Molluscs & Annelids

Zoology LS2014, Donald Winslow

3 March 2008,

Following Hickman, et al., 2008

Ch. 16 (pp 332-344, 346-357)

Ch. 17 (pp 363-378)

Molluscs & Annelids

• Protostome

• Spiral cleavage

• Mosaic development

• Eucoelomate

• Schizocoelous coelom formation

• Many have trochophore larvae

Phylum Mollusca

• Coelom usually only around heart

• Unsegmented with organ systems

• Trochophore & veliger larvae in many

• Gills (ctenidia) or lung

• Circulatory system usually open

• Variation in body size

• Mantle, mantle cavity, shell, foot, head

Molluscan characteristics

• Radula & odontophore

• Visceral mass

• Surface epithelium, cilia & mucous glands

• Metanephridic kidneys (drain from coelom)– Empty liquid waste into mantle cavity

• Usually nerve ring & paired ganglia

• Mostly dioecious

Phylum Mollusca

• Classes Caudofoveata & Solenogastres

• Class Monoplacophora—one shell plate

• Class Polyplacophora—8 shell plates

• Class Scaphopoda—tusk shells

• Class Gastropoda—snails & slugs

• Class Bivalvia—clams, mussels, etc.

• Class Cephalopoda—squid, octopus, etc.

Class Gastropoda

• Snails w/ coiled or uncoiled shells

• Slugs w/ no shells

• Pulmonate land snails & slugs

• Marine nudibranchs (no shell, “sea slugs”)

• Poisonous cone shells

• Conchs, whelks, limpets, abalones

Class Bivalvia

• Clams, scallops, mussels, oysters

• Zebra mussels, shipworms, giant clams

Class Cephalopoda

• Giant squid

• Cuttlefish

• Octopus

• Chambered nautilus

Phylum Annelida

Developmental characteristics

• Spiral cleavage & mosaic development

• Protostome

• Eucoelomate

• Schizocoelous coelom formation

• Segmentation (metamerism)– External rings called annuli– Few differences between segments

Morphological characteristics

• Bilateral symmetry

• Chitinous setae (bristles)

• Parapodia in Class Polychaeta

• Cerebral ganglia

• 2 ventral nerve cords with giant axons

• Ganglia & lateral branches in each segment

Specialization of annelid head

• Tactile organs

• Taste buds

• Statocysts

• Photoreceptors

Annelid circulatory system

• Closed

• Dorsal blood vessel pumps blood

• Aortic arches control blood flow

• Ventral blood vessel

• Respiration by skin, gills, or parapodia

Digestion and excretion

• Complete digestive tract

• Unsegmented digestive tract

• Nephridia in each segment remove waste from blood.

Reproduction of annelids

• Complete regeneration when injured

• Monoecious or dioecious sex

• Trochophore larvae in some taxa

• Budding in some taxa

Sections of annelid body

• Prostomium (“head”)

• Somites (“body”)

• Pygidium (“tail”)

Annelid coelom

• Septa divide coelom and separate segments.

• Fluid in coelom provides hydrostatic pressure for “hydrostatic skeleton”.

• Epidermis secretes cuticle.

• Peritoneum (visceral & parietal)

• Mesenteries (dorsal & ventral)

Phylum Annelida

• Class Polychaeta– Mostly marine– Some tube-dwelling– Mostly dioecious (separate sexes)

• Class Oligochaeta– Earthworms and other terrestrial & freshwater

species

• Class Hirudinida—leeches and relatives

Examples of polychaetes

• Nereis

• Eunice viridis—Samoan palolo worm

• Featherduster worms

• Chaeopterus

Earthworms

• Mix, aerate, hydrate, & fertilize soil

• React to many stimuli, learn

• Hermaphroditic, testes, ovaries

• Seminal vesicles, oviducts, seminal receptacles

• Clitellum, fertilization in cocoon

• Freshwater oligochaetes with gills

Leeches

• Ectoparasitic

• Anterior & posterior suckers

• Medicinal uses