Main invertebrate groups

As we discussed there are main groups under invertebrates. Let us consider each one of them. 1. Cnidaria 2. Annelida 3. Mollusca 4. Arthropoda Cnidaria Cnidarians are invertebrates such as jellyfish and corals. They belong to the phylum Cnidaria. All cnidarians are aquatic. Most of them live in the ocean. Cnidarians are a little more complex than sponges. They have radial symmetry and tissues. There are more than 10,000 cnidarianspecies. They are very diverse.

Cnidaria Anatomy

Annelida The phylum Annelida is made up of segmented worms such as earthworms. Segmented worms are divided into many repeating segments. There are roughly 15,000 species of annelids. Most belong to one of three classes.

Structure and Function of Annelidas Annelids range in length from less than 1 millimeter to over 3 meters. They never attain the large size of some mollusks. Like mollusks, however, they have a coelom. In fact, the annelid coelom is even larger, allowing greater development of internal organs. Annelids have other similarities with mollusks, including: A closed circulatory system (like cephalopods). An excretory system consisting of tubular nephridia. A complete digestive system. A brain. Sensory organs for detecting light and other stimuli. Gills for gas exchange (but many exchange gas through their skin).

Mollusca Have you ever been to the ocean or eaten seafood? If you have, then you probably have encountered members of the phylum Mollusca. Mollusks include snails, scallops, and squids. There are more than 100,000 known species of mollusks. About 80 percent of mollusk species are gastropods.

Structure and Function of Mollusks Mollusks are a very diverse phylum. Some mollusks are nearly microscopic. The largest mollusk, a colossal squid, may be as long as a school bus and weigh over half a ton! The basic body plan of a mollusk is shown in Figure below. The main distinguishing feature is a hard outer shell. It covers the top of the body and encloses the internal organs. Most mollusks have a distinct head region. The head may have tentacles for sensing the environment and grasping food. There is generally a muscular foot, which may be used for walking. However, the foot has evolved modifications in many species to be used for other purposes. Basic mollusk body plan

Arthropoda There are more than a million known species of arthropods. There may actually be ten times that many. Arthropods include insects, spiders, lobsters, and centipedes.

Structure and Function of Arthropods Arthropods range in length from about 1 millimeter to 4 meters (about 13 feet). They have a segmented body with a hard exoskeleton. They also have jointed appendages. The body segments are the head, thorax, and abdomen (see Figure below). In some arthropods, the head and thorax are joined together as a cephalothorax.