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There are about 10 invertabrate phyla containing more than 95% of all animal species.  Invertebrates have a wide range of characteristics, the lack of a backbone being the major common trait.

Some invertabrate phyla:

Porifera
Sponges are aquatic animals that represent a transition from unicellular to multicellular life. Of the 10,000 species of sponges, only about 150 species live in fresh water. Adult sponges are sessile, meaning that they attach themselves to a surface and do not move. Because they are sessile, sponges are filter feeders, screening food out of the water that flows through their body.
Cnidaria
Freshwater hydra, jellyfish, and corals go through a transition in body forms. The medusa stage (bell-shaped) is specialized for swimming and the polyp stage (vase-shaped) is specialized for a sessile life. All members of the phylum have these two stages at one time in their life, even though the medusa stage may be found only as an embryo. All cnidarians have tentacles containing cnidocysts and nematocysts, specialized stinging cells for defense and capturing food.
Nematoda
Roundworms have a long, slender body that tapers at both ends. They range in length from 1 millimeter to 4 feet in length. This phylum is the first to have a digestive tract with two openings, which is a major advancement over the phyla up to this point. The vast majority of these animals are free-living, but there are about 150 species that are plant and animal parasites. It has been said that if all things except roundworms were erased from the earth, one could recognize where plants, animals, and even cities were by looking at the roundworms.
Mollusca
Snails, clams, and octopi are the first animals with a true coelom, a hollow, fluid-filled cavity completely surrounded by the mesoderm. The phylum is divided into three diverse classes.
• Gastropoda: most members of this class have a one-piece, external shell. Gastropods include snails and slugs.
• Bivalvia: members have an external shell divided into two halves that are connected by a hinge. Bivalves include clams, oysters, and scallops.
• Cephalopoda: these marine mollusks are the most advanced group in the phylum. Their nervous and circulatory systems are highly advanced and they have an internal shell supporting the body. Octopus and squids are cephalopods, including the giant squid - the world's largest known invertebrate.
Annelida
Segmented worms are more advanced than roundworms and have several visible characteristics that distinguish them. They are named for the many body segments that make an annelid look like it is composed of a series of "rings". Unlike roundworms, the segmented worms have an obvious head and tail. All segmented worms have one of two types of structures extending from their bodies that are used for locomotion. Most annelids have hair-like bristles known as setae and some have fleshy protrusions called parapodia.

The body extensions are used to divide annelids into three classes:

• Oligochaeta: members of this class have few setae on each segment. The earthworm belongs to this class and has two pair of setae on each segment.
• Polychaeta: two-thirds of all annelids are polychaetes. These marine animals have many setae projecting from parapodia. Polychaetes also differ from other annelids in that they have antennae and specialized mouth parts.
• Hirudinea: the smallest class of annelids contains the leeches, which have no setae or parapodia. Most leeches live in calm, fresh water and some live in moist vegetation on land, but none live in salt water. Many leeches are carnivores that feed on small invertebrates, but some species are parasites that suck blood from other animals.
Arthropoda
Three-fourths of all animal species, including spiders, crayfish, and insects, are arthropods. This phylum is composed of a very diverse group of animals that are bilaterally symmetrical. The following characteristics have allowed arthropods to adapt to almost every environment on Earth:
• Jointed appendages: the phylum name means "jointed foot". All the appendages (body extensions) of arthropods are jointed, giving them a wide range of controlled motions.
• Exoskeleton: the exoskeleton provides support and protection. It is composed of three layers that are secreted by the epidermis.
• Segmented body: the segments in the arthropod body make movement possible, even with the hard exoskeleton.
Echinodermata
Starfish, sand dollars, and sea urchins are found in marine environments ranging in depth from shallow tide pools to 10,000 meters. Most of the 7,000 species of echinoderms have a type of radial symmetry called pentaradial symmerty, in which the body parts extend from the center along five spokes. In addition to their radial symmetry, echinoderms have three other major characteristics that are not shared by any other invertebrate phylum:
• They have an endoskeleton composed of calcium carbonate plates known as ossicles. These may be attached to spines that protrude through the skin. The phylum name echinoderm means "spiny skin".
• They have a water-vascular system - a network of water-filled canals inside their body.
• They have many small, movable extensions of the water-vascular system called tube feet, which aid in movement, feeding, respiration, and excretion.

• Sponges - Glasgow University
• About Octopi - Mote Marine Laboratory
• Invertebrate Anatomy & Dissection Guides - Lander University
• Marine Reef Organisms - Gray's Reef National Marine Sanctuary
• The World of Parasites - By J. M. Smith
• Parasitism and Symbiosis - By J. M. Smith
• Tapeworms & Parasites - Ohio State Univeristy
• Pheromones - Lund University, Sweden
• Snails - Frescargot Farms
• Insect Orders - California Academy of Sciences
• Other Invertebrate Links - Frontier High School

Animals with a hard exoskeleton must shed the exoskeleton through a process of "molting" before they can grow in size.
 
 
 
 
 
 
 
 
 
 
 
 
 
 

While there may be more than one way to describe animal success, insects are the most numerous and diverse animals on earth today.