Biology - Chapter 27

Mollusks and AnnelidsCharles Page High SchoolStephen L. Cotton

Section 27-1MollusksOBJECTIVES:Explain how mollusks perform their essential life functions.

Section 27-1MollusksOBJECTIVES:Describe and give examples of the three major classes of mollusks.

Section 27-1MollusksOBJECTIVES:Discuss how mollusks affect humans and other living things.

Section 27-1MollusksPhylum Mollusca- soft-bodied with an internal or external shellevolved in the sea over 600 million years agohave had a long and successful adaptive radiationmore than 100,000 species today

Section 27-1Molluskslive everywhere, from deep ocean trenches to tree topsrange in size from snails as small as grains of sand to giant squids more than 20 metersdifferently looking; but grouped together because of similar development patterns

Section 27-1MollusksMost have a special kind of larvae called a trochophore- swim in open water, and feed on tiny floating plantsFigure 27-2, page 586trochophore larvae also present in the annelids, which are also in this chapter

Section 27-1MollusksMollusks are soft-bodies animals with either an internal or external shellname comes from the Latin word molluscus, meaning softsome today lack shells (like slugs), but thought to have evolved from shelled ancestors

Section 27-1MollusksBody plan consists of four basic parts: (Figure 27-3, page 586)1. Foot2. Mantle3. Shell4. Visceral mass

Section 27-1Mollusks1. Foot- the soft muscular foot usually contains the mouth and other structures associated with feedingcan take many different shapes- flat for crawling; spade-shaped for burrowing; tentacles for capturing prey

Section 27-1Mollusks2. Mantle- a thin, delicate tissue layer that covers most of a mollusks body- much like a cloak3. Shell- found in almost all mollusks; made by glands in the mantle that secrete calcium carbonate (CaCO3)

Section 27-1Mollusks4. Visceral mass- located just below the mantle in most mollusks- this area contains the internal organsThese basic body parts have taken on different forms as mollusks adapted to different habitats

Section 27-1MollusksThe type of foot, and the kind of shell that mollusks have are used to group them into the classes we will studyFeeding: many types of feeding have developed, and they feed on many different kinds of food

Section 27-1MollusksMost mollusks are herbivores, carnivores, or filter feeders- but a few are detritus feeders and others are parasitessome, such as the snails and slugs, feed with a tongue-shaped structure called a radula, which is rough like sandpaperFigure 27-4, page 587

Section 27-1MollusksMollusk herbivores use the radula to scrape algae off rocks in the water, or to eat the buds, roots, and flowers of plantsMollusk carnivores use the radula to drill through the shells of other animals, then swallow the soft tissues inside

Section 27-1MollusksIn a carnivorous snail called cone shells, the tiny rasping teeth of the radula have evolved into long, hollow darts that are attached to a poison gland- can stab prey such as small fishoctopus and sea slugs typically use sharp jaws to eat their prey

Section 27-1MollusksMollusks such as clams, oysters, and scallops are filter feeders, using their feathery gills to sift food from the watercilia on the gills move the mixture of mucus and food into the mouth

Section 27-1MollusksRespiration: gills serve as the organs of respiration, as well as filters for foodmost only use gills for breathingaquatic mollusks include snails, clams, octopus- use gills inside the mantle cavity

Section 27-1MollusksLand mollusks such as land snails and slugs breathe by using a specially adapted mantle cavity that is lined with many blood vesselsthis surface is kept moist so that oxygen can enter the cellssnails and slugs live in moist areas for this reason

Section 27-1MollusksInternal Transport: oxygen and nutrients are transported by the blood to all parts of the bodypumped by a simple heart through an open circulatory systemthe blood is not always in blood vessels, but in sinuses

Section 27-1MollusksOpen circulatory systems work well for slow-moving or sessile mollusks like snails or clamsbut faster-moving mollusks such as the octopi and squids have a closed circulatory system, in which blood always moves inside blood vessels

Section 27-1MollusksExcretion: like other animals, mollusks must eliminate wastesundigested food becomes solid waste that leaves through the anus in the form of fecescellular metabolism produces nitrogen-containing waste as ammonia

