Phylum Mollusca

Biology 112

Mollusks

Snails, slugs, clams, octopus??? Do diverse, yet the same phylum??? Most mollusks have soft bodies and some

type of protective shell 3 classes:

1. Class Gastropoda – snails and slugs

2. Class Pelecypoda – bivalves

3. Class Cephalopoda – squid and octopus

Class Gastropoda: snails and slugs Largest mollusk group 35,000 species Aquatic (marine and fresh), and the only terrestrial gastropods are

snails and slugs Aquatic species have gills for gas exchange. They absorb oxygen from

the water and release carbon dioxide into it. Terrestrial species do not have gills. Instead, they have evolved a

simple lung where gases diffuse between the air and the blood. Variety of feeding habits; plants, meat, dead organisms, and a few are

parasites. Bilateral symmetry Has a feeding device called a radula that has hard, toothlike projections

that acts like a file scraping food from surfaces. Travels on a large muscular foot that secretes mucus from glands that

help aid in mvmt.

Circulation in Mollusks

Different from annelids (which has a closed circulatory system – blood contained in vessels)

Mollusks have an open circulatory system in which blood is not confined to vessels after leaving the heart. A two chambered heart pumps blood into the around the coelom.

Mollusk blood has an oxygen carrying molecule like hemoglobin, called hemocyanin – although it is blue in color, not red.

Class Pelecypoda: Bivalves

Oysters, clams, scallops and mussels Mollusks with two shells or valves; therefore

called bivalves. The two valves are hinged so they can open

or close through the action of several large muscles

When closed they serve as protection from predators.

Class Cephalopoda Squid and octopus Most highly specialized mollusks Have a well developed nervous system with a large brain and

complex sensory organs. In cephalopods, the foot has evolved into long arms that project

from the head. The arms are equipped with suction disks. These are used to grasp prey and pull food toward the mouth.

They are all carnivorous The mouth of cephalopods has a pair of hard, beaklike teeth

used for biting and tearing prey. They are active movers Only mollusks with a closed circulatory system

                                                                                        

                    

Phylum Arthropoda

Make up about 80% of the known animal species

Insects are by far the most common species of arthropods

Subdivided into two subphyla: the chelicerates and the mandibulates.

Cherlicerates: spiders, ticks Mandibulates: crayfish, centipedes,

millipedes, insects

Arthropods

General Arthropod Characteristics: Most prominent characeristic is their outside

skeleton, or exoskeleton. Exoskeleton provides support, protection and

attachment site for muscles. Made of chitin (strong, flexible, polysaccharide)

All arthropods molt, or shed their exoskeletons periodically. After it molts, it grows before the new skeleton hardens.

Have joints between body sections Have jointed appendages (a structure such as a leg

that grows out from the main part of the body) Have segmented bodies

Characteristics cont…

Cephalization is more prominent than in annelids (brain and complex sense organs in the head region)

Special organs sense touch, vibration, and chemicals. The eyes of many arthropods are particularly specialized.

Open circulatory system. The dorsal heart pumps blood from the posterior end of the animal to the anterior end.

Subphyla Chelicera

The body of chelicerates has two major parts. 1. The cephalothorax is a fused section composed of the head and any body segments that have legs attached. 2. The abdomen consists of posterior segments that contain most of the internal organs.

The chelicerates include the class arachnida (spiders) and other chelicerates include mites and ticks.

Subphyla Mandibulata:

Unlike chelicerates, mandibulates have - mandibles, or jaws, for chewing food.

All mandibulates have antennae – segmented sense organs on the head

They have three or more pairs of walking legs There are 4 major classes of mandibulates:1. The crustaceans (crab, lobster, and **crayfish**)2. The diplopods or (millipedes)3. The chilopods or (centipedes)4. Insecta

The Crustaceans:

Crayfish dissection on Friday!

Assignment

Complete worksheet on molluscs and arthropods reading information from your textbook.

Pick up crayfish dissection worksheet to review before tomorrow’s class!