Evolutionary PerspectiveOctopuses, squids, and cuttlefish (all

cephalopods) are some of the most adept predators of the invertebrate world

Class Cephalopoda once numbered approx. 9000 species but now only have about 550 species

Numerically, the mollusks are twice are successful as vertebrates

There are nearly 100,000 species of mollusc. Most belong to two classes: Gastropoda and Bivalvia

E.P. con’tMolluscs and all animals from this point on

are triploblastic.They are the first to possess a coelomCoelom performs multiple functions (refer to

previous notes for these functions)Molluscs are protostomate animalsCoelom forms by the splitting of the

mesoderm. The unique and successful body form of the

molluscs resulted in reduced size and importance of the coelom.

Molluscan Characteristics1. Body of two parts: head-foot and visceral mass2. Mantle that secretes a calcareous shell and covers the

visceral mass3. Mantle cavity functions in excretion, gas exchange,

elimination of digestive wastes, and release of reproductive products

4. Bilateral symmetry5. Protostome characteristics – trochophose larve, spiral

cleavage, and schizocoelous coelom formation6. Coelom reduced to cavities surrounding heart, nephridia,

and gonads7. Open circulatory system in all but one class (Cephalopods)8. Radula usually present and used in scraping food

Three Main Regions1. Head-footElongate with an anterior headContains mouth and nervous and sensory

structuresElongate foot – used for attachment and

locomotion2. Visceral MassContains the organs of digestion, circulation,

reproduction, and excretionPositioned dorsal to the head-foot

Con’tMantleUsually attaches to the visceral mass, enfolds most

of the body, and may secrete a shell that overlies the mantle

Shell is secreted in 3 layers:a. Outer layer – periostracum. Secreted by the mantle’s outer

margin cellsb. Middle layer – prismatic. Thickest of the three. Consists of

calcium carbonate and organic materials. Secreted by mantle’s outer margin cells

c. Inner layer – nacreous layer. Formed from thin sheets of calcium carbonate alternating with organic matter. Secreted by the entire epithelial border of the mantle. Nacre thickens the shell

Con’tIn addition to the three main body regions,

there is the mantle cavity and the radula.Mantle cavity – space between mantle and the

foot. It is open to the outside. Functions in gas exchange, excretion, elimination of digestive wastes, and release of reproductive products

Radula – made up of achitinous and posteriorly curved teeth. Overlays a fleshy tongue like structure called an odontophore. The odontophore can be protruded from the mouth. Food is scraped off and passed posteriorly to the digestive tract

Class Gastropoda Includes snail, slugs, and limpetsOver 35,000 living speciesLargest and most varied molluscan classFound in marine, freshwater, and terrestrial habitatsEscargot is considered a delicacy in restaurants

Con’tCone shells are the only members of the

gastropod class that may be seriously harmful to man. The venomous sting of some cone shell species may be deadly even for an adult.

Impacts on HumansGarden pestsServe as an intermediate hosts for trematode

parasites of humansFasciola hepatica, the liver fluke, has a

complex life cycle, requiring water snails as intermediate hosts. Humans and other mammals acquire the organism by eating cyst-contaminated water plants. Watercress is a common source of the parasite for humans. The cysts release immature flukes that migrate to the liver and gallbladder. A high load of the parasite may obstruct the biliary tract.

TorsionTorsion positions the gills, anus, and openings from

the excretory and reproductive systems the head and nerve cords.

Digestive tract becomes U shapedThree advantages:1. Head enters shell the first. Provides protection from

predators. Some snails have a operculum that closes the opening of the shell and prevents dessication

2. Allows clean water from the front of the snail to enter the mantle cavity

3. It makes the snail more sensitive to stimuli coming from the direction in which it moves

Shell CoilingAsymmetrically coiled into compact formsLeaves less room for organs so now organs

are single, not paired

LocomotionFlattened footOften ciliated and covered with gland cells Creeps across the substrateSmall gastropods use cilia to propel

themselves over mucous trailsLarger gastropods use wave-like muscular

contractions The foot can be used for clinging or for

swimming

Feeding and DigestionMost feed by scraping algae and other organisms

from the substrateOthers feed on larger plantsSome are scavengers, parasites, or predatorsSome have an extensible proboscis (contains the

radula) to get to those hard-to-reach placesDigestive tract is ciliated. Digestive gland releases enzymes and acid into the

stomachFood trapped on the protostyle is freed and

digestedWastes form fecal pellets in the intestines

Maintenance FunctionsGastropods have one gillSome have a siphon. Used by burrowing species to

extend to the surface of the substrate and bring in water

Gills are lost or reduced in land snails – have a rich vascular mantle for gas exchanged between blood and air

Open circulatory system. Blood leaves the vessels and directly bathes cells in tissue spaces called sinuses. Heart is a single, muscular mass consisting of ventricle and two auricles. Some have one ventricle, one auricle due to coiling

Con’tBlood acts like a hydraulic skeleton. They contract muscles to

force fluid into distant structures to push it forwardThe nervous system of the modern gastropod is a concentration

of nervous tissues into large ganglia, especially in the headWell developed sensory structures:1. Eyes at/near the base of the tentacles2. Statocysts are in the foot (used for equilibrium and balance)3. Osphradia – chemoreceptors in the mantle cavity that detect

sediment and chamicals in inhalant water or air. In predatory gastropods, it helps detect prey

4. Nephrida – modifies excretory wastes by selectively reabsorbing certain ions and organis molecules. Aquatic gastropods excrete ammonia, terrestrial gastropods convert ammonia into to uric acid (a semisolid form that helps conserve water)

Reproduction/DevelopmentMany marine snails are dioecious. Ducts

release gametes into the sea for external fertilization.

Other snails are monoecious. Internal, cross-fertilization occurs. Fertilized egg is protected by a capsule.

Snails that are monoecious can be protandric. Testes develop first, and after they degenerate, ovaries mature.

Eggs are shed singly or in mass for external fertilization

Con’tEggs are deposited into moist environments

(leaf litter)Calcareous shell may encapsulate them