Section 27-1MollusksAmmonia is quite poisonous, thus must be removed from body fluidsthis is done by tube-shaped organs called nephridiathe nephridia remove ammonia from the blood, and release it to the outside

Section 27-1MollusksResponse: mollusks vary greatly in complexities of their nervous systems, and ability to respond to environmental conditionsclams and other two-shelled mollusks are basically inactive, burrowing in mud or sandhave simple nervous systems

Section 27-1MollusksThese inactive forms may have several small ganglia near the moutha few nerve cordssimple sense organs such as chemical and touch receptors; statocysts (for balance); and ocelli (eyespots)

Section 27-1MollusksOctopi and other tentacled mollusks are active and intelligent predatorshave the most highly developed nervous systems of all members of their phylummay be more intelligent than even some vertebrates!

Section 27-1MollusksThey have numerous sense organs that help them distinguish shapes by sight, and texture by touchOctopi can be trained to perform different tasks in order to obtain a reward, or avoid punishment

Section 27-1MollusksReproduction: in most mollusks, the sexes are separate, and fertilization is externalin snails and almost all two-shelled mollusks, they release eggs and sperm into open water in enormous numbers

Section 27-1MollusksIn tentacled mollusks and certain snails, fertilization takes place inside the body of the femalesome other hermaphroditic mollusks, such as certain oysters, switch from one sex to the other- sometimes producing eggs, and sometimes sperm

Section 27-1Mollusks1. Class Gastropoda- are called gastropods, the name meaning stomach footappropriate name because they move by means of a broad, muscular foot located on the ventral (stomach) side

Section 27-1MollusksGastropods include familiar pond snails and land slugs, as well as more exotic mollusks such as abalones, sea butterflies, sea hares, and nudibranchsFigure 27-8, page 590

Section 27-1MollusksMany gastropods have a one-piece shell that protects their soft bodieswhen threatened, many snails can pull up completely into their coiled shellssome have small shells, or like slugs- lack shells completely

Section 27-1MollusksMost of these without shells are not completely helplessthey are protected by behavior, spending daylight hours under rocks or logssome sea hares have a special ink they squirt into the surrounding water

Section 27-1MollusksOthers, such as the sea butterflies, escape predators by swimming rapidlymany nudibranchs (or sea slugs), have chemicals in their bodies that taste bad or are poisonous; some even have nematocysts to sting with

Section 27-1MollusksThe bad-tasting, poisonous, stinging, or otherwise booby-trapped nudibranchs are usually bright colored- this warns the predators to stay awaythus, the shell-less gastropods do have means of protection

Section 27-1Mollusks2. Class Bivalvia- have two shells that are hinged together at the back and held together by one or two powerful musclescommon bivalves include clams, oysters, and scallopsmay be tiny, or like the giant clam 1.9 meters in length

Section 27-1MollusksAlthough bivalve larvae are free-swimming, they soon establish life at the bottom of a body of watersome secrete sticky threads to attach themselves to rocksmost sessile; however scallops can flap their shells to move

Section 27-1MollusksMantles of bivalves, like other mollusks, contain glands that manufacture the shells, and also keep the shells inside surface smooth and comfortable by secreting layers of mother-of-pearl

Section 27-1MollusksIf a foreign object (sand or a pebble) gets caught between mantle and shell- the mantle covers it with this secretionafter time, these objects become completely coated, and are called pearloften very valuable

Section 27-1Mollusks3. Class Cephalopoda- called cephalopods, are among the most active and interestingincludes octopi, squids, cuttlefish, and nautilusescephalopoda means head-foot; their head is attached to the foot, divided into tentacles

Section 27-1MollusksCephalopods range in size from tiny cuttlefish less than 2 cm long, to giant squids which are thought to grow greater than 20 meters longthey have flexible tentacles equipped with round sucking disks to grab fish and prey

Section 27-1MollusksMost modern cephalopods have small internal shells, or no shells at allthe only present-day cephalopod with a shell are a few species of nautilusesFigure 27-11, page 592

