Hypothetical ancestral mollusc Mollusca

1 hypothetical ancestral mollusc

1.1 mantle , mantle cavity
1.2 shell
1.3 foot
1.4 circulatory system
1.5 respiration
1.6 eating, digestion, , excretion
1.7 nervous system
1.8 reproduction

hypothetical ancestral mollusc

because of great range of anatomical diversity among molluscs, many textbooks start subject of molluscan anatomy describing called archi-mollusc, hypothetical generalized mollusc, or hypothetical ancestral mollusc (ham) illustrate common features found within phylum. depiction visually rather similar modern monoplacophorans, , suggest may resemble molluscs.

the generalized mollusc bilaterally symmetrical , has single, limpet-like shell on top. shell secreted mantle covering upper surface. underside consists of single muscular foot . visceral mass, or visceropallium, soft, nonmuscular metabolic region of mollusc. contains body organs.

mantle , mantle cavity

the mantle cavity, fold in mantle, encloses significant amount of space. lined epidermis, , exposed, according habitat, sea, fresh water or air. cavity @ rear in earliest molluscs, position varies group group. anus, pair of osphradia (chemical sensors) in incoming lane , hindmost pair of gills , exit openings of nephridia ( kidneys ) , gonads (reproductive organs) in mantle cavity. whole soft body of bivalves lies within enlarged mantle cavity.

shell

the mantle edge secretes shell (secondarily absent in number of taxonomic groups, such nudibranchs) consists of chitin , conchiolin (a protein hardened calcium carbonate), except outermost layer in cases conchiolin (see periostracum). molluscs never use phosphate construct hard parts, questionable exception of cobcrephora. while mollusc shells composed of aragonite, gastropods lay eggs hard shell use calcite (sometimes traces of aragonite) construct eggshells.

the shell consists of 3 layers: outer layer (the periostracum) made of organic matter, middle layer made of columnar calcite, , inner layer consisting of laminated calcite, nacreous.

foot


a 50-second video of snails (most natica chemnitzi , cerithium stercusmuscaram) feeding on sea floor in gulf of california, puerto peñasco, mexico

the underside consists of muscular foot, has adapted different purposes in different classes. foot carries pair of statocysts, act balance sensors. in gastropods, secretes mucus lubricant aid movement. in forms having top shell, such limpets, foot acts sucker attaching animal hard surface, , vertical muscles clamp shell down on it; in other molluscs, vertical muscles pull foot , other exposed soft parts shell. in bivalves, foot adapted burrowing sediment; in cephalopods used jet propulsion, , tentacles , arms derived foot.

circulatory system

molluscs circulatory systems open. although molluscs coelomates, coeloms reduced small spaces enclosing heart , gonads. main body cavity hemocoel through blood , coelomic fluid circulate , encloses of other internal organs. these hemocoelic spaces act efficient hydrostatic skeleton. blood contains respiratory pigment hemocyanin oxygen-carrier. heart consists of 1 or more pairs of atria (auricles), receive oxygenated blood gills , pump ventricle, pumps aorta (main artery), short , opens hemocoel.

the atria of heart function part of excretory system filtering waste products out of blood , dumping coelom urine. pair of nephridia ( little kidneys ) rear of , connected coelom extracts re-usable materials urine , dumps additional waste products it, , ejects via tubes discharge mantle cavity.

respiration

most molluscs have 1 pair of gills, or 1 gill. generally, gills rather feathers in shape, although species have gills filaments on 1 side. divide mantle cavity water enters near bottom , exits near top. filaments have 3 kinds of cilia, 1 of drives water current through mantle cavity, while other 2 keep gills clean. if osphradia detect noxious chemicals or possibly sediment entering mantle cavity, gills cilia may stop beating until unwelcome intrusions have ceased. each gill has incoming blood vessel connected hemocoel , outgoing 1 heart.

eating, digestion, , excretion

snail radula @ work

  = food       = radula

  = muscles

  = odontophore belt

members of mollusc family use intracellular digestion function. molluscs have muscular mouths radulae, tongues , bearing many rows of chitinous teeth, replaced rear wear out. radula functions scrape bacteria , algae off rocks, , associated odontophore, cartilaginous supporting organ. radula unique molluscs , has no equivalent in other animal.

molluscs mouths contain glands secrete slimy mucus, food sticks. beating cilia (tiny hairs ) drive mucus towards stomach, mucus forms long string called food string .

at tapered rear end of stomach , projecting hindgut prostyle, backward-pointing cone of feces , mucus, rotated further cilia acts bobbin, winding mucus string onto itself. before mucus string reaches prostyle, acidity of stomach makes mucus less sticky , frees particles it.

the particles sorted yet group of cilia, send smaller particles, minerals, prostyle excreted, while larger ones, food, sent stomach s cecum (a pouch no other exit) digested. sorting process no means perfect.

periodically, circular muscles @ hindgut s entrance pinch off , excrete piece of prostyle, preventing prostyle growing large. anus, in part of mantle cavity, swept outgoing lane of current created gills. carnivorous molluscs have simpler digestive systems.

as head has largely disappeared in bivalves, mouth has been equipped labial palps (two on each side of mouth) collect detritus mucus.

nervous system

simplified diagram of mollusc nervous system

the cephalic molluscs have 2 pairs of main nerve cords organized around number of paired ganglia, visceral cords serving internal organs , pedal ones serving foot. pairs of corresponding ganglia on both sides of body linked commissures (relatively large bundles of nerves). ganglia above gut cerebral, pleural, , visceral, located above esophagus (gullet). pedal ganglia, control foot, below esophagus , commissure , connectives cerebral , pleural ganglia surround esophagus in circumesophageal nerve ring or nerve collar.

the acephalic molluscs (i.e., bivalves) have ring less obvious , less important. bivalves have 3 pairs of ganglia— cerebral, pedal, , visceral— visceral largest , important of 3 functioning principal center of thinking . such scallops have eyes around edges of shells connect pair of looped nerves , provide ability distinguish between light , shadow.

reproduction














trochophore larva

the simplest molluscan reproductive system relies on external fertilization, more complex variations. produce eggs, may emerge trochophore larvae, more complex veliger larvae, or miniature adults. 2 gonads sit next coelom, small cavity surrounds heart, shed ova or sperm. nephridia extract gametes coelom , emit them mantle cavity. molluscs use such system remain of 1 sex lives , rely on external fertilization. molluscs use internal fertilization and/or hermaphrodites, functioning both sexes; both of these methods require more complex reproductive systems.

the basic molluscan larva trochophore, planktonic , feeds on floating food particles using 2 bands of cilia around equator sweep food mouth, uses more cilia drive them stomach, uses further cilia expel undigested remains through anus. new tissue grows in bands of mesoderm in interior, apical tuft , anus pushed further apart animal grows. trochophore stage succeeded veliger stage in prototroch, equatorial band of cilia nearest apical tuft, develops velum ( veil ), pair of cilia-bearing lobes larva swims. eventually, larva sinks seafloor , metamorphoses adult form. while metamorphosis usual state in molluscs, cephalopods differ in exhibiting direct development: hatchling miniaturized form of adult.