Chapter 35

Mollusks and Annelids

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I. Phylum Mollusca Things you should know about mollusks after this chapter

• Key characteristics of mollusks.

• The body plan of different mollusks.

• Name three classes of mollusks.– Gastropods – Bivalves– Cephalopods

A. Characteristics of Mollusks:

• 2nd largest animal phylum (Arthropoda = largest)– Invertebrates such as clams, snails, slugs, squids, and octopuses.

• from Latin molluscus, which means “soft.– Many have soft bodies /some have a hard shell.)

• Feeding– Some are sedentary filter feeders– others are fast-moving predators.

• Mollusks are true coelomates.

• Trochophore larval stage – Most aquatic mollusks and annelids (NOT ALL) have this stage

B.     Mollusk Body Plan• Divided into 2 parts:

– Visceral mass• which contains the heart and the organs of digestion, excretion,

and reproduction

– Head-foot, which consists of:• the head, contains the mouth (including Radula- a tongue

like feeding adaptation with tiny teeth that point backwards) and sensory structures

• the foot, a large, muscular organ for locomotion

• Coelom -limited to space around heart.

• Mantle -a layer of epidermis that covers the visceral mass. (secretes the shell)

C.    Classes1. Gastropoda

(snails, slugs, albalone, conch)

2. Bivalvia (clams, oysters, scallops)

3. Cephalopoda (octopuses, squid, cuttlefish, chambered nautiluses)

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Class Gastropoda

• The largest and most diverse class of mollusks

• Most, including snails, abalones, and conchs, have a single shell. Slugs have no shell.

• Undergo the process of torsion during larval development.

• Open circulatory system in which a heart pumps hemolymph from gills or lungs into the hemocoel.

Class Bivalvia• Aquatic mollusks

(clams, oysters, & scallops)

• Shell is divided into two valves connected by a hinge.

• Most are sessile filter feeders.

• Bivalves lack a distinct head

Clams• The mantle cavity of a clam is sealed except for

a pair of hollow, fleshy tubes called siphons.– Water enters through the incurrent siphon. – Water leaves through the excurrent siphon.

Other Bivalves• Oysters attach to a hard surface

• Scallops can move through the water by repeatedly opening their valves and snapping them shut.

Class Cephalopoda• Means “head-foot.” • Free-swimming predators • Have tentacles & beaklike

jaws on the head. • Nervous system is very

advanced.

• Have a closed circulatory system.

• Many use pigments to hide and disguise themselves.

Squids• 10 tentacles.• Propels by pumping jets of water with the mantle through an

excurrent siphon.

Octopuses• 8 tentacles • They often crawl along the ocean bottom or lie in wait for prey.

Chambered Nautiluses• The only existing cephalopod with external shell.• Shell is coiled & divided into chambers. • The body is confined to the outermost chamber.

Cephalopod Intelligence• Considered the most intelligent invertebrates.• Examples:

– Dexterity, tool use and manipulation. Suction cups & arms are as efficient as human’s hand.

– Can learn through observation (choose colored balls)– Can “Hide” & act like sea weed, – Assess their prey. – Scientists saw octopus complete & remember mazes & patterns

• Notes John Messenger, a neurobiologist at the University of Sheffield. They learn these things rather faster than a vertebrate will – like a pigeon or a rat.“ "That is quite impressive." He also points out, however, that although cephalopods learn faster at first, their skills level off. A trained octopus will always make more mistakes than a trained rodent, he says.

http://en.wikipedia.org/wiki/Cephalopod_intelligence , http://www.fortunecity.com/emachines/e11/86/cephpod.html

II. Phylum AnnelidaThings you should know:

• Advantages of body segmentation.

• Structures used to divide annelids into three classes.

• Structures in earthworms

• Name 3 classes of annelids.

A. Characteristics of Annelids

• Bilaterally symmetrical, segmented worms. (earthworms, feather-duster worms, & bloodsucking leeches.

• Annelids means “little rings” (many body segments).

• True coelom that is divided into separate compartments by partitions.

• Most annelids have external bristles called setae • Some have fleshy protrusions called parapodia

Class Oligochaeta

• Live in the soil or in fresh water • Have no parapodia.

• Oligochaeta means “few bristles”; have a few setae on each segment.

• Most familiar is the earthworm.

Structure and Movement• An earthworm’s body has over 100 nearly-identical

segments.

• Circular and longitudinal muscles line the interior body wall.

• Locomotion is made possible by segmentation.

Earthworms Feeding /DigestionIngest soil as they burrow through it.

• Soil is moved through these structures:

– mouth

– pharynx

– esophagus

– crop

– gizzard

– intestine• includes the typhlosole

– anus • Earthworms play an important role in the condition of soil.

Closed circulatory system.• Contractions of the aortic arches and the dorsal

blood vessel force blood through body vessels

Respiration and Excretion• O & CO2 diffuse through moist skin, • Cellular wastes and excess water are excreted

through nephridia.

Neural Control• Consists of a chain of ganglia connected by a ventral

nerve cord. • Sensory structures are found in all segments but are

concentrated at the anterior end.

Reproduction

• Earthworms are hermaphrodites, but an individual worm cannot fertilize its own eggs.

– During mating, earthworms press their ventral surfaces together.

– Held together by their setae and by a film of mucus secreted by each worm’s clitellum.

Class Polychaeta • Most annelids are members of the class Polychaeta,

which means “many bristles.” • Polychaetes differ from other annelids in that they have

antennae and specialized mouthparts.• Are the only annelids that have a trochophore stage.

Class Hirudinea

• Hirudinea is the smallest class, consisting of about 500 species of leeches.

• Leeches have no setae or parapodia. • Many leeches are carnivores but some are parasites that

suck blood from other animals.

Earthworm diagrams

Haemophagic Leeches • Attach to their hosts & remain until

they become full & they fall off to digest.

• Bodies are 34 segments. • Have an anterior (oral) sucker formed

from the first six segments of their body, used to connect to a host for feeding and releases an anaesthetic to remain unnoticed by the host.

• Use a combination of mucus and suction to stay attached and secrete an anti-clotting enzyme into the host's blood stream.

• http://en.wikipedia.org/wiki/Leech

•Reading assignment: page 712

Comparing some Mollusks & Annelids

Same in both:– Both phyla are true coelomates– Most aquatic forms develop from a trochophore larvae

• Some Differences:– Annelids are segmented, flexible & soft– Mollusks are not segmented but divided into 2 areas-

a head/foot & a visceral mass

– Many mollusks have a shell (not octopus or slug) , annelids –no shell