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By Spencer Raak and Jonathan Lowe
• Mollusks include a variety of invertebrates such as clams, octopuses, squid, snails, and slugs. The word mollusk comes from the Latin word molluscus which means “soft”.
Mollusks first appear in the fossil record about 545 million years ago.
During the Ordovician ( about 505-438 million years ago) thousands of species of mollusks appeared.
Aplacophora (shell-less, worm-like)
Polyplacophora (chitons)
Monoplacophora (primitive limpet-like)
Gastropoda (snails and slugs)
Cephalopoda (octopus and squid)
Bivalvia (mussels, clams, scallops)
Scaphopoda (tusk shell)
-Most Mollusks have a calcareous shell.
-The shell provides protection and holds the internal organs.
-Some have tentacles
-The body is turned 180 degrees behind the head in snails
-Can differ between Mollusks
-The pattern of the stomach varies according to the mollusks diet.
-Radula is used in all Mollusks
-Complete digestive system
-Food is taken up by the cells lining the digestive glands of the stomach
-All Mollusks have a muscular foot which they use to move.
-Movement is determined by the structure of the foot
-Herbivorous forms usually move by in waves of muscular contraction
-Cephalopods swim by jet propulsion
-Some rely on lateral fins
-Gas exchange by lungs, gills, and through the body surface
-Land Mollusks have an adapted mantle cavity for breathing through the skin
-They have an open or closed (only cephalopods) circulatory system which includes a heart, blood vessels, and sinuses
-The closed circulatory system of cephalopods has a system of continuous vessels and accessory hearts to supply the organism with the much needed oxygen
-Some have kidneys which filter wastes from the body
-Ammonia excretion typical of aquatic species becomes uric acid with land species
-Do not have a bladder
-Some Mollusks are hermaphrodites, but most have separate sexes.
-Some have trocophore larva that hatch from eggs.
-No organism can reproduce with itself despite being hermaphrodites
-Mollusks eat a variety of animals and plants.
-Most bivalves and chitons are herbivorous.
-All cephalopods and more than half of the gastropods are carnivorous.
CaudofoveataAplacophoraPolyplacophoraMonoplacophoraBivalviaScaphopodaGastropodaCephalopoda
-There are two other classes that are extinct.-Rostroconchia-Helcionelloida
-There are 100 known species
-Largest mantle cavity of all Mollusks
-Gills also used to collect food
-Burrowers
-During periods of scarce water, they retain moisture by being totally inactive
-The shell has a protrusion called the umbo
-The umbo has seasonal growth lines which makes it easier to tell the age
-Can rapidly shut the valves of the shell
-Most have one pair of gills
-Cilia are used to bring water into the inhalant chamber which is where the gills are oxygenated
-Older species of calms elongate their mouth to form a proboscis which surrounds the food
-Over 40,000 species
-The body behind the head is twisted 180 degrees
-Has specialized nerve cells and a complicated nerve system
-Nervous system is ganglionated
-Have eyes and tentacles
-Some are hermaphrodites and others have two sexes
-Fertilization can be external or internal
-Only nautilids have a shell
-Some have an internal shell
-Only 400 species in existence
-Can reach up to 60 feet, but there is a possibility that they can be larger
-Swim by expelling water from their mantle cavity (jet propulsion)
-They have a cartilaginous brain case and a well developed nervous system
-Can exhibit complex behavior such as the ability to solve puzzles and remember that ability
Invertebrates such as earthworms and leeches are a part of the phylum Annelida.
The word annelid in comes from a Latin word annulus which means “little rings”.
-Some of the earliest animals burrowed in the soft sea-floor mud.
-They were segmented worms, as shown by fossil worm tunneling tracks, dated to Ediacaran times (540-650 million years ago).
-Descendants of these earliest worms can be found today living in the mud of quiet bays.
