Animals are nearly all multi-cellular with a central chamber or tube serving in acquiring and digesting food. Aside from sponges, they typically have distinct tissues including nerves and muscles that coordinate movement. Most thus act as a single functional unit, in some ways more like protozoans than complex plants, chromists, and fungi.
Adults vary from the size of ciliates to dozens of metres in length. Reproduction is commonly sexual, but the smallest kinds are often parthenogenetic, with females able to lay eggs without mating. Many other animals have a specialized larval stage, particularly those from oceans and lakes.
Cnidaria have distinct tissues but no complex organ systems. Most have radial symmetry around the mouth, which is used for both ingestion and excretion, and is surrounded by stinging tentacles that capture prey. There are two main forms, attached polyps and free-swimming medusae.
Greek hydra, water serpent
Hydra are slender freshwater polyps up to 2 cm in length. They are contractile and can crawl by forming attachments at each end in alternation. They usually reproduce by budding.
Adults also have gonads, with both testes nearer and ovaries farther from the mouth. Most kinds are whitish or brown, but one species usually has green zoochlorellae.
Phylum Gnathifera – rotifers & allies
The remaining animals are usually bilaterally symmetric and have tissues arranged into distinct organs. Most have a digestive tract with two openings, and in some groups an internal body cavity or coelom. In smaller kinds, though, the latter is reduced or absent.
Rotifers are some of the most common animals less than 1 mm in size. The head has cilia around the mouth and pharyngeal jaws, and many have an adhesive foot.
Classes included here: Bdelloidea,
Gastrotrichs are aquatic and mostly less than 1 mm long. Most resemble rotifers and often have posterior adhesive glands, but no jaws or defined foot. Their bodies have cilia along the underside, scales on the back, and often sensory bristles around the head.
Greek chaitē, long hair, nōton, back
Chaetonotus have a slender body with a forked posterior, making two toes. They differ from relatives in having spines formed from dorsal scales.
Phylum Platyhelminthes – flatworms
Flatworms are largely aquatic carnivores or parasites, though some are terrestrial or more herbivorous. The digestive tract is sometimes branched but characteristically has a single main opening, or may be reduced or absent. Most mobile forms have cilia though they may also crawl or even swim using muscular contractions.
Greek stenos, narrow, stoma, mouth
Stenostomum are slender with a pair of sensory pits at the front but no eyespots. There are also new pits near the middle, where they grow into head-to-tail chains that then separate.
Greek makros, long, stoma, mouth
Macrostomum have the mouth as a front-to-back slit set close behind two eyespots. They are typically flat with rounded ends and usually a slightly bulged tail.
After Sir John Dalyell, 1775-1851
Dalyelliids have an anterior mouth and barrel-shaped pharynx. The body is more or less cylindrical with a tapering tail. Most tend to be brownish or green and grow 1-5 mm.
Greek phainein, to reveal, korē, pupil of eye
Phaenocora have a forward-directed pharynx and flattened underside. The body tends to widen toward the posterior, which is pointed or round except for a minute tail.
Fish Creek - body about 380 µm
Larvae: Trematoda ☣
Greek trēma, perforation
Trematodes are parasitic. Digenetic flukes after hatching first infect and multiply in molluscs. These release free-swimming larvae, often with a distinctive forked tail, called cercariae.
Cercariae then enter vertebrates, where they mature, or other intermediate hosts. A few tropical species infect humans; elsewhere some may enter skin but die leaving small rashes.
Bryozoa are sedentary filter-feeders that grow into colonies. Each individual is polyp-shaped with many ciliated tentacles, which can be drawn into a protective case. Most freshwater kinds have tentacles in a horseshoe pattern, directly connected bodies, and produce dormant buds called statoblasts inside the colonies.
Latin plumatus, feathered, -ella, dimn.
Plumatellids have both larger attached and smaller floating statoblasts. The latter are oval, with a distinct outer ring of gas-filled chambers but not hooks or spines.
Phylum Annelida – segmented worms
Annelids include a variety of worms with bodies divided into repeating segments. Each of these usually has bristles called setae, often on lateral appendages called parapodia. The mouth is behind an anterior projection called the prostomium, which in some is part of a developed head. Most kinds are flexible and move by contractions.
Aphanoneura include a few small aquatic species. In these the prostomium is muscular and ciliated, used for both sucking up food and swimming, and the segments are indistinct. They usually reproduce by simple fission.
Greek aiolos, nimble, sōma, body
Aeolosoma are 1-10 mm long and occur in freshwater. They have hair-like setae in ventral and dorsal pairs, and normally small coloured fat droplets spread throughout the body.
Clitellata include some small detritus-feeders along with larger earthworms and leeches. The prostomium is reduced or occasionally proboscis-like, and sexual reproduction involves a characteristic band called the clitellum, which secretes a cocoon for the eggs. However, smaller kinds usually reproduce by fission.
Greek chaitē, long hair, gastēr, belly
Chaetogaster have short hook-like setae in ventral bundles, but none on the back. They are usually 1-20 mm long, and often occur in chains before separating.
Molluscs have a mantle forming a chamber enclosing more or less of the body, including the reproductive and excretory organs, and typically fine gills. There is usually a ventral foot used to crawl or adhere to surfaces, and often a distinct a head, with a tongue-like radula used to rasp food. The smallest grow a few millimetres long.
Snails and other gastropods twist to one side during development. Most have a helical shell that they can retract into, and often a lid or operculum that closes it behind them. However lids are lacking in pulmonate groups, which breathe air or water through a lung-like chamber opening on one side rather than the usual gills.
Lymnaeidae – pond snails
Greek limnē, pond
Pond snails live in freshwater and breathe on the right. They usually have helical shells with dextral growth, i.e. turning clockwise as viewed from the tip or spire.
This sort of shell is the most common among snails in general. However, it only occurs in a few other freshwater pulmonates, exclusively in the southern hemisphere.
Physidae – tadpole snails
Greek physa, bellows
Tadpole snails are similar but mirrored, with sinistral shells and breathing on the left. In many the mantle also has finger-like projections around the opening, which act much like gills.
Planorbidae – ramshorn snails
Latin planus, flat, orbis, orb
Ramshorn snails also breathe on the left, but the shell is typically flattened into a planar spiral. This form separates them from most other modern aquatic snails.
Phylum Nematoda – roundworms
Roundworms are typically long and slender. Along with the following phyla, they moult several times during growth, and have no cilia. Instead they move by whipping back and forth.
Free-living kinds are usually less than 2 mm long and are most common in soil, but there are also many aquatic and parasitic types.
Tardigrades have four pairs of stubby appendages with claws or adhesive pads. Most kinds are less than 1 mm in size, and occur among submerged moss and debris.
Phylum Arthropoda – insects, crustaceans & allies