1. Molluscs

2. Development of the coelom
In the precambrian era, the most complex animals in the seas were acoelomate
With time, some animals developed a more elaborate anatomy, a space called a coelom
It is lined with mesoderm, and organs within it are suspended by mesenteries
The coelom is an efficient hydrostatic skeleton, with circular and longitudinal muscles acting against each other
Development of the coelom was a major step in the evolution of the larger and more complicated phyla, beginning with the molluscs

3. Characteristics of the Phylum Mollusca
A very large phylum with 90,000 living species and 70,000 extinct fossil species
Many more molluscs have not been formally named yet
While many have a shell, the body is soft
This phylum includes chitons, tooth shells, snails, slugs, nudibranchs, sea butterflies, clams, mussels, oysters, squid, octopuses and nautiluses

4. Representative Molluscs

5. Mollusc Characteristics
Some molluscs such as Tridactna (the giant clam) may weight (counting the shell) as much as 900 pounds.
Squid may be over 50 feet long; many molluscs, however are about 2 inches
Shell collecting has been a popular hobby for many people for hundreds of years
Native Americans in the Gulf of Mexico region used seashells as money in trade with neighboring tribes

6. Classes of Molluscs Gastropoda - snails with shells and slugs
Bivalvia - clams, oysters
Polyplacophora – chitins
Cephalopoda – squid, nautiluses, octopuses
Monoplacophora
Scaphopoda
Caudofoveata
Solenogastres

7. Relationships among Molluscs

8. Ecological Relationships
Molluscs are found from the tropics to the polar seas
Most live in the sea as bottom feeders, burrowers, borers, grazers, carnivores, predators and filter feeders
Fossil evidence indicates that molluscs evolved in the sea and most have remained marine
Some bivalves and gastropods have moved to estuaries (brackish) and freshwater
Only snails have been successful living on land, they are most common where the habitat is moist and sheltered, with calcium available in the soil for their shells

9. Economic Importance
Oysters “on the half-shell” are a way of life in Florida (and other states as well)
Digging clams at low tide in coastal states provides much needed income
Culturing of oysters for pearls is probably a billion dollar industry; in Japan in the 1960’s, you could pay $1 for an oyster and were guaranteed a pearl
- There are freshwater pearls too!!
Teredo, the burrowing worm (but really a mollusc), causes untold damage to wooden docks, as well as wooden boats
Snails and slugs are unwelcome guests in the garden

10. Mollusc Body Plan
The head-foot portion contains the feeding, sensory, and locomotor organs
The visceral mass portion contains digestive, circulatory, respiratory, and reproductive organs
The protective mantle is formed by two folds of skin
The space between the mantle and body wall is the mantle cavity
Within the mantle cavity are the gills (or lungs, in the case of pulmonate snails)
In most molluscs, the mantle secretes a protective shell over the visceral mass

11. Anatomy of a generalized Mollusc

12. The head-foot complex
Most molluscs have a well-developed head bearing the mouth and some sensory organs
Photosensory receptors range from simple eyes to complex eyes such as in the squid
Tentacles may be present
Posterior to the mouth is the chief locomotor organ, the foot

13. The Radula (Fig. 10.4)
The radula is found only in the molluscs; it is found in all except bivalves, some gastropods and solenogasters
It is a protruding, rasping (like a wood file), tongue-like organ
On the radula are rows of tiny teeth, up to 250,000, pointed backward
By a filing action, the radula rasps off fine particles of food material from surfaces, but can also drill a hole through hard shells
The radula moves the particles of food to the digestive tract
Movements of the radula are supported by complex muscles and cartilages (called the odontophore)

14. The molluscan foot The foot is usually ventrally located
It can be used to attach to the bottom of the ocean or lake (called the substratum) or can also be used in location (clam vs squid or the tortoise and the hare)
Modifications of the foot include the attachment disc of limpets, and the siphon jet of squid)
The foot often secretes mucus which aids in adhesion or in helping some molluscs glide on cilia

15. The Visceral Mass Mantle and Mantle Cavity
A mantle is a sheath of skin on each side of the body, it secretes the shell when present
The mantle cavity contains the gills or lungs that develop from the mantle
The exposed surface of the mantle also serves for gaseous exchange
Digestive, excretory, and reproductive products are emptied into the mantle cavity
In aquatic molluscs, a continuous flow of water brings in oxygen and food, and flushes out wastes
Cephalopods (squid) use the head and mantle cavity to create jet propulsion

