1. Chapter 12 Molluscan Success

2. Evolutionary Perspective
Octopuses, squids, and cuttlefish (all cephalopods) are some of the most adept predators of the invertebrate world
Class Cephalopoda once numbered approx 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

3. E.P. con’t
Molluscs and all animals from this point on are triploblastic.
They are the first to possess a coelom
Coelom performs multiple functions (refer to previous notes for these functions)
Molluscs are protostomate animals
Coelom 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.

4. Molluscan Characteristics
1. Body of two parts: head-foot and visceral mass
2. Mantle that secretes a calcareous shell and covers the visceral mass
3. Mantle cavity functions in excretion, gas exchange, elimination of digestive wastes, and release of reproductive products
4. Bilateral symmetry
5. Protostome characteristics – trochophose larve, spiral cleavage, and schizocoelous coelom formation
6. Coelom reduced to cavities surrounding heart, nephridia, and gonads
7. Open circulatory system in all but one class (Cephalopods)
8. Radula usually present and used in scraping food

5. Three Main Regions 1. Head-foot Elongate with an anterior head
Contains mouth and nervous and sensory structures
Elongate foot – used for attachment and locomotion
2. Visceral Mass
Contains the organs of digestion, circulation, reproduction, and excretion
Positioned dorsal to the head-foot

6. Con’t
Mantle
Usually 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:
Outer layer – periostracum. Secreted by the mantle’s outer margin cells
Middle layer – prismatic. Thickest of the three. Consists of calcium carbonate and organic materials. Secreted by mantle’s outer margin cells
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

7. Con’t
In 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

8. Class Gastropoda Includes snail, slugs, and limpets
Over 35,000 living species
Largest and most varied molluscan class
Found in marine, freshwater, and terrestrial habitats
Escargot is considered a delicacy in restaurants

9. Con’t
Cone 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.

10. Impacts on Humans Garden pests
Serve as an intermediate hosts for trematode parasites of humans
Fasciola 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.

11. Torsion
Torsion positions the gills, anus, and openings from the excretory and reproductive systems the head and nerve cords.
Digestive tract becomes U shaped
Three 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

12. Shell Coiling Asymmetrically coiled into compact forms
Leaves less room for organs so now organs are single, not paired

13. Locomotion Flattened foot Often ciliated and covered with gland cells
Creeps across the substrate
Small gastropods use cilia to propel themselves over mucous trails
Larger gastropods use wave-like muscular contractions
The foot can be used for clinging or for swimming

14. Feeding and Digestion
Most feed by scraping algae and other organisms from the substrate
Others feed on larger plants
Some are scavengers, parasites, or predators
Some have an extensible proboscis (contains the radula) to get to those hard-to-reach places
Digestive tract is ciliated.
Digestive gland releases enzymes and acid into the stomach
Food trapped on the protostyle is freed and digested
Wastes form fecal pellets in the intestines

15. Maintenance Functions
Gastropods have one gill
Some 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

16. Con’t
Blood acts like a hydraulic skeleton. They contract muscles to force fluid into distant structures to push it forward
The nervous system of the modern gastropod is a concentration of nervous tissues into large ganglia, especially in the head
Well developed sensory structures:
Eyes at/near the base of the tentacles
Statocysts are in the foot (used for equilibrium and balance)
Osphradia – chemoreceptors in the mantle cavity that detect sediment and chamicals in inhalant water or air. In predatory gastropods, it helps detect prey
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)

17. Reproduction/Development
Many 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

18. Con’t Eggs are deposited into moist environments (leaf litter)
Calcareous shell may encapsulate them