Higher Invertebrates

K.2 Phylum Mollusca

1. What are the essential characteristics of molluscs? Exoskeleton: the shells or “suits of

armour” that protects the organism Require a balancing act between

protection and movement Mantle: specialized tissues that create

the shell – secretes CaCO3 (calcium carbonate)

Foot: used for locomotion Soft bodied Visceral mass – most of this tissue is the

edible portion of molluscs

2. Describe the origins of the class names of three types of molluscs.1) Class Bivalvia (Pelecypoda): includes clams and oysters

Pelycypoda: means “hatchet foot”; the shape it takes on when filled with blood

2) Class Gastropoda: includes slugs and snails Gastro refers to “stomach” and pod refers to “foot”; thus these animals

appear to crawl around on their stomach as the foot comprises the soft tissue

3) Class Cephalpoda: includes squid and octopi Can be translated to “head-foot”; the foot is divided into tentacles with

suckers and have a very large head

3. Describe the characteristics of Class Bivalvia (Pelecypoda). Have a variety of lifestyles and range of habitats that made possible by the

presence of a siphon and the shell Siphon: a double tube that can be extended into the ocean from the organism’s

location Incurrent tube: takes in water rich in oxygen and tiny organisms used for

food Bathes the gills for gas exchange; and delivers food to the labial palps

(lips) to begin digestion

Shell: two shells that are hinged together; very important for those that live in the intertidal zone as it can be closed completely to retain enough moisture to survive until the high tide comes back

All bivalves have a smooth interior shell Nacre: the shiny “mother-of-pearl” lining that creates a smooth surface

on the interior of the shell Used by oysters to make pearls

4. Describe the circulatory system in bivalves. These animals exhibit an open circulatory system Blood is delivered to sinuses (cavities) in the tissues Blood is pumped through short arties from a two-

chambered heart The blood soaks the tissues and allows O2 and CO2

to be exchanged as well as other metabolic waste before returning to the heart

There is a major blood sinus in the foot; by filling this cavity with blood, the foot can be extended and anchored

When the sinus is contracted, the blood returns to the heart and the foot contracts; allowing the animal to move

5. Describe the remaining body systems of bivalves.1) Reproductive: are broadcasters; have separate

sexes but must broadcast sperm and ova for fertilization

2) Zygotes form trochophore larvae that are ciliated and spherical; eventually the become large enough to settle and mature into adults

3) Nervous: not highly specialized; have a brain are and some major nerve cords to operate the foot and siphon

4) Excretory: nephridia are localized into one region

5) Digestive: is short with a stomach and an intestine

6. Describe the characteristics of Class Gastropoda. Are thought to be “typical molluscs” because of the average level of

specialization they exhibit is between the bivalves and octopi

Snails: the mantle is located at the open of the shell; produces additional shell material to accommodate growth

Slugs: the mantle is vestigial so does not produce shell material Movement: done via a series of muscular undulations in the foot Slime: produced by the foot to aid in movement – acts as a lubricant

7. Describe the body systems of gastropods. Have similar organization and specialized tissues and organs that are very similar to

molluscs The note-worthy differences include:

Many organisms in this class are herbivores and but all lack feeding appendages

Radula: a tongue-like structure with tiny bony plates the acts like a file to rasp off bits of vegetation

They are hermaphrodites with a mating system similar to annelids

Have a more specialized nervous system with light sensitive “eyes” on long antennae (snails)

Display cephalization

8. Describe the characteristics of Class Cephalopoda. The most highly evolved and specialized members

of this phylum Are active hunters with well-developed nervous

system and can move through the water very quickly

The mantle is an internal tissue that produces a bony plate in the anterior end to protect the brain

They have a very efficient closed circulatory system to deliver oxygen and nutrients to their tissues

9. How do cephalopod eyes work? Have very well-developed and specialized eyes that can exceed the visual acuity in

some mammals The eye is a fluid-filled chamber that is enclosed by the outer skin with the exception

of a small opening

The eye structure has the following components: Retina: nerve endings clustered together to help form an image

Lens: a flexible structure that separates the interior fluid-filled chamber from the outside environment

Cornea: a clear membrane that covers the outer portion of the eye and is exposed to the water

Iris: pigment folds of tissue that can open and close the opening to the lens and control the amount of light that enters the eye