Mollusks

The pulmonata

Defining characteristics -mantle cavity highly vascularized and otherwise modified to form a lung -mainly only interdidally and in estuaries -escargot is a terrestrial member -coiled shell is present but the shell is reduced, internalized, or completely lost in others (slug) -only a few species have the operculum on the foot -possess a long radula -head commonly bears two pairs of tentacles -torsion is limited to about 90 degrees so the nervous system is not so greatly twisted and the mantle cavity opens on the right side of the body -lack ctenidtia -monecious -have "love darts" Mantle Cavity Functioning as Lung -downward movement of the floor of the mantle cavity increases the cavity's volume so that air or in some cases water is drawn into the mantle cavity for respiration -the fluid is then expelled by decreasing the volume of the mantle cavity -air or water flows into and out of the lung through a single small opening called the pneumostome

The prosobranchs

Defining characteristics -most primitive of gastropods that is other gastropod most likely evolved from prosobranch-like ancestors -mantle cavity generally anterior due to torsion -most species are marine -generally free living and mobil although some species have evolved sessile or even parasitic lifestyles -a number of carnivorous species produce potent venoms that are injected into fish, molluscans or annelid prey through a harpoon-like hollow radular tooth -the various toxins exert their effects by binding to very specific classes of cell surface receptors and ion channels -some species contain symbiotic bacertia living in the gill tissue and the bacteria may provide the snail with nutrients overview -possess a well-developed shell, mantle cavity, osphradium, and radula, and the foot usually bears a rigid disc of protein called the operculum -typical posobranch gill is the ctenidia, consisting of a series of flattened, triangular sheets (filaments) lying adjacent to the next Operculum -when the foot is withdrawn into the shell, the operculum may completely seal the shell aperture, thus protecting the snail from predators and from such physical stresses as dehydration and low salinity Blood Flow -deoxygenated blood enters an afferent blood vessel from the animals open system of blood sinuses (HEMOCOEL) -once distributed to the individual sheets of the ctenidium the blood moves through the sheet where it becomes oxygenated and then on to the auricle of the heart through an efferent blood vessel. -from the auricle the blood is pumped into the single associated ventricle and is then distributed to the tissues through a single aorta leading to the blood sinuses of the hemocoel. -primitive prosobranch species possess a pair of auricles, a pair of efferent blood vessels and a pair of ctenidia Water Flow -water is drawn into the mantle cavity and across the gill sheets by the movements of gill cilia -in some species a portion of the mantle is drawn out into a cylindrical extension called the siphon and water is drawn through this siphon by the action of the gill cilia, into the mantle cavity and across the osphradium (a chemical and tactile receptor organ) -the snail moves the muscular siphon back and forth, sampling the water from different directions -in burrowing species the siphon is extended through the substrate to the water above -the gastropod siphon is especially well developed in carnivores and scavengers - which often hunt their prey by chemical sensing - generally reduced or absent in suspension feeders, herbivores and deposit feeders -water usually enters the mantle cavity at the left side of the head, passes over or between the gill filaments, and exits at the right side of the head

Class Scaphopoda

Defining characteristics -tusk-shaped, conical shell, opens at both ends -development of anterior, threadlike adhesive feeding tentacles -live on soft sediment -no gills -no ctindia -feeding tentacle - captaculae

