Evolutionary Perspective Octopuses, squids, and cuttlefish (all cephalopods) are some of the most...
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Transcript of Evolutionary Perspective Octopuses, squids, and cuttlefish (all cephalopods) are some of the most...
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