16. The Shell
When present, the shell is secreted by the mantle and is lined by it
The outer layer of the mantle is called the periostracum, and is composed of conchiolin (a tough membrane made of protein)
The middle of the shell is the prismatic layer made of calcium carbonate
The periostracum protects the shells of freshwater molluscs from acid caused by decaying leaves
The shell appears during the larval period and grows throughout life

17. The shell and mantle of a bivlave

18. Internal Structure and Function
The open circulatory system includes a pumping heart, blood vessels and blood sinuses
The open circulatory system is the most common type in molluscs
Most cephalopods have a closed system with a heart, vessels, and capillaries
The digestive system is quite complicated and specialized for different feeding habits

19. Internal Structure and Function
Most molluscs have a pair of kidneys called metanephridia
The kidney ducts also discharge sperm and eggs
The nervous system has pairs of ganglia with various specialized sense organs
Most molluscs are dioecious (separate sexes) but some gastropods are hermaphroditic
The egg hatches into a free-swimming trochophore larvae
In many gastropods and bivalves, there is an intermediate stage, the veliger, wich occurs next

20. Trochophore Larva

21. Class Monoplacophora
Thought to be extinct, a living specimen was found in 1952; a dozen species are now known
Small molluscs with rounded shells resembling limpets
Some indication of segmentation (or metamerism) suggesting the ancestor was also segmented
Neopilina, however, only shows pseudometamerism

22. Class Monoplacophora

23. Class Polyplacophora (chitons)
Flattened mollscs about 2 inches long, with 8 overlapping dorsal plates
Most prefer rocky intertidal surfaces, and survive by scraping algae from the rocks with their radula
The mantle extends around the chiton margin, forming a girdle
Gills are suspended from the roof of the mantle cavity and grooves form a closed chamber so water flows from anterior to posterior
The plates have photosensitive structures to perceive light
Osphradia serve as a sense organ to sample water in the mantle cavity
Sexes are separate; a trochopore larva is present

24. Anatomy of a Chiton

25. Scaphopoda (the tooth shells)
Live in the marine environment
Slender body covered with a mantle; the shell is open at both ends
Foot protrudes from the larger end to burrown into md
The foot and ciliary action move respiratory water through the mantle cavity
Gills are absent and gaseous exchange occur via the mantle
Detritus and protozoa are caught on cilia on the foot or the mucous-covered knobs of the tentacles

26. Dentalium, a scaphopods

27. Class Gastropoda
The most diverse of the molluscs; contains 70,000 living and 15,000 extinct species
Includes snails, slugs, limpets, whelks,conches, periwinkles, sea slugs, sea hares, sea butterflies
From marine forms with ancestral features to air-breathing terrestrial snails and slugs
Gastropod shells are one-piece univalves, coiled or uncoiled

28. Torsion in the gastropods
All living gastropods have descended from ancestors which were both coiled and torted, whether they show those features today or not
Torsion is a twisting process that begins with the veliger larva, changing it from a bilateral to an asymmetrical form
The mantle cavity moves from the rear position (shown in the picture of the ancestral mollusc) and comes to rest over the head, facing towards incoming water
When torsion is completed, the gills, anus, and reproductive glands

29. Torsion in the gastropods
It is not known what the original advantages were to ancestral gastropods in adopting torsion
It may have some advantages and disadvantages
An advantage may be that the gills face fresh incoming water with high oxygen content
The disadvantage is the waste products from the anus come out just above the head where sensory organs are located
However, the mantle cavity is designed in such a way that wastes are quickly dispersed into the water
Also, the sperm and eggs are also sent out into the water in the same way

30. Torsion and coiling in the gastropods

31. Feeding habits of gastropods
Having a radula results in much variation in how snails feed
Many are herbivorous; some graze on plankton
Some scavenge on decaying flesh, others are active carnivores using the radula to ear apart prey
Oyster drills and moon snails drill holes in the shells of bivalves and insert their proboscis to such out tissues and juice
Species of Conus can deliver a lethal stink to capture prey

32. Other gastropod structures
Land snails that are pulmonates have a highly vascular area in the mantle that acts as a lung
Aquatic pulmonates come to the surface to expel a gas bubble and then inhale by forming a siphon with their mantle
The sense organs of gastropods include eyes, statocysts, tactile organs and chemoreceptors
Gastropods include both monecious and dioecious species

