Mollusks and AnnelidsChapter 35

Examples with Scientific NamesScientific Name Common NameMollusks:Athleta toumeyi Fossil GastropodCaprinuloidea perfecta Rudist Polysacos vickersianum Fossil ChitonSnail in Amber Fossil SnailAnnelids:Polychaeta PolychaeteCanalipalpata CanalipalpataFeather duster worms Sabellidae

Examples of Mollusks

Evolution• Mollusks•Sipuncula are the sister taxon of Mollusca• (2) the two aplacophoran taxa,Neomeniomorpha and Chaetodermomorpha, are monophyletic with a commonneomenioid-like ancestor, and of the two taxa,• Chaetodermomorpha are more divided, Aplacophora and Polyplacophora are sister taxa and form a clade, Aculifera•Aculifera are the sister group of the remainingextant mollusks, Conchifera•Aplacophora are progenetic Aculifera.

Evolution (Cont.)• Annelids•Some of the earliest animals burrowed in the soft sea-floor mud.

•They were segmented worms, as shown by fossil worm tunnelling tracks, dated to Ediacaran times.

•Descendants of these earliest worms can be found today living in the muddy substrate of quiet bays -

Evolution Diagram

Symmetry of Body• Mollusks•The body symmetry of Mollusca and Annelida is a figure of the same bilateral appearance when it is cut in half longitudinally.

• The body symmetry of Mollusca and Annelida when cut in half is the same mirror image of the other half.

•The body symmetry of Mollusca and Annelida is typically a short soft body with a very small coelom .

Body cavity for Mollusks

Body cavity for Annelids

This is the coelom, found in Mollusks and Annelids

Structural Support• Mollusks• The basic body structure, consisting of a foot, visceral mass, and mantle, is similar in most mollusks.

•The foot is located below the visceral mass.

•The external shell consists of three layers. The thin outer layer, called the periostracum, is made of a tough hornlike material and serves to protect the lower layers.

Structural Support (Cont.)•Annelids •Elongate body usually round.

• For polychaetes and clitellidae, head has prostomium and peristomium (which usually has the mouth); anus is in pygidium.

• Trunk segmentation reflected in external annuli, internally by series of coelomic compartments separated by septae (mostly in oligochaetes, less so in others).•Some are homonomous, with body segments mostly similar, others are heterononmous, with specialized segments.

Nutrition and Digestion• Mollusks

• Process and Structures• macrophagy (predation and herbivory) – use either radula to scrape food or

harpoon-like poisoned teeth• microphagy (filter feeding)- modifications of gills, which are covered by

sheets of mucus which trap food, then are passed into the stomach• A Mollusk has food brought in through a siphon system and then to its mouth.

It then goes to a digestive gland and to its intestine. Wastes leave via the anus.

• Nutrition• They eat plants, animals, and dead things• Many cephalopods eat fish• Bivalves usually eat phytoplankton

Digestive tract for Mollusks

Nutrition and Digestion (Cont.)

• Annelids• Structures and Process

• They have either a straight or coiled gut. • Some have no known digestive system.• Some use a raptorial method by capturing and swallowing prey• Deposit feeders ingest sand/dirt and extract nutrients from it. • Suspension feeders use tentacles or mucus webs to filter nutrients from

fluid environment. • The sediment is ingested and any digestible organic material is assimilated

as it passes through the alimentary canal• Nutrition

• Many eat sediment• Uses non-selective deposit feeding

Digestive tract for Annelids

Transportation and Circulation

• Mollusks• Structures

• 3-chambered heart• Circulatory system (except for cephalopods) is open • Cephalopods have a closed circulatory system and an extra pair of

powerful gill hearts.• Uses one large foot muscle

• Process• Blood from the paired gills enters the atria, leaves the ventricle.• the artery from the ventricle opens and releases blood into tissue-

bathing sinuses, eventually draining back into gills.

Mollusk Circulatory system

Transportation and circulation (Cont.)

• Annelids• Structures

• Well-developed and closed in most annelids• Some have some vessels capable of functioning as "hearts". • Some have no mesodermally derived blood vessels, but have blood

"channels”• The nervous system has a solid, ventral nerve cord from which lateral

nerves arise in each segment. This controls movement.• Process

• Carries blood from the dorsal to the ventral vessel in the head region.• Every segment has an autonomy; however, they unite to perform as a

single body for functions such as locomotion

Annelid circulatory system

Respiration• Mollusks

• Structures and Process• Molluscs use true gills, with extensive capillary network and

countercurrent flow for gas exchange. • A Mollusk has tubes called siphons that bring water in and out of their

bodies so that they can get oxygen and give off carbon dioxide. They have gills that help with this.

• Annelids• Structures and process

• Respiration may occur through gills • In other types, the cilia are the most likely site for respiration.• Many also respire through the skin

Annelid Reproduction

• Annelids may reproduce asexually or sexually.• Most annelids can regenerate in many different ways. Certain species, such

as Dodecaceria can regenerate an entire individual from a single segment.• Asexual reproduction is usually by fragmentation.• Most polychaetes, chiurans, and pogonophorans are gonochoristic.• Gonochorism is the differentiation of male and females of a species by their

reproductive organs. For example, most mammals are gonochoristic

Annelid Reproduction

Mollusk reproduction

• Mollusks come in hermaphroditic and gonochoristic forms. Gastropods can be either.

• When a Gastropod is Hermaphroditic, they usually have only one kind of gonad, and are protandrous.• Protandrous means that the male sex organs are

produced before female organs.• Most bivalves are gonochoristic; though a few are hermaphroditic, like

oysters.

Reproduction of Mollusks

Nervous system•In mollusks, the brain and one or two large ventral nerve cords, are connected by commissures (which is a ladder form).

• Cephalopods can be said to have a true brain, capable of learning and complex behavior.

Nervous system (cont.)•All annelids show a degree of cephalization. A well-developed circumesophageal ganglion in the head region leads to paired ventral nerves with ganglia in each segment controlling the activity of that segment via lateral nerves and coordinating with other segments. Many use giant axons (e.g., Lumbricus) for escape reactions.

Unique characteristics of Mollusks•Mollusks have a soft body that is protected by a hard shell •Mollusks are bilaterally symmetrical.•Mollusks are capable of living on either land or in water.

• They have lungs that can function in both water and air.

Unique characteristics of Annelids

•Like mollusks, annelids are bilaterally symmetrical.•The Annelid moves by alternating circular and longitudinal muscles.

• The circular muscles surround the body wall and contractions cause the body to become long and thin.

• The longitudinal muscles run the length of the body, and cause the body to become shorter and fatter.

•The body cavity is a true coelom, divided by internal septa.•Body possesses 3 separate sections, a prosomium, a trunk and a pygidium.•They live in most environments.

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