Mollusca pt.1

• Phylum Mollusca


  • Body plan: head, foot, visceral mass
  • Mantle (pallium)
  • Calcareous exoskeleton
  • Radula, supported by an odontophore (muscles)
  • Radula made by teeth like structure; Bivalves don't have radula
  • Ctenidia (gills)
  • Marine molluscs occupy a variety of substrates (rocky shores, coral reefs, mud flats and sandy beaches)
  • Reduced coelom
  • Generally the mollusc reduced and coelom
• Aplacophora


  Worm like, deep water species and absence of shell
• Monoplacophora


  Single shell of bulb
• Gastropoda


  Divided because of gills
• Bivalvia


  Divided because of dentation or teeth (pseudo cardinal teeth) and abductor muscle
• Chaetodermomorpha


  They feed on decaying matter or detritus
• Neomeniomorpha


  Solenogastres (synonyms) feed on small Cnidarians
• Nautiloidea


  Retained shells
• Coleoidea


  Reduce or loss squids and octopus
• Torsion in Gastropods


  1. Rotation of visceral mass, mantle and shell 180 degrees with respect to the head and foot

  2. Brings the mantle cavity and anus to an anterior position above the head
• Causes of Torsion


  • Growth of the foot muscle and its subsequent contraction
  • Differential tissue growth of the left-hand side of the gastropod compared to the right-hand side
• Advantages of Torsion


  • The animal can retract its head into its shell since the mantle cavity is now located over the head
  • The back-and-forth motion of the shell during movement would tend to block the mantle opening if it was in a posterior position. With the mantle anteriorly positioned, ventilation is better esp for terrestrial species
  • The anterior position of the osphradium (olfactory sense organ) enables the gastropod to locate food and avoid predators
  • The large unmanageable pretorsion shell was solved; with the obliquely positioned shell, the animal can now move with relative ease
• Disadvantage of Torsion


  • Fouling problem (because of aftermath of the torsion, before the anus was at the rear, it moved to near the mouth)
• Adaptation to the fouling problem

  1. Formation over the mantle cavity of a cleft or split in the shell and mantle

  2. Other gastropods developed an inhalant water circuit to the side of the head

  3. Some underwent detorsion, thus, reversing the twisting process (mantle cavity and anus once again opened posteriorly)
• In octopuses they exhibit chromatophores all over the body, a pigment cell for camouflage and communication
• Subclass Prosobranchia


  • Largest group; mostly marine; operculated; shell spirally-coiled, cupshaped or tubular; eyes on tentacles; mantle cavity anteriorly directed and near the head
  • Exhibit streptoneurous condition of CNS
  • 3 Orders: Archaeogastropoda, Mesogastropoda, and Neogastropoda
  • Includes abalones, limpets, periwinkles, cowries, whelks, top shells, helmet shells, etc.
• Order Archaeogastropoda


  • With linear series of perforations or slits in the shells
  • With two gills, usually two-chambered heart
  • Herbivorous; fertilization external
• Order Mesogastropoda


  • Chiefly marine; usually w/o nacre (not shiny innermost layer of the shell); mostly herbivorous; radula taenioglossate, with 7 teeth in each transverse row; about 30,000 species
  • Gills, kidney and osphradium displaced to the left side of the body; wastes discharged to the right side of the head-foot
  • Largest & most diverse order
  • Development of internal fertilization
• Ranellidae: Charonia tritonis


  Natural predator of crown of thorns sea stars (Acanthaster planci)
• Order Neogastropoda


  • About 16,000 species; With well-developed siphonal canal
  • Bipectinate osphradium (olfactory structure (smell) at the base of siphon
  • Rachiglossate (rasp-like) radula (3 denticles in each transverse row)
  • Sexes separate (dioecious)
  • All marine except Clea
  • Mostly predators, some saprophagous
• Conidae


  The aperture is elongated and the outer lip is prominent, the spire is also low, the body whorl has undergone development
• Piscivorous feeding


  Feeding on fish
• Molluscivorous feeding


  Feeding on molluscs (similar to but smaller than the molluscs)
• Vermivorous feeding


