Ansc Nutrition

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Phylum Molusca

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Molluscs, snails, slugs, clams, chitons, octopi, squid, etc...

- 50,000 spp. (+35,000 fossil spp.)

- diverse in body form, ecology, feeding habits.

- most marine, but some in freshwater and terrestrial systems.

- many are of considerable economical importance as food and pests.

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Phylum Mollusca : The true body cavity.

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- Eucoelmate animals

-- have secondary body cavity that develops after the blastocoel has filled with mesoderm

- Eucoelmate cavities arise in two different ways

-- Schizocoely- Mesoderm splits apart on each side; the splits enlarge to form the cavity

- occurs in molluscs, annelids, and anthropods

-- Enterocoely- body cavity arises from outpocketing (coelomic vesicles) of the embryonic gut.

- Occurs in echinoderms and chordates.

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Characteristics of Mollusca

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- Defined on the basis of the ancestral body plan; modern groups have diverged in various ways.

- Primitive body plan:

-- body divided into head, foot, and visceral mass

-- presence of a shell to enclose a soft body

-- the shell is secreted by a modified epidermis, the mantle.

-- along the overhang of the shell, the mantle partially surrounds a space called the mantle cavity

-Mantle cavity:

-contains gills, the anus, and the excretory pores

- a feeding structure called the radula.

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The molluscan shell

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Shell has three distinct layers

- outer periosracum, made of conchiolin protein.

- a middle layer, the prismatic layer, consisting of a blend of conchiolin and CaCO3 (chalky in consistency)

- an inner layer, the nacreous layer, made of CaCO3 crystals (and sometimes some conchiolin)

-- pearly in texture.

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Phylum Mollusca : Integument

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- Epidermis is a single cell layer.

-- under the shell, the epidermis is a thickened mantle

-- the mantle secretes the shell

- Epidermis on the underside of the foot id frequently ciliated and supplied with mucous glands.

- enables molluscs to glide over surfaces (ciliary swimming in a slime trail).

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Phylum Mollusca : Skeletal system

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- the shell of molluscs serves as an exoskeleton

-- it provides from muscle attachment

-- it provides for body support

-- it also provides protection for the soft tissue underneath.

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Phylum Molusca : Muscular system

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- musculature is complex and regionally specialized.

- body wall generally bears an outer circular layer, a middle diagonal layer, (sometimes 2) and an inner longitudinal layer.

- also, specific muscles to....

-- move the radula and odontophore

-- extend and retract the foot

-- close the shell (bivalves)

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Phylum Molusca : Feeding and digestion

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Snails: herbivores (grazers), scavengers, and predators - use their radula in various ways in feeding.

Chitons: grazers- use radula to scrape algae from rock surfaces

Cephalopods: predatory- use arms & tentacles to capture prey, beak and radula to dismember it.

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The Molluacan Radula

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- conaists of a "belt" of chitinous teeth that slides over a cartilaginous tongue like odontophore.

- specialized muscles pull the radula back and forth over the odontophore.

- other muscles enable the odontophore to be protruded and retracted.

Bivalves:

- filter feeders- use mucus on their gills to trap food and particles.

- deposite feeders- use tentacles flanking the mouth to extract food particles from sediments.

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Phylum Mollusca : Digestive system

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- mouth (with or without radula)

- esophagus

- stomach

-- sorting & extracellular digestion

-- associated digestive glads- more extracellular and/or intracellular digestion.

- intestine

- anus

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Phylum Mollusca : Excretion

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- the metanephridium- true kidney

- has two open ends

-- the first receives the wastes from the coelomic (pericardial) cavity, which leads into a coiled nephridial tubule.

-- the second is a nephridial pore emptying out in the mantle cavity.

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Phylum Mollusca : Gas exchange

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- Gills or Lungs

Gills:

- gills occur in all molluscan classes

-- primitively associated with the mantle cavity

-- consists of a series of filaments projecting from each side of a supporting axis.

-- filaments are ciliated, which generates a water current over the gill surfaces.

-- the gills are vascularized to allow gas exchange with the blood.

Lungs: pulmonate snails

- mantle cavity has been sealed off from the outside , leaving only a small opening, the pneumostome.

-inside lining of mantle cavity is moist and highly vascularized to facilitate gas exchange with air.

- some pulmonates are secondarily aquatic, but still breath air.

- others have developed secondary gills in the lining of the mantle cavity and exchange gases with water.

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Phylum Molusca : Circulatory function

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Open system:

-blood is not channeled into capillary beds, but is emptied into tissue spaces within or surrounding the organs themselves.

-blood spaces are interconnected and collectively = the hemocoel.

Dorsal Heart:

-one or two posterior auricles and an anterior ventricle

- an aorta extends anteriorly from the ventricle.

--ventricle pumps blood into the aorta, which empties into the anterior part of the hemocoel;

-- after circulating through the hemocoel, the blood is collected by veins that carry it to the auricles.

-- the auricles pass the blood bsck to the ventricle.

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Nervous and Sensory System

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Ancestrally

- a circumesophageal nerve ring

- two pairs of nerve cords extend posteriorly (one pair into the foot and the other into the visceral mass (often forming a loop).)

