Invertebrates form a large part of fauna of the world as well as of Finland. Among invertebrates insects are the largest and the most diverse group. 

SPONGES (Porifern, sienieläimet) are very simple animals in their structure. Sponges do not have any specialised organs. Spongilled (Spogilla, järvisieni) is common in inland waters. 

CNIDARIANS (Cnidaria, polttiaiseläimet) are water animals, with nettle cells in their tentacles. Cnidarias can be seen in the sea in Southeast Finland. 

FLATWORMS (Platyhelminthes, laakamadot) are small water predators. Many flatworms are parasites like the fish tapeworm (Diphyllobotharium, lapamato). 

ROUNDWORMS (Nematoda, sukkulamadot)are abundant in nature. Pinworm (Entrebius vermicularis, kihomato) is the most common roundworm parasite in human body. 

ANNELIDS (Annelida, nivelmadot)include the earthworms (Lumbricidae, lierot) and the leeches (Hirundae, juotikkaat). 

MOLLUSCS (Mollusca, nilviäiset) are usually herbivores. Some typical species of Finnish mollusc are gastropod (Gastropoda, kotilot) and snails (etanat). 

CRUSTACEANS (Crustacea, äyriäiset), are very diverse group in life-style; they are herbivores, predators or parasites. Most of the crustaceans have also metamorphosis. Crustaceans have two pairs of antennas and usually many pairs of legs. Many crustaceans are water animals such as crayfish (Autacus fluviatilis, rapu) and lobsters (Homarus vulgaris, hummeri). 

SPIDERS (Arachnida, hämähäkit). The body of the spider has two parts with four pairs of legs. Spiders are typically predators. The common spider of Finland belongs to the phalangid group: (Phalangid, lukki).