periostracum on the shell surface)
Calcite (may be pure calcite, or else low-Mg or high-Mig calcite)
Aragonite
Molluscan Shell Microstructure:
Prismatic, Foliated, Nacreous
Prismatic:
Layer consisting of tiny, vertical prisms of calcite or aragonite
Foliated:
Layer consisting of imbricated plates of calcite.
Nacreous (“mother-of-pearl”):
Interlaminated sheets of aragonite and organic conchiolin.
Mollusscan Shell Shape:
Planispiral, Conispiral
Planispiral:
Coiled in a single plane (e.g., nautiloids & ammonoids)
Conispiral:
Coiled along a translating axis to form a cone (e.g., gastropods)
Apex:
Pointed end of the shell
Aperture:
Opening in the end of the shell opposite the apex.
Radial Ribs:
Raised ridges that radiate from the apex towards the aperture.
Growth Rings:
Ring-like ridges that parallel the aperture and represent shell growth.
Spines:
Long, sharp projections on the shell exterior.
Nodes:
Short, blunt projections on the shell exterior.
Class Gastropoda:
Mollusks with a distinct head that usually possesses eyes and sensory tentacles; mouth possesses a radula; foot adapted for crawling in most forms, but is adapted for swimming or burrowing in some forms; nervous system of head & body are distinct; heart has 2 or 3 chambers; general bilateral symmetry, which is secondarily reorganized during ontogeny by “torsion”; most (but not all) members of the class have a single, univalved, conispiral shell with one aperture; the shell is always composed of aragonite; 3 very diverse subclasses
Class Gastropoda Geologic Range
Cambrian to Recent
Three main subclasses of Class Gastropoda:
Subclass Prosobranchia Subclass Opisthobranchia Subclass Pulmonata
Subclass Prosobranchia:
Gastropods with gills in front next to head; a robust, external, conispiral shell; commonly have an operculum; very good fossil record
Subclass Prosobranchia Geologic Range
Upper Cambrian to Recent
Three main orders of Subclass Prosobranchia
Order Archaeogastropoda
Order Mesogastropoda
Order Neogastropoda
Order Archaeogastropoda:
Two gills; 3-chambered heart; no inhalent siphon; shell has a circular or oval aperture; mainly herbivores; mostly marine (a few fresh-water species)
Order Archaeogastropoda Geologic Range
Upper Cambrian to Recent
Order Mesogastropoda:
One gill; 2-chambered heart; well-developed siphon; shell typically has a siphonal notch; includes herbivores, scavengers & predators; exclusively marine
Order Mesogastropoda Geologic Range
Lower Ordovician to Recent
Order Neogastropoda:
One gill; 2-chambered heart; well-developed siphon; shell typically has a siphonal canal; highly specialized predators; exclusively marine
Order Neogastropoda Geologic Range
Upper Jurassic to Recent
Subclass Opisthobranchia:
Advanced marine gastropods with gills behind head; either a very thin, internal shell or no shell at all; most are active predators; exclusively marine; typically a very poor fossil record
Subclass Opisthobranchia Geologic Range
Middle Devonian to Recent
Subclass Pulmonata:
Advanced terrestrial gastropods with either a thin external shell or no shell at all; mantle adapted for breathing in air; mainly herbivores; continental environments; a very spotty fossil record found only in terrestrial sediments
Subclass Pulmonata Geologic Range
Pennsylvanian to Recent
Visceral Mass:
Torsion brings the stomach up and over the head
Foot:
Muscular organ modified mainly for crawling, but also for swimming and/or burrowing.
Gills:
Leaf-shaped structures of blood-filled tissue that serve as respiratory organs.
Mantle:
Tissue that lines inside of shell of prosobranchs or envelopes shell of opisthobranchs.
Siphon:
Fold in the mantle forming a single inhalent tube.
Radula:
Rasping, tongue-like structure in the mouth of all gastropods.
Protoconch:
Earliest portion of the shell, located at the apex.
Whorl:
One complete revolution of the conispiral shell.
Columella:
Pillar of shell material at the axis of coiling of the shell.
Aperture:
Large opening in the shell through which the foot protrudes.
Callus:
Thickened region of the shell along the inside margin of the aperture.
Lip:
Thickened region of the shell along the outside margin of the aperture.
Operculum:
Calcareous or chitinous “door”, which seals off the aperture when the foot retracts.
Anal Slit:
Notch in the lateral side of the aperture (only in pleurotomariids & some primitive limpets) to accommodate excretion from the anus.
Anal Aperture:
Second opening in apex of some limpets to accommodate excretion from the anus.
Siphonal Notch:
Notch in the anterior end of the aperture of most mesogastropods to allow the siphon to protrude.
Siphonal Canal:
Groove-like or tubular extension of the notch in the anterior end of the aperture of most neogastropods to conceal and protect the long siphon as it protrudes.
Ornamentation:
Ribs, spines, nodes, etc., on the exterior of the shell.
