Originally Marine Biology - now Zoology

The majority of animal phyla are found in the sea. Arthropods, molluscs and annelids are introduced in an article called 'Invertebrate Classification' (at: http://fishinsects.suite101.com/article.cfm/invertebrate_classification). That article concentrates on land invertebrates, so several major aquatic phyla were not even mentioned.

Echinoderms
The Echinodermata are all marine. They have no head or tail, and are said to have a 'five-fold' symmetry. One of the best known is the starfish, but the group also includes the sea-urchins and sea-cucumbers. Curiously their larvae vaguely resemble very small fish and they are thought to be quite closely related to the vertebrates.

Coelenterates
The Coelenterata (an old term) includes all the jellyfish and their allies, as well as the comb-jellies. These have what is called 'radial symmetry'. Their generations alternate between a swimming jellyfish-like and a settled sea anemone-like stage. This is one way of allowing 'settled' animals to move into the water and colonise new areas. (The other way is to have planktonic larvae.)

Sponges
Porifera are among the simplest animals on earth. They are as much a collection of individual single-celled organisms as they are whole animals, but if the whole animal is carefully mashed up and passed through a sieve these 'individual' cells often creep back together to re-form the whole! All are aquatic (the vast majority marine), and they nourish themselves by 'filter-feeding', usually straining plankton out of the water.

Vertebrates
Marine Mammals are covered in a previous article (at: http://marine-life.suite101.com/article.cfm/marine_mammals), but there are also marine birds, marine reptiles, and - of course - marine fish.

Marine Birds. The penguins are the only birds that can be said to be fully aquatic, although there are many other types of bird that spend most of their time flying over the sea and feeding there.

Marine Reptiles (at: http://marine-life.suite101.com/article.cfm/marine_reptiles).
Most of the sea-snakes spend all their time in the sea, even reproducing there. The marine turtles and marine crocodiles need to come ashore to lay their eggs, and the marine iguana of the Galapagos Islands spends most of its time ashore, only going into the sea to browse algae.

Marine Amphibians. There are no truly marine amphibians, although some, (like the Marine or Cane Toad Bufo marinus ) can tolerate brackish water.

Other Phyla
The majority of animal phyla have not even been mentioned here. Most are not represented on land, and they are therefore not familiar to most people. This does not mean that they are unimportant - the Arrow Worms (phylum Chaetognatha) for example - are major predators in the plankton, eating many young fish larvae.