They lack tissues; there are a few specialized cell types.

Fertilization is external:

Cnidarians are diploblastic and may exist in polyp and medusa stages in their life cycle.

Tentacles are armed with cnidocytes to capture prey.

The gastrodermis lines a gastrovascular cavity for digestion; a single opening to this cavity functions as both mouth and anus. A gelatinous mesoglea is sandwiched between the epidermis and gastrodermis.

Both medusa and polyp stages present; the polyps are often colonial.

Hydra eating (polyp):

Portuguese man-of-war (medusa):

Jellies,\ and sea nettles have free-swimming medusae.

The polyp stage is often absent or reduced.

Moon jelly:

Thimble jellies:

Box jellies and sea wasps have box-shaped medusae and complex eyes.

Their tentacles are highly toxic; some are more potent than cobra venom.

Sea wasp:

Sea anemones, sea fans and most corals have only the polyp stage and are often colonial and sessile.

Sea anemone:

Planarians have a mouth at the tip of a pharynx to take food into the gastrovascular cavity (intestines).

Waste is excreted through the mouth.

The nervous system is organized into two nerve cords and clustered in anterior ganglia.

Eyespots (ocelli) can detect light.

Planaria:

Trematodes (flukes) are parasites that use two suckers to attach to primary host (human).

Sexual reproduction takes place in the primary host: a female fits into a groove on the male's body.

Juveniles mature in intermediate hosts such as snails.

The motile larvae that escape from the intermediate host are called "cercaria".

Tapeworms are parasites that use a scolex to attach to host intestines with hooks and suckers.

Many segments of proglottids produce eggs and break off after fertilization.

They are covered with a tough exoskeleton (cuticle) which must be shed or molted through a process called ecdysis as the animal grows.

Trichinella can cause trichinosis when they are ingested by eating undercooked pork or other meat.

Adults mate in the intestine, producing juveniles, which encyst in muscles.

Molluscs include snails, clams, and octopuses.

Snails and slugs secrete slime on which they crawl.

Slugs have lost their outer shell.

Nudibranchs are sea slugs that use gills for gas exchange.

Nudibranch predator:

Clams, mussels, scallops, oysters have a shell with two valves and paired gills.

They feed by passing water through siphons, and often anchor themselves in the substrate with their muscular foot.

Clam locomotion:

Squids, octopuses, cuttlefish, chambered nautiluses are the only molluscs that exhibit cephalization: development of a head.

They have a closed circulatory system that enables them to move quickly by jet propulsion using siphons.

Tentacles are used to capture prey.

Annelids are coelomates that exhibit segmentation in their body plan.

Earthworms have "few chaetae" (setae), which are bristles made of chitin that aid in anchoring the body to burrow.

The body is segmented with separate mouth and anus.

Circular and longitudinal muscles enable it to move by peristaltic locomotion.

Earthworm locomotion:

These marine segmented worms have a well-developed head.

Each segment has a pair of parapodia ("almost feet") with "many chaetae".

Leeches have flattened segments with no chaetae.

Suckers at the anterior and posterior ends attach to food.

Some species such as Hirudo medicinalis secrete hirudin to prevent blood from coagulating, and are used as medicinal leeches to drain blood from an injury.

Arthropods belong in the clade Ecdysozoa.

The exoskeleton requires them to molt as they grow, in a process called ecdysis.

Horseshoe crabs, scorpions, spiders, ticks, mites have chelicerae and pedipalps in their mouth parts that may be modified as pincers.

They possess four pairs of legs.

Ticks and mites:

Millipedes (class Diplopoda) have two pairs of legs on each trunk segment. Centipedes (class Chilopoda) have one pair of legs per trunk segment.

Crabs, shrimp, and barnacles have two pairs of antennae and three or more pairs of legs.

Many are decapods and have ten legs.

Barnacles:

Decapods:

Echinoderms are spiny-skinned animals.

These deuterostomes reproduce by external fertilization.

Sea urchin spawning:

Sea stars have star-shaped body with multiple arms; suckers on tube feet.

Brittle stars have a distinct central disk; long, flexible arms; tube feet lack suckers.

Sea urchins are roughly spherical or disk-shaped; five rows of tube feet on the spines enable movement.

Sea urchin locomotion:

Sea lilies have feathered arms surrounding upward-pointing mouth.

Sea cucumbers have five rows of tube feet and additional tube feet modified as feeding tentacles.

Sea cucumbers:

Sea daisies have a disk-shaped body ringed with small spines.