Annelids and Mollusks: A Comprehensive Guide to Their Biology
Annelids
Phylum Annelida
Partitions called septa divide the body internally; the segmented coelom acts as a hydrostatic skeleton.
Feeding
The digestive system is a tube extending from mouth to anus: Mouth – Pharynx – Esophagus – Crop – Gizzard – Intestine – Anus.
- Pharynx: Muscular front portion of the system that can extend through the mouth. Carnivores have two or more sharp jaws that shoot out, grabbing prey. Herbivores have jaws to tear up plants. Detritus feeders use thick mucus to suck up food. The pharynx acts as a pump in earthworms and leeches.
- Crop: Food storage
- Gizzard: Grinds up food further
- Intestine: Digestion & absorption
Respiration
Aquatic annelids use delicate gills and filter feed. Land annelids use diffusion to breathe. Their skin needs to stay moist. A cuticle provides a protective coating to keep them moist.
Circulation
A closed system delivers blood to every segment. Two blood vessels run the length of the body.
- Dorsal Vessel: Carries blood to the head.
- Ventral Vessel: Carries blood to the body.
- Ring Vessels: Connect the ventral and dorsal vessels and supply blood to internal organs. Five pairs of larger and more muscular rings near the front of the worm act as hearts.
Waste
Solid waste is expelled through the anus. Other waste is expelled by nephridia, with one pair per segment.
Response
The brain is above the pharynx in the anterior of the body. Two large nerves pass around the gut to the pair of ganglia below. Ganglia merge to form the ventral nerve cord, which runs the length of the body. Each segment has a pair of small ganglia that carry information from the sense organs to the ventral nerve cord and then to the brain.
Free-living marine species have the best-developed sense organs, including many sensory tentacles, statocysts (for vibrations), chemical receptors, and two or more pairs of eyes. Most eyes are simple receptors that can detect shadows and are near the mouth. Few can see objects. Other sense organs are scattered along their epidermis.
Land species have simpler sensory systems.
Reproduction
A few annelids reproduce via budding, but many reproduce sexually. Marine species have separate sexes and external fertilization. Some are hermaphrodites and can self-fertilize or pair up to exchange sperm. Received sperm is stored in the seminal receptacle. When eggs are ready to be fertilized, a band of thickened segments called the clitellum secretes a mucus ring, where sperm and eggs are released. This forms a cocoon.
Classes
- Polychaeta: Many hairs (setae) that anchor the worm. They possess chitinous jaws that capture and eat small animals and crustaceans. Polychaetes show cephalization.
- Oligochaeta: Small setae, no cephalization. When the body bulges, setae protrude in small clusters to grip soil and withdraw when the body contracts.
- Hirudinea: Mostly freshwater, some land and marine. No setae. They produce hirudin, a powerful anticoagulant that prevents clotting.
Both annelids and mollusks have trochophore larvae, suggesting a common ancestor.
Mollusks
Phylum Mollusca
Mollusks are soft-bodied animals with an internal or external shell. They have a coelom and exhibit bilateral symmetry.
Form
Mollusks have four main body parts:
- Foot: Contains the mouth and other feeding structures.
- Mantle: Thin, delicate tissue that covers the body. It can be membranous or muscular, encloses the viscera, and may secrete a shell.
- Shell: Made by glands in the mantle that secrete calcium carbonate.
- Visceral Mass: Contains the internal organs and is soft.
Mollusks are grouped into classes by the type of foot and shell.
Feeding
Most mollusks are herbivores, carnivores, or filter feeders. A few are detritivores and parasites. They feed with a radula, placing the tip of the radula on prey and pulling back and forth. Herbivores scrape algae or eat plants. Carnivores drill through shells. Filter feeders use gills; mucus sticks onto food, and cilia move the food towards the mouth.
Respiration
Gills are inside mantle cavities. Land snails and slugs breathe using an adapted mantle lined with many blood vessels and wrinkled to increase surface area. The surface is kept moist to allow oxygen to enter cells.
Excretion
Similar to annelids.
Circulation
Blood is pumped by one heart. Most mollusks have an open circulatory system, where blood travels through tissues in open spaces called sinuses. Cephalopods like octopi and squid have a closed circulatory system.
Response
Bivalve mollusks have sense organs near the mouth, a few nerve cords, and simple sense organs (statocysts, ocelli, touch and chemical receptors).
Octopi and other tentacled mollusks can remember information. They have many complex sense organs to help them see shapes and textures and are capable of learning.
Reproduction
Most mollusks have separate sexes. Snails and bivalve mollusks have external fertilization, where eggs develop into free-swimming larvae. Snails and tentacled mollusks have internal fertilization in the female. Hermaphroditic mollusks reproduce via internal fertilization and pair to fertilize each other’s eggs. Oysters can change sexes.
Classes
- Bivalvia: Clams, oysters, mussels, scallops. They have two shells held together by one or two powerful muscles. They lack cephalization and have three pairs of ganglia. They have an open circulatory system, and the heart is in the pericardial cavity (the coelom remains). Two kidneys lie below the heart to filter blood. Gills exchange gases. Bivalves are filter feeders: food moves through the incurrent siphon towards the gills. The anus empties into the excurrent siphon. The larva is free-swimming. Most bivalves are sessile, but scallops can move around by flapping their shells. Their shells are made of calcium carbonate. Mantle glands secrete layers of mother-of-pearl.
- Cephalopoda: Tentacled mollusks: octopi, squid, cuttlefish, nautilus. The foot has evolved into tentacles. They use a beak and radula to tear prey. Their advanced eye evolved separately from vertebrates. The brain consists of many fused ganglia. Large nerves control the contraction of the mantle for jet propulsion. Most have eight tentacles with suction cups attached, which helps grab and hold prey. Only the nautilus has a shell. For protection, cephalopods move quickly, release ink, and change color.
- Gastropoda: Snails, slugs, abalones, nudibranchs, etc. They have a stomach foot and move by using a muscular foot on the ventral surface (stomach) that contracts. They have a one-piece shell, though some have a small shell or no shell at all. In conches and snails, a coiled mass contains the visceral mass. Some have vessels under the mantle that function as a lung on land. Gastropods use various forms of protection, including hiding under rocks, releasing ink (sea hares), swimming away, using poisonous chemicals, and employing nematocysts (nudibranchs) or shells.