  Feeding on marine worms (Polychaetes)
• Subclass Opisthobranchia


  • Most of members are highly reduced or reduction or loss of shell
  • With chemosensory organ (sense of smell) called rhinophore
  • Exhibit detorsion
  • Gills located behind the heart
  • Hermaphroditic (monoecious)
  • Mostly marine
• Subclass Pulmonata


  • The gills become lungs
  • Some serve as host in Trematode larvae
• Class Bivalvia


  • Shell of two hinged valves (connected dorsally); body laterally compressed
  • No head or radula
  • Foot adapted for burrowing except in sedentary species; some attached to surfaces via byssal thread, or by cementing themselves to hard substances; scallops can swim by clapping movement
  • Exhibit various life habits
  • With strong adductor muscles (muscles that closes their valves)
  • Incurrent and excurrent siphon (use of siphon in Bivalves exchange of gases of food getting)
  • Trochophore and veliger larvae (free swimming)
  • Include mussels, clams, scallops, oysters, and shipworms (worm like if bivalve, capable of borrowing in wood)
• Dentition


  Cardinal teeth (near the umbo) and lateral teeth (far from the umbo)
• Equilateral


  If the umbo is in the middle
• Inequilateral


  If the umbo is leaning to the left or right
• The teeth are not used for cutting food, their function is to ensure the two valves lock in exactly
• Life habits of Bivalves

  • Epifaunal suspension feeder, one valve cemented to substrate (oysters)
  • Epifaunal suspension feeder, attached by byssus thread (mussels)
  • Infaunal siphonate suspension feeders, rock boring (Pholas)
  • Infaunal nonsiphonate suspension feeder, partly buried (penshells)
  • Infaunal nonsiphonate deposit feeder, partly buried (Nucula)
  • Infaunal siphonate deposit feeders, completely buried (Yoldia, Tellina)
  • Infaunal mucus tube feeder, deeply buried (lucinids)
  • Infaunal siphonate suspension feeder, deeply buried (Mya, Mercenaria)
  • Infaunal siphonate carnivores (Cuspidaria)
• Cephalopoda


  • Shell divided by septa, with chambers connected by the siphuncle (internal tubes that connects the chamber of the shell)
  • Shell reduced or lost in many species
  • Closed circulatory system (blood remains inside the blood vessels)
  • Hemocyanin (copper containing pigment) in blood
  • Complex chambered heart; 3 chambers (2 oracles, 1 ventricle)
  • Absence of cilia in gills
  • Foot modified to form flexible arms and siphon
  • Ganglia fused to form a large brain encased in a cartilagenous cranium
  • Well-developed sensory and motor function (considered to be the most cognitively advanced group of invertebrates)
  • Usually dioecious; spermatophore common; without larva
  • Chitinous beak / radula
  • Chromatophores/ink gland; carnivorous (they produce pigments to help the animal achieve general resemblance to the substrate, it also allows the animal to change color)
  • 17,000 named species of fossil cephalopods and 800 identified living species
• Evolute


  Coiled shell in which the whorls touch but do not overlap; all whorls are exposed in side-view. Also called serpenticone.
• Convolute


  Tightly coiled shell in which the last whorl covers much of the preceeding whorl
• Involute


  Tightly coiled shell in which the last whorl covers all of the preceeding whorls, as in Nautilus.
• Cephalopod eyes


  • Only one type of photo receptors, only capable of seeing in grey scale
  • Presence of ring shaped muscular fold of skin around the eye that can close to protect the eye
• Squid


  • Arms used for prey handling, manipulating objects, and reproduction
  • Arm #4 hectocotylus secretes sperms
  • Chromatophores
  • Swimbladder for buoyancy
• Nautiloids


  • Septa simple and concave, and produce simple sutures
  • Siphuncle has septal necks that point to the rear (directed backwards)
  • Predatory, have horny and beak-like jaws
• Ammonoids


  • Septa with intricate folds called lobes and saddles, as well as lacey patterns on the outer shell
  • 3 basic patterns for ammonite septa: goniatite (irregular zigzag), Ceratite (regular wavy), and ammonite (feathery or fern like patterns)
  • Patterns of septa can be reflected on the outside of the shell called sutures