Section 27-1MollusksCuttlefish have small shells that are found inside their bodies- thin and coiledthis is the cuttlebone on which pet birds condition their beaksthis helps the nautilus and cuttlefish to remain upright and allow it to float in the water - Figure 27-12, page 593

Section 27-1MollusksHowever, there are other means of protection for cephalopodscan move quickly, either by swimming or crawling, or a form of jet propulsion of drawing water in and then outchanging their color; releasing dark, foul-tasting ink

Section 27-1MollusksHow Mollusks fit into the World:feed on plants; prey on animals; clean up their surroundings by eating detritussome are hosts to symbiotic algae or to parasites; others are parasites themselvesfood for many, even humans

Section 27-1MollusksCan be a check on pollution levels, since some are filter-feeding bivalvessnails and other mollusks never seem to develop cancer; why is that? Scientists are interested in finding out!

Section 27-1MollusksAlthough beneficial in many ways, there are also some negative relationships with humansland slugs and snails are plant eaters that do damage to cropsshipworms will damage wood ships- termites of the sea

Section 27-1MollusksSince many are filter-feeders, if we eat them, we are likely to get high concentrations of pathogens (things that cause disease), toxins, or pollutants that can result in sickness or even death

Section 27-2AnnelidsOBJECTIVES:Describe how annelids perform their essential life functions.

Section 27-2AnnelidsOBJECTIVES:List and give examples of three classes of annelids.

Section 27-2AnnelidsPhylum Annelida - segmented worms, or annelidsapproximately 9,000 speciescan live everywhere in the world, except Antarctica and Madagascarmany are aquatic, thus only a few species are familiar to us

Section 27-2AnnelidsThey are round, wormlike animals with a long, segmented bodyfrom Latin annellus- which means little ring, and refers to the ring-like appearance of the body segments

Section 27-2AnnelidsRange in size from tiny aquatic worms less than half a millimeter long to giant earthworms more than 3 metersvary greatly in color, patterning, number of bristles, and other superficial featuresmost appear very wormlike

Section 27-2AnnelidsThe many segments of an annelids body are separated by internal walls called septamost of the body segments are identical to each other, but some are modified to perform special functions- first few may have eyes, antennae, etc.

Section 27-2AnnelidsFeeding: the digestive tract (gut) is a long tube within the body cavity that extends from the mouth to the anusFigure 27-14, page 595pharynx is one structure that has evolved many different forms in different annelids

Section 27-2AnnelidsPharynx is a muscular front end of the digestive tubemany can extend it through the mouth- in many carnivorous annelids it usually has two or more sharp jaws attached to help grab prey

Section 27-2AnnelidsOthers use the pharynx to tear off bits of algaesome detritus feeders have the pharynx covered with sticky mucus to grab food particles; other detritus feeders use it as a pumpleech sucks blood from a host

Section 27-2AnnelidsRespiration: aquatic annelids often breathe through gillsthe feather-duster worms have brightly colored gills that protrude from their burrowFigure 27-15, page 595others may have delicate gills located on the side of the body

Section 27-2AnnelidsMany annelids can exchange gases through their skin; but the skin must remain moist to make gas exchange possibleterrestrial annelids (such as the earthworm) secrete a thin protective coating called a cuticle to hold moisture

Section 27-2AnnelidsInternal Transport: annelids typically have closed circulatory systems organized around two blood vessels that run the length of their bodies (Fig. 27-16, p.596)blood moves toward the head in the dorsal vessel; toward the posterior in the ventral vessel

Section 27-2AnnelidsEach segment has a pair of smaller vessels called ring vessels that connect the dorsal and ventral blood supplyin annelids such as earthworms, several ring vessels are larger, more muscular- often called hearts

Section 27-2AnnelidsExcretion: like other animals, there are two types of wastesolid waste passes out through the anus at the end of the gutwaste from cellular metabolism are eliminated by nephridia- a pair of which are in each body segment

Section 27-2AnnelidsResponse: many annelids are active animals with well-developed nervous systemsbrain sits on top of the gut at the front end of the bodytwo large nerves pass around the gut, and connect brain with a pair of ganglia below