33. Class bivalvia
This class includes mussels, clams, scallops, oysters and shipworms (Teredo)
They range from 1 mm to hundreds of pounds
Most are sedentary suspension feeders dependent on ciliary currents to bring in small particles of food
They lack a head, radula, or any other kind of cephalization
Most are marine; some live in freshwater streams, ponds and lakes

34. Shell morphology
The two shells or valves are held together by a very strong hinge ligament
The valves are drawn together by strong adductor muscles
The umbo is the bulge, and is the oldest part of the shell with growth occurring outward in rings

35. Body and Mantle
A visceral mass is suspended from the dorsal midline, with gills hanging down on each side
The foot is attached anteroventrally
The posterior edges of the mantle form excurrent and incurrent openings
These “siphons” can be very long in clams that burrow in sand or mud
Bivalves have a three-chambered heart that pumps blood to the gills, mantle, and kidney

36. Feeding mechanism of freshwater clams

37. Anatomy of a freshwater clam

38. Feeding and digestion
Suspended organic matter enter the incurrent siphon
Gland cells on the gills secrete mucous which entangles the food particles
The food particles then slide to food grooves at the bottom of the gill
Cilia and grooves on the labial palps direct the mucous mass into the mouth
In the stomach, the mucus and food particles are kept mixed by the crystalline style which produces enzymes
The stomach sorts food particles and directs them to the digestive gland for intracellular digestion

39. Locomotion in the bivalves
The slender foot is extended out between the valves
Blood is pumped into the foot; it swells and anchors the bivalve in the mud
Shortening the foot then pulls the clam forward
Locomotion in scallops is accomplished by clapping their valves to create a jet propulsion effect, the mantle guides the stream of water
Some mussels don’t move once they complete the larval stage; mussels attach by byssal threads

40. Reproduction and development
Sexes are usually separate, oysters undergo protandry where they start out as males and then change sex and become females
Fertilization of eggs by sperm is external
Embryos develop as trochophore larva, some then develop into veligers, and end up as spat larval stages before settling down to the bottom and metamorphosing into adults
The larvae of freshwater clams develop into glochidia which attach to the gills of fish where they live for a short time as parasites

41. The glochidia of freshwater clams
The glochidia eventually leave the gills of the fish and settle to the bottom; this hitchhiking helps to increase the distribution of the species
Freshwater clams are highly sensitive to pollution and thus their survival is in question

42. Class Cephalopoda
This class includes squids, octopuses, nautiluses, devilfish and cuttlefish
All are marine predators
The giant squid is the largest invertebrate known
The eye of the giant squid (Architeuthis”) is 10 inches in diameter and very sophisticated in structure

43. Class Cephalopoda
Cephalopods feed on fish, mollusks, crustaceans, and worms
The foot is in the head region and is modified for expelling water from the mantle cavity
Octopuses and cuttlefish secrete venom from the salivary glands
The beaklike jaws grasp prey; the radula tears off pieces of flesh

44. The shell of the cephalopod
The shell of the cuttlefish and squid are enclosed internally within the mantle cavity
The octopus has completely loss the shell
The Nautilus in ancient times, had a shell which was straight but overtime, has become coiled
The shell of the Nautilus fills with gas which causes it to stay upright; they can swim
Because the Nautilus may be found swimming as deep as 400 meters, the atmospheric pressure on the shell may be as high as 600 pounds per square inch

45. The chamber Nautilus

46. Body and Mantle of the Cephalopod
Octopods have 8 arms with suckers; squid have 10 arms, 8 arms with suckers and pair of long retractile tentacles
Tentacles grasp food causing it to adhere with secretions, not suckers

47. Body and Mantle of the Cephalopod
Water enters the mantle cavity through the neck region
Contraction of the mantle edges about the neck expels water through the funnel
Water currents provide oxygen to the gills, they also create water propulsion, and carry waste and reproductive products away from the body
The cephalopod brain is the largest of an invertebrate
Sense organs are well developed, the eyes are complex, complete with cornea, lens and retina

48. The eye of the cuttlefish

49. Special features of cephalopods
Cephalopods can change color by expanding and contracting chromatophores in the skin
This process is controlled by the nervous system
Many of the color changes are associated with alarm signals or mating
Some deep-sea squids are bioluminescent
Many cephalopods can eject ink which forms a smokescreen that may confuse an enemy when released

50. Reproduction Sexes are separate Males display and court females
Fertilization is internal and are attached to stones or other objects to develop
Some of the octopii provide parental care to their eggs until they hatch