- each pair of nerve cords is connected by transverse nerves, producing a "ladder type" arrangement.

Gastropods

- three pairs of anterior ganglia

-- a pair of dorsal cerebral ganglia

-- a pair of lateral pleural ganglia

-- a pair of ventral pedal ganglia

- posteriorly, paired parietal ganglia and visceral ganglia.

Sensory Systems

-snail tentacles: mechano- and chemoreceptors

- arms and suckers of cephalopods - highly sensitive to both chemical and mechanical stimulation.

-Osphradia: a patch of sensory epithelium

-- surface is often rigid or folded to increase surface area.

-- in gastropods

--- highly developed in predators and scavengers

--- serves as a chemoreceptor to locate prey or dead animals in the water.

--- located on the mesenteries attached to the gills.

-- in bivalves

--- serves not only as a chemoreceptor, but also measures the amount of sediment entering the siphons.

--- located inside the exhalent siphon.

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Molluscan eyes

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-eyes developed in snails, cephalopods

- lenses form images

- the cephalopod eye (octopus and squid only) is built much like that of a vertebrate

-- photoreceptors directed toward the source of light, rather than toward the bscj of the eye as in vertebrates.

-- focusing is done by muscles moving the lens back and forth in the cavity of the eyeball, rather than by changing the shape of the lens.

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Reproduction and development

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- Gasteopods are mostly hermaphroditic; all other groups mostly dioecious.

- different methods of sperm transfer.

-- primitively, eggs and sperm are shed into the water

--- in some, fertilization may occur in the females mantle cavity and the developing of eggs can be brooded there.

-- in many gastropods, there is copulation.

--- the male system has a penis.

--- female system has glands to add nutrients and form a capsule around the eggs.

-- in cephalopods, sperm is placed in a packet (spermatophore) and transferred to the female with the tip of a modified tentacle.

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Reproductive systems

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- consists of an anterodorsal gonad (ovary or testis) and a system of gonoducts (oviduct or vas deferens).

-- gonoducts may empty to the outside via a separate gonopore.

-- or they may empty into the nephridial tube ( in this case the gametes leave through nephridiopore).

- a more complex hermaphroditic system is seen in the pulmonate snails.

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Larval stages

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- Two characteristic larval stages, not present in all groups.

--Trochophore : top shaped, with a band of cilia in the middle, and ciliary tufts at both ends.

--Veliger: shell is beginning to develope, has a ciliated velum (consisting of two flaps) allows for swimming.

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Class Polyplacophora

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- chitons

- characterized as follows:

-- shell consists of eight dorsal plates surrounded by a fleshy girdle (Mantle).

-- foot is flattened, used for locomotion.

-- mantle cavity is a groove around the foot underneath the shell; gills lie in the mantle cavity

-- Found in rocky intertidal habitats.

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Class Gastropoda

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- marine snails, land and aquatic snails, sea slugs, land slugs.

- 40,000 species (plus 15,000 fossil spp.)

- shell, when present is univalved and usually coiled

- exhibit a developmental phenomenon called torsion.

-- 180° counterclockwise rotation of the visceral mass.

-- moves the mantle cavity anteriorly, as well as the anus.

- foot is flattened and used for locomotion

- use radula to feed as herbivores, scavengers, or predators.

SUBCLASS Prosobranchia

-marine snail

-distinguished by the following characters:

-- shell present, although it may be reduced

-- shell typically has an operculum

--- a horny plate attached to the foot, which seal the shell when the animal is withdrawn inside.

-- gills present in mantle cavity l, moved into a forward position.

-- mostly dioecious.

SUBCLASS Opisthobranchia

-sea slugs (nudibranchs), sea hares.

- Opisthibranchs are distinguished by the following characters:

-- reduction or loss of shell

-- partial or complete detorsion ; anus and gills on right side of body.

-- gills exposed along dorsum (the name "nudibranch" means naked gill).

-- hermaphroditic

SUBCLASS Pulmonata

-land snails, aquatic snails, land slugs

-Pulmonates have lungs (see respiratory system)

- may be secondarily modified for breathing aquatically

- shells lack an operculum

- hermaphroditic

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Class Bivalvia

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- clams, oysters, mussels, scallops, etc.

- have a bivalved shell, with the two valves held together by hinge teeth And powerful adductor muscles.

- inner surfaces of shell covered by a thin mantle

- siphons Bring water in and out of the mantle cavity.

- use their gills for filter feeding on particulate matter suspended in the water. Radula has been lost.

- foot is spade shaped and used for digging.

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Class Cephalopoda

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- Nautiluses, octopuses, squid, cuttlefishes, and extinct ammonoids.

- foot produced into arms and tentacles that surround the mouth, as well as a ventral siphon.

-- water in the mantle cavity forces out through the siphon to generate jet propulsion (excellent swimmers)

- shell present (ammonoids. Nautiloids) reduced and covered by the mantle (squid, cuttlefish) , or absent (octopuses).

- excellent vision (eyes similar to those of vertebrates), highest intelligence among invertebrates.

- dioecious; sperm transfer via a spermatophore placed inside the females mantle cavity using a modified tentacle.