Class Gastropoda

Defining characteristics -visceral mass and nervous system become twisted during embryonic development -proteinaceous shield on the foot (operculum) Dextral- shells are right-handed, they coil to the right Sinistral -left handed in their coiling Visceral Mass -typical sail consists of a visceral mass sitting atop a muscular foot -the mass is commonly protected by a univalves shell that is typically coiled, probably as an adaptation for efficient packaging of the visceral mass -for shelled species the snail is attached to the inside of its shell by a columellar muscle which extends from within the animals foot to the central axis of the shell called the columella - important in most major body movements, protraction from the shell, retraction into the shell, twisting, raising the shell above the substratum and lowering it back down -most shells coil clockwise to the right - shells are dextral -as a consequence of space limitations within the coiled shell, the ctenidium, osphradium, kidney (nephridium) and heart auricle on the right side of the body tend to be reduced or absent Behavioral and Chemical defenses in 3 ways 1. gastropods senses the presence of potential predators, either chemically or by touch and initiates appropriate escape, avoidance, etc. 2. the gastropod chemically senses the presence of injured individuals of its own species and initiates appropriate escape behavior 3. the gastropod accumulates noxious organic compounds in its tissues thereby becoming distasteful to potential predators Torsion -a counterclockwise 180 degree twist of the head and foot relative to the shell, mantle, and the rest of the body (visceral mass) during early development -as a consequence of torsion, the nervous and digestive system because obviously twisted and the mantle cavity moves from the rear of the animal to become positioned over the head -through torsion the ctenidia and osphradia come to be located a the front of the animal in the direction of locomotion but torsion also shifts the anus so that is discharges over the head creating a potentially serious sanitation problem. Locomotion -small species may move largely through the action of cilia located on the ventral surface of the but but most species move through PEDAL WAVES of muscle contraction along layers of adhesive mucus secreted by the mouth or foot -pedal waves do not involve circular muscles or muscular contractions of great magnitude and they are restricted to the central portion of the foot's ventral surface Foot -at the start of the pedal wave, the dorsoventral musculature contracts at the anterior portion of the foot. -the transverse muscles do not relax meaning that the foot can't widen instead the foot is squeezed forward -a wave of contraction of the dorsoventral musculature then moves posteriorly allowing the rest of the foot to catch up with the anterior -the edges of the foot are temporarily sealed against the substrate with mucus so a small negative pressure is generated in the space between the substrate and the raised portion of the foot. -the dorsoventral muscles are reextended when they relax -this small space thus acts as a hydrostatic skeleton seen though it is external to the body allowing the musculature at the forward edge of the wave to antagonize that at the trailing edge -this all applies to retrograde waves - the wave of muscular contraction travels in the direction opposite that in which the snail is moving -pedal waves may also be direct - moving in the same direction of the animal Reproduction -the first larval stage is the so-called trochophora

Mollusks

First coelomate group

what does HAM stand for

Hypothetical Ancestral Mollusc

prismatic layer

a thick calcareous middle layer

mantle

both the organic and inorganic components of the shell are secreted by specialized tissue known as the mantle

Mantle cavity

characteristic cavity lying between the mantle and the viscera, which usually houses the comb-like molluscan gills known as the ctenidia and also serves as the exit site for the excretory, digestive, and reproductive systems

Septibranch Ctinedium

creates pressure changes within the mantle cavity and produces sudden inrushes of water, carrying prey into a funnellike inhalant siphon ( Cuspidaria). Food is then pushed into the mouth by the palps and foot. Others evert the inhalant siphon

coelem

is very small being restricted largely to the area surrounding the heart and gonads

osphradium

-a chemoreceptor/tactile receptor which is generally located adjacent to the ctenidium

hemocoel

-blood sinuses -serves as a hydrostatic skeleton in the locomotion of some mollusks

Defining Characteristics

-dorsal epithelium forming a mantle, which secretes calcareous spicules or one or more shells -cuticular band of teeth (radula) in the esophagus, used for feeding (----not present----lost?-----in bivalves) -ventral body wall muscles develop into a locomotory or clinging foot

countercurrent exchange

-exception of squid and other cephalopods, molluks gills work in a system that generally increases the efficiency of gas exchange between blood flowing within the ctenidial filaments and the water flowing over them. In this system, blood and water flow in opposite directions

radula

-feeding structure -consists of a firm ribbon, composed of chitin and protein along which are found numerous rows of sharp, chitinous teeth -the ribbon is produced from a radular sac and it underlain by a supportive cartilage- like structure called the odontophore -the odontophore - radular assembly together with its complex musculature is known as the buccal mass or the odontophore complex -for feeding the buccal mass is protracted so that the odontophore extends just beyond the mouth

Feeding

-for feeding the buccal mass is protracted so that the odontophore extends just beyond the mouth -the radular ribbon is then moved forward over the leading edge of the supporting odontophore and then pulled back -as each row of teeth passes back over the edge of the odontophore the teeth automatically stand upright and rotate laterally, rasping food particles from the substrate and bringing them into the mouth as the radula is withdrawn

