Phylum Mollusca Gills Molluscan Diversity Examples of Cephalopods

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Phylum Mollusca Gills Molluscan Diversity Examples of Cephalopods

Sykes, Ricky, Music Director has reference to this Academic Journal, PHwiki organized this Journal Phylum Mollusca Introduction Includes animals such as squids, snails, oysters, clams in addition to slugs. Most are marine, but many are freshwater in addition to some live on the l in addition to Despite the diversity of as long as m in addition to function among the molluscs, all members of this group have the same basic body plan. This is often indicated by presenting a hypothetical ancestral mollusc (HAM) HAM is hypothetical primitive ancestor that has characteristics that appear among most members of the mollusca A Closer look at HAM The foot – a broad, flat muscular organ that is adapted as long as locomotion in addition to attachment The visceral mass – contains the internal organs The mantle – a fold of tissue that drapes over the visceral mass; space between the mantle in addition to the visceral mass is called the mantle cavity

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The Shell The mantle is responsible as long as secreting the shell. The shell is comprised of three layers: The outside of the shell is covered by an organic layer – periostracum The middle prismatic layer is characterized by densely packed prisms of calcium carbonate laid down in a protein matrix The inner nacreous layer is composed of calcium carbonate sheets laid down over a thin layer of protein The gills of HAM are often indicated as one or more pairs of bipectinate gills, – flattened filaments attached to a longitudinal axis on either side Gills The Radula The mouth cavity of HAM possesses a specialized rasping organ called the radula; sits on a cartilaginous structure – odontophore Particles of food brought into the mouth are bound in mucous secreted by the salivary gl in addition to s

Other Features of HAM Nervous system consists of a nerve ring in addition to 2 longitudinal nerve cords Coelom is reduced Open circulatory system The excretory organs of the molluscs are metanephridia; inner ends open into the coelom via a ciliated funnel called the nephrostome; wastes leave the body via the nephridiopore Molluscan Larval Stages Most molluscs produce a free-swimming ciliated larvae called the trochophore larvae In some molluscs the trochophore develops into the adult, but in other molluscs (e.g., gastropods) there is a second larval stage called the veliger Molluscan Diversity

Class Monoplacophora A few centimeters in length Dorsal surface is covered with a shield-shaped shell; apex with slight anterior peak Ventral surface is broad in addition to flat, with the mantle cavity in the as long as m of 2 grooves located to either side of the foot Mantle groove with 5 or 6 pairs of monopectinate gills There is serial repetition of certain body parts It’s unclear whether serial repetition (= pseudosegmentation) Class Monoplacophora cont. Neopilina sp. Evolutionary relations with other Molluscs Embryological data does hint to a phylogenetic relationship among the annelids in addition to molluscs Monoplacophorans are thought to be ancestral to several other molluscan classes

Class Polyplacophora (Chitons) Common on the rocky surfaces of the intertidal zone Head is poorly developed; ventral surface occupied by a broad, flattened foot Has a dorsal shell composed of 8 overlapping plates, arranged linearly along the anterior-posterior axis Lateral margins of the plates are overgrown to varying degrees by the girdle Mantle cavity is limited to two lateral troughs between the foot in addition to the mantle edge = pallial grooves Within the grooves lie many bipectinate gills Class Gastropoda Three evolutionary innovations occurred among the gastropods: changes in the shell, increased development of the head, the embryonic process of torsion 1. Changes in the Shell The shell became higher in addition to conical with a reduced aperture The shell also became coiled Shells initially were planospiral – bilaterally symmetrical shell with the whorls lying in the same plane Modern day shells are asymmetrical – each successive coil is a little outside in addition to offset a little above the one below Why was there a change in shell architecture Planospiral shell was not as compact as the asymmetrical shell But, this change in symmetry of the shell created a shift in the weight to one side of the animal To achieve a better weight distribution, the shell needed to shift upward in addition to posterior The shell axis then became oblique to the longitudinal axis of the foot (= bilateral asymmetry) The weight in addition to bulk of the main body whorl, pressed on the right side of the mantle cavity; thus, many of the organs on the right side became lost during evolution