Slow-moving, sediment-eating worms
Active, highly mobile marine worms
Sessile, suspension-feeding worms
Earthworms and relatives
Primitive leech-like annelids
True leeches
Polychaeta
Oligochaeta
Hirudinea
-Annelids are segmented creatures.
-The segments are separated by vertical mesenteries and another mesentery that runs from one end of the animal to the other.
-Polychaeta have appendages called parapodia are used for movement
-Leeches have suckers on both ends of their bodies
All Annelids have:
-A complete digestion system.
-Unsegmented gut that runs through the middle of the body from the mouth to the anus.
-The gut is separated from the body by the coelom.
-Leeches possess sharp jaws to puncture the skin of their host
-Polychaeta also have strong jaws to grab on to their prey
-Annelids possess both circular and lengthwise muscle fibers. These, combined with their segmentation and hydrostatic skeleton, allow for great flexibility in movement.
-One part of the body is able to contract, without affecting the rest of the body.
-Use parapodia to move
-Coordinated muscle contractions
-Most have external bristles called setae
Respiration in annelids occurs primarily through diffusion in their moist skin, although certain species have evolved gills or use parapodia in gas exchange.
Parapodia is used primarily for movement, but can be used in breathing.
-Earthworms are an example of annelids that respire through their skin.
-Reproduction is either sexual or asexual.
-Asexual reproduction is by fragmentation, budding, or fission.
-Among sexually reproducing annelids, hermaphrodites are common, but most species have separate sexes.
-Fertilized eggs of marine annelids usually develop into free-swimming trochophore larvae.
-Eggs of terrestrial forms are enclosed in cocoons and hatch as miniature versions of the adults.
Types of feeding:
-Raptorial feeding-In raptorial feeding, worms seize their food. The Polychaetes
who feed this way have developed jaws that allow them to either grab their food or tear off pieces of it.
-Non-Selective Deposit Feeders-These annelids eat mud, sand, or soil; this is a very common
type of feeding among Annelids. Muddy, shallow-water soil tends to have the most non-selective deposit feeders. However, because the amount of food is still only about 1-2%, non-selective deposit feeders have to eat almost all the time.
-Selective Deposit Feeders-Selective deposit feeders pick and choose their meals. It sorts out
the organic material before eating the soil. Selective deposit feeders typically live in tubes.
-Filter Feeders-In this type of feeding, the worms use their bristles to create a
current of water around themselves, then they trap food particles. Smaller pieces are eaten while larger ones are pushed away. All types of filter feeding worms live in tubes
-Bacterial Symbiosis -In this type of feeding, bacteria within the worms provide the worm with the majority of the nutrition they need. Sometimes, these worms live in harsh and extreme conditions.
-Parasites-Some leeches have developed suckers on one or both of the ends
of their bodies that allow them to suck the blood of other organisms for nutrition.
-The blood system in Annelids is closed.
-It is arranged throughout the animal in its segments.
-Blood flow is towards the head
-Earthworms have 5 pairs of aortic arches which act as hearts
-Excrete wastes and excess water through excretory tubules called nephridia
-Some water is released through pores on the ventral surface
-No bladder
Polychaeta
Oligochaeta
Hirudinea
Mollusks and Annelids share a common ancestor who exhibited:-Bilateral symmetry-A true coelom-Ganglia-Cephalization-A complete digestive system-Sensory organs-External fertilization-Trochophore larva
Annelids and Mollusks both have bilateral symmetry.
Some benefits of being bilaterally symmetrical-Cephalization-Streamlining-Helps with movement
Annelids and Mollusks are coelomates
Having a coelom, Annelids and Mollusks have better flexibility due to the support that the coelom gives.
Some cephalopods can squirt ink if they are attacked.
The ink contains a compound called tyrosinase which irritates the predator’s eyes and disables the sense of smell.
Certain Mollusks can also change color for:-communication-camouflage-show moods
Leeches can suck up to 5 times their body weight in blood
The diversity of mollusks is greater than that of fish and is exceeded only by the large number of insects that inhabit the planet.