Class Polyplacophora Overview

-known as the "chitons" -found close to shore particularizing in the inter dial zone -live on only hard substrates especially rocks -most distinctive external feature: occurs as a series of eight overlapping and articulating plates covering the dorsal surface -these plates are partially or largely embedded in the mantle tissue that secretes them -because the body is multi-sectional, the body can bend and conform to a wide variety of underlying substrate shapes -thick lateral mantle is called the girdle - most species contains numerous calcareous spicules, secreted independently of the shell plates Mantle Cavity: -takes the form of two lateral grooves, one on each side of the body -80 bipectinate ( double-combed - two branched) ctenidia hang down from the rood of each groove, dividing each elongated mantle cavity into incurrent and excurrent chambers Countercurrent exchange system -faciliates gas exchange as discussed earlier Foot -extends along the animals entire central surface and is completely covered by the overlapping shell and girdle Locomotion -accomplished by subtle waves of muscular activity called "pedal waves" -when disturbed the chiton can press the girdle tightly against the substrate, then lifting up the central portion of the foot while retaining a tight seal against the substrate along the entire outer margin of the foot (and girdle) the chiton can generate suction, aided by mucus secretions that holds the animal tightly again the substratum - the ability to cling tightly to the substrate is a particularly effective adaptation for life in areas of heavy wave action Nervous System -ganglia are lacking in many species and only poorly developed in others -sensory systems are also reduced - adult chitons lack statocysts, tentacles, and eyes on the head -AESTHETES - abundant organs derived from mantle tissue and extending through holes in the shell plates Digestion -mouth and anus on opposite ends of the body -food particles are scraped from the substrate by a radula/odontophore complex although a few species are carvinores -pair of pharyngeal glads often called sugar glands release amylase-containing secretions into the stomach

Class Bivalva Subclass Protobranchia

-live in soft sediment -food collection by long, thin, muscular extensions of tissue surrounding the mouth called palp proboscides -rejected material that has never been ingested - PSEUDOFECES -type of feeding in which sediment is taken in and the organic fraction is digested is called DEPOSIT FEEDING

Overview

-no 'typical' mollusk because each class is so different, but generally they all have shells at some part of their life span and the shells is made of CaCO3 and protein.

Class Polyplacophora Defining Characteristics:

-shell forms as a series of seven to eight separate plates

Periostracum

-the shells of most mollusks have a thin, outer organic layer

Nacreous layer

-thin innermost calcareous layer

Class Aplacophora

-vermiform molluscs contain no shell -found in all oceans mostly in deep water - entirely marine -body is unsegmented and bears numerous calcareous spines or scales are embedded in an outer cuticle - spines or scales are secreted by individual cells in the underlying epidermis there is no true shell -some species possess a style sac, a small posterior mantle cavity with ctenidia, a radula, although in most spices the radula is used for grasping rather than rasping. -have no conspicuous foot, although the members of one group have a homologous to the foot of other mollusks -ONLY CLASS OF MOLLUSKS TO HAVE LEFT NO FOSSIL RECORD -burrow into or meander about on mud - other species live on cnidarians - soft coral in which they prey Nervous system -consists of paired cerebral ganglia, giving rise to four linear hanglionated nerve core - two lateral - two ventral -form calcareous spicules through extracellular secretions from single cells

ctenidium

-when present may have a purely respiratory function or may also function in the collection and sorting of for particles

Cephalopoda

Defining Characteristics -shell divided by septa with chambers connected by the SIPHUNCLE - a vacsularized stand of tissue contained within a tube of calcium carbonate -closed circulatory system -foot modified to form flexible arms and siphon -ganglia fused to form a large brain encased in a cartilaginous cranium -large eyes with no lens -4 ctinedia -only group with more than 1 pair -chitonous sheets called pens -a heart and also a branchial heart (sometimes 2) -contain brain