2. The Increased Development of the Head The head bears 2 pairs of tentacles, with the eyespots at the base of one pair eyespots tentacles 3. The Embryonic Process known as Torsion During embryonic development, 1 side of the visceral mass grows at a much faster rate than the other. Causes the visceral mass to rotate 180 degrees relative to the head-foot. Advantages: head retracted first; gills receive water currents; the osphradium is now directed anteriorly Disadvantage: may cause fouling Adaptations to Avoid Fouling Improved separation of inhalent in addition to exhalent water flow In some of the more primitive gastropods (keyhole limpets), the shell contains a hole at the top through which the exhalent water stream exits In the more advanced gastropods, water is brought into the mantle cavity on the left side, passes over a single gill, in addition to exits the right side

Shell Most have a single, spiraled shell in addition to can move the entire head in addition to foot into this shell as long as protection. Also, many gastropods have a hardened plate called the operculum on the back of the foot that plugs the shell aperture when the body is withdrawn Nutrition Many gastropods are herbivores in addition to use their radula scrap algae from surfaces of rocks Some gastropods are active predators in addition to in these the radula is often highly modified, e.g., as a drill (oyster drills) or harpoon (venomous gastropods) Cone snail Respiration Aquatic gastropods possess gills as long as respiration Terrestrial gastropods obtain oxygen via a well vascularized mantle gills Vascularized mantle

Class Bivalvia Shells divided into 2 equal halves or valves Mantle tissue is indented in the anterior-posterior margins, with 2 centers of calcification Shells joined at the dorsal midline by a non calcified protein ligaments called the hinge Pallial muscles insert on the underside of the shell in addition to are attached to the free edge of the mantle; pull the mantle under the shell Muscles fused across the width (from left to right) at 1 anterior in addition to posterior position in addition to as long as m adductor muscles; connect the 2 shell across their width; close the shell When relaxed, shell swings open due to elastic ligaments of the hinge Protobranchs (Subclass Protobranchia) Possess a small foot Gills are bipectinate; cilia on the face of the gills – lateral cilia – generate water currents Protobranchs are deposit feeders A pair of elongate, ciliated palp probosci are extended from the animal into the substrate Each tentacle is associated with 2 labial palps During feeding the probosci are extended into the sediment in addition to the cilia bring sediments with food toward the palps Be as long as e material enters into the mouth it is sorted by cilia on the palps Rejected material is pseudofeces

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Lamellibranchs (Subclass Lamellibranchia) Gills play an important role in feeding: of the total volume of water that is processed by gills only 5% is required as long as gaseous exchange; 95% of the volume is used to supply the animal with food There were 2 principal modification of in the lamellibranch gill: 1. Lengthening in the anterior-posterior perspective, as long as ming a series of gill filaments 2. Flattening in addition to folding of the gill filaments, greatly increasing surface area More Regarding Gills in addition to Filter Feeding The long folded filaments are supported by the development of cross connections between the two halves, by connections between adjacent filaments, in addition to by connection of the tips of the filaments to the foot or mantle wall More Regarding Gills in addition to Filter Feeding Lengthened filaments in addition to their attachment to one another give the gills a sheet-like appearance Cilia in between the gill filaments generate the water current in addition to other cilia are used to filter out food from the Where adjacent filaments are tightly connected, openings (ostia) remain as long as the passage of water between the filaments The interior space between the two folded halves of the filaments as long as ms water tubes, which connect with the suprabranchial cavity

Movement of the Ventilating Currents In lamellibranchs, the ventilating currents enter posteriorly in addition to ventrally Upon reaching the gills, there are cilia that bring it in through the ostia in addition to into the water tubes Now water flows upward to the suprabranchial cavity, where it turns posteriorly in addition to flows outward through the shell gape Movement of the Ventilating Currents Some kinds of cilia on the gills are used to trap food particles suspended in the water in addition to move the trapped food over the surface of the gills toward food grooves Once here, food is directed anteriorly toward the mouth On route to the mouth cells in the gills secrete copious amounts of mucous, in which the food particles become entangled Prior to entering the mouth the mucous food thread is first sorted by the labial palps Movement of the Ventilating Currents

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