Some cool invertebrate facts:

• One entire phylum of invertebrates, the Cycliophora, is known to live in an extremely specialized habitat: on the mouthparts of lobsters.
   
• One of the largest solitary invertebrate is the giant squid Architeuthis dux (phylum Mollusca). The largest confirmed specimen measured 20 meters long including the tentacles. A close second is the jellyfish Cyanea arctica, which can be longer-nearly 40 meters including tentacles, but is not as bulky.
   
• One of the most venomous creatures in the sea is the box jellyfish or sea wasp, Chironex fleckeri (phylum Cnidaria). Found in Australia, the venom from these animals is a potent neurotoxin that can cause heart failure. However, the species of box jellyfish found in Hawaii and the Indo-Pacific region, like Carybdea spp.) are not as toxic.
   
• The Great Barrier Reef, off the northeastern coast of Australia, is the largest structure on Earth built by living organisms (SF Fig. 3.1). It can be seen from space. Corals look like rocks, but are actually invertebrate animals within the phylum Cnidaria.
   
• Ocean quahog clams (Arctica islandica; phylum Mollusca) are suspected to be one of the longest-lived invertebrates. Scientists have aged one ocean quahog to 507 years old.
   
• Roundworms in the phylum Nematoda may be the most numerous animals on Earth. A handful of farmland soil contains thousands of small nematodes. Nematodes are also very specialized. One German species is known only to inhabit the cardboard coasters used in pubs.
   
• Krill are shrimp-like crustaceans (phylum Arthropoda) that generally live planktonic lifestyles feeding on microscopic phytoplankton (SF Fig. 3.2). They are very very abundant in open oceans and important to the diet of large filter-feeding whales.