Mollusca and Annelida

Tom McTernanBrian Munger

Mollusca General Info

• 150,000 known species• Most mollusks are marine, but some live in

fresh water environments• Mollusks are soft-bodied animals that are

sometimes protected by a hard shell, depending on the class

Symmetry in Mollusca

• In the beginning of the life cycle of a mollusk, bilateral symmetry is present, but this may be lost by adulthood

Body Development in Mollusks• 3 main parts: muscular

foot, a visceral mass, and a mantle

• The muscular foot is used for movement

• The visceral mass contains most of the internal organs

• The mantle is a fold of tissue that secretes the shell if present, covers the visceral mass

• The shell secreted by the mantle is made of CaCO3

Movement• In Polyplacophora,

Gastropoda, and Bivalvia, a muscular foot is used for locomotion

• In Cephalopods, siphon made from the mantle allows jet propulsion

Germ Layers and Coelom

• Mollusks are triploblastic, the condition of the blastoderm is divided into 3 layers: ectoderm, ectoderm, and mesoderm

• Open circulatory system, where the body cavity is a blood-filled hemocoel filled with hemophyll (blood and lymph between organs)

Life Cycle/Reproduction

• Starts with a ciliated larva, otherwise known as a trochophore

• No true segmentation unlike Annelids

• Normal sexual reproduction is present, although some are hermaphrodites

• Most have external fertilization

Specialized Tissue

• The radula is a tissue unique to the phylum Mollusca, and is a tongue-like structure that scrapes up food

• The mantle cavity is a water-filled chamber that houses the gills, anus, and excretory pores

• Torsion is a process in Gastropoda where one set of muscles grows faster than the other, which creates the shell to grow in a spiral pattern

Chiton (Polyplacophora) Octopus (Cephalopoda)

Scallops (Bivalvia)Squid (Cephalopoda)

Snail (Gastropoda)

Annelids General Info

• There are about 15,000 species of Annelids that range from less than 1mm to 3m

• They live in the sea, many freshwater habitats and soil

• Annelida means “little rings” for the segmentations present in all species

Symmetry in Annelids

• Annelids posses bilateral symmetry

• They never lose this symmetry, or gain another type throughout their lifetime

Body Development in Annelids

• The Coelom in an Annelid is sectioned off by septa, but the digestive tract, nerve cords, and blood vessels run the length of the whole organism

Digestive System• There are a few

specialized regions in the digestive system of an Annelid; the pharynx, the esophagus, the crop, the gizzard, and the intestine. The metanephridia excrete waste from the blood and coelom fluid our of the worm

Germ Layers/Coelom

• Annelids are triploblastic, the condition of the blastoderm is divided into 3 layers: ectoderm, ectoderm, and mesoderm

• They have closed circulatory systems, and have true segmentation

• Oligochaeta and Polychaeta have more spacious coeloms while the coeloms in leeches (Hirudinea) are filled with tissue

Movement• While there are three

classes of Annelids, only two have setae, which are bristle-like structures that assist in burrowing

• The parapodia are the actual extensions on either side of the organism from which the setae attach to

• Earthworms (Oligochaeta) eat their way through soil

Circulatory system• Consists of networks of

vessels that carry hemoglobin

• Dorsal and ventral vessels are connected by other segmental pairs of vessels

• Five pairs of vessels and the dorsal vessel pump blood throughout the circulatory system

Life Cycle/Reproduction• Some Annelids use fission, or breaking off of the main

body to create another organism• Earthworms cannot reproduce like, this but do have

the ability to regenerate if a segment is amputated• Some Annelids are hermaphroditic while others are

different sexes within a species• Polychaeta have ciliated larva, just like Mollusks• Oligochaeta and Hirudinea are hermaphroditc and

mate throughout the year• The clitella is the thickened band on a part of a

segmented worm that secretes a viscid sac where eggs are deposited

Bristle worm (Polychaeta)

Leech (Hirudinea)

Christmas Tree Worm (Polychaeta)

Earthworm (Oligochaeta) Marine Leech (Hirudinea)