Section 27-2AnnelidsFrom these ganglia, a ventral nerve cord runs the entire length of the wormSense organs are well-developed in the free-living marine formssensory tentacles; statocysts; chemical receptors; two or more pairs of eyes (light detectors)

Section 27-2AnnelidsMany other annelids have much simpler sensory systemsearthworms have no specialized sense organs, but rely on simple sensory cells in the skinlittle protection from predators; burrow or swim away

Section 27-2AnnelidsBut, some annelids do fight backseveral carnivorous annelids use their sharp jawsthe marine fireworms have tufts of poisonous bristles that easily break off and penetrate skin, causing painful sores and a burning sensation

Section 27-2AnnelidsMovement: two major muscle groups in their body wallslongitudinal muscles run from the front to the rear; when they contract, the worm gets shortera group of circular muscles; when they contract they make the worm skinnier

Section 27-2AnnelidsReproduction: some are able to reproduce asexually by buddingmost reproduce sexuallysome species have separate sexes and external fertilization happens in open waterproper timing is essential

Section 27-2AnnelidsIn the South Pacific, islanders eagerly await the autumn spawning season of the annelids called palolo wormsthey are considered a great delicacy to humans, as well as sea birds and fishes that gather to feed

Section 27-2AnnelidsSome such as leeches and earthworms are hermaphroditicundergo internal fertilizationhowever, they rarely fertilize their own eggs - page 599when eggs are ready, and band called the clitellum secretes a mucus ring

Section 27-2AnnelidsThe eggs and sperms are secreted into the clitellumand then the ring slips off the worms body and forms a cocoon that shelters the eggs

Section 27-2Annelids1. Class Polychaeta - many common and important marine worms; many ocean plankton are small polychaetespolychaetes have paired paddle-like appendages on their body segments; tipped with bristles that give their name

Section 27-2AnnelidsNote Figure 27-19, page 599 of the sea mouse- bristles are so long they look like hair or furthe polychaetes live in cracks and crevices in coral reefs; in sand, mud, or piles of rocks; or even in open watersome colorful; others dull

Section 27-2Annelids2. Class Oligochaeta - contains earthworms and related speciesexamples are earthworms and tubifex worms- red threadlike aquatic worms sold as tropical fish food in pet storesoligo means few, so these have fewer bristles

Section 27-2AnnelidsThe bristles (called setae) are felt as roughness on the ventral side of the earthworm; help anchor it in its burrowmostly found undergroundsquiggles of mud are sometimes seen above ground, and called castings

Section 27-2AnnelidsEating materials in the soil, this passes through; some will be digested- other indigestible materials pass out the anus in large quantities- the castingssome tropical varieties castings are enormous- 18 cm long, and 2 cm in diameter!

Section 27-2Annelids3. Class Hirudinea - contains the leeches, most of which live in tropical countriestypically no more than 6 cm long; some tropical species as long as 30 cmmost are freshwater organisms

Section 27-2AnnelidsUsually exist as external parasites, sucking blood and body fluids from the hostroughly 1/4 of all leeches are carnivores rather than parasites; feeding on snails, worms, and insect larvae; either swallow whole or suck soft parts from the body

Section 27-2AnnelidsLeeches have powerful suckers at each end of the bodypenetrate either by: a) muscular proboscis, or b) slice into the skin with razor sharp pair of jawsuses its muscular pharynx to suck blood from the area

Section 27-2AnnelidsA special secretion from the salivary gland keeps the blood from clotting, and it may also anesthetize the wound- thus the host does not even know it has been bitten!Can swallow as much as 10 times its weight in blood

Section 27-2AnnelidsSuch a huge meal may take up to 200 days to digest with the help of symbiotic bacteria in the gutmay live up to a year before needing to feed again

Section 27-1MollusksHow Annelids Fit Into the WorldAnnelids are important in many habitatssome are members of the ocean plankton, where they are food for many fishes, crab, and lobster

Section 27-1MollusksHow Annelids Fit Into the Worldearthworms condition soil by burrowing; helps aerate soilspeed the return of nitrogen and other important nutrients from dead organismsthus, spraying insects is not always good