Mollusca and Annelida Tom Mc Ternan Brian Munger

Mollusca and Annelida Tom Mc. Ternan Brian 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 Ca. CO 3

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, 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) Snail (Gastropoda) Squid (Cephalopoda) Scallops (Bivalvia)

Annelids General Info • There about 15, 000 species of Annelids that range from less than 1 mm to 3 m • 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 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, 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) Christmas Tree Worm (Polychaeta) Leech (Hirudinea) Earthworm (Oligochaeta) Marine Leech (Hirudinea)