The Opisthobranchia

Defining Characteristics -mantle cavity lateral or posterior due to detorsion or lost -includes the sea hares, sea slugs, and bubble shells are all marine Distinguishes from the Prosobranchs -a trend toward reduction, internalization, or loss of the shell -reduction or loss of the operculum -limited torsion during embryogenesis -reduction or loss of the mantle cavity -reduction or loss of the ctenidia Nudibranchs -gas exchange occurs across brightly colored dorsal projections called cerata which also contains extensions of the digestive system Cerata -shell reduction or loss potentially increases vulnerability to predators and it is reasonable to expect that pressures selecting for alternate means of dense have been quite strong -the cerata houses unfired defensive organelles. Rhinophores -typical bear in addition to a pair of tentacles adjacentt to the mouth, a second pair of tentacle-like structures located dorsally called rhinophores -believes to be chemosensory making them analogous to the osphradium of prosobranch gastropods Sea Slugs -do not contain mantle cavity, ctenidia, osphradium, shell or operculum -few species possess a conspicuous, external, spirally coiled shell with operculum -snails often show little sign of detorsion Locomotion -generally by means of cilia and pedal waves along the ventral surface of the foot, Pteropods -can swim in short spurts by flapping folds of the foot called parapodia -all permanent members of plankton -no specialized respiratory organs so gas exchange is accomplished across the general body surface

Class Monoplacophora

Defining Characteristics -three to six pairs of ctenidia, 6 to 7 pairs of nephridia -multiple pairs of foot (pedal) retractor muscles -all marine -produce single, unhinged, cap-shaped shells as do many. -the shell of the adult monoplacophorans are flattened rather than spirally wound, although the LARVAL STAGE is spiral -the foot is flattened Mantle Cavity -takes the form of two lateral grooves, and 3-5-6 pairs of gills hang down within the mantle grooves -uncertain if these gills are homologous with the ctenidium -in addition to gills, the pedal retractor muscles, auricles and ventricles of the heart, gonads, and nephridia occur in multiple copies -both the radula and a crystalline style are present -the gut in linear - mouth is anterior and anus posterior Nervous System -includes both lateral and pedal nerve cords

Class Bivalvia

Defining Characteristics -two-valved shelled -body is flattened laterally -hinged shell, the two sides of which are closed by one or two adductor muscles -lateral compression of the body and foot -lack cephalization, virtual absence of a head and associated sensory structures -a spacious mantle cavity, relative to that found in other molluscan classes -a sedentary lifestyle -the absence of a radula/odontophore complex -includes clams, scallops, mussels, and oysters Overview -suspension feeders using their gill cilia to drive water through the mantle cavity and capture photoplankton and other microscopic particles from the seawater adductor muscle is what attaches the namila to the shell is all bivalve species Adductor Muscles -a springy ligament that springs that shell calves apart when the adductor muscle relaxes -contraction of the adductor muscles also pumps blood into the foot Shell -hinged portion of the bivalved shell is dorsal -the shell valves are on the animals left and right sides -the shell opens ventrally -distinct growth lines typically run parallel to the sells outer margins - the age of the shell can be determined by counting growth lines because the pattens of shell growth are seasonal Siphon -water typically enters and exits the mantle cavity posteriorly -the water generally enters through an incurrent siphon and passes dorsally between adjacent gill filaments and then exits through a more dorsally located excurrent siphon Redroduction -A clam's reproductive system lies in the visceral mass. -Most Bivalves have separate genders, and when the time comes they release both eggs and sperm at the same time and fertilization happens in the open water. -The larva develop in the water, floating along like plankton and other microscopinc organisms.

The Lamellibranchs

Defining characteristics -gills modified to collect suspended food particles, in addition to serving as gas exchange surfaces -secretion of proteinaceous attachment material, usually in the form of threads,by a specialized gland (bryssus gland) on the foot Siphon -water typically enters and exits the mantle cavity posteriorly -the water generally enters through an incurrent siphon and passes dorsally between adjacent gill filaments and then exits through a more dorsally located excurrent siphon Bryssal Threads -some species such as mussels live attached to hard substrates by means of proteinaceous secretions called byssal threads -a proteinacous liquid is secreted by a byssal gland at the base of the foot within the mantle cavity where is it quickly transported to the substrate along a groove in the foot