Echinoderms and Crustaceans: Morphology, Classes and Groups

Echinoderms and Crustaceans: Morphology and Groups

Echinoderm Classes

Class Echinoidea (Sea Urchins)

Class Echinoidea: Present morphology is radially symmetrical, composed of five ambulacral rows of pores through which the ambulacral feet protrude. When the skeleton (test) is examined, these five rows of pores and the interambulacral areas are especially noticeable. Embedded in these areas is a tangle of spines, short or long, which give this group its name. These animals move and bury themselves; their tube feet also allow them to stand on flat surfaces.

Class Holothuroidea (Sea Cucumbers)

Class Holothuroidea: Commonly called sea cucumbers. They have evolved a different solution for life on the seafloor: the body becomes cylindrical and the five radial areas characteristic of echinoderms form a ventral support surface. One end bears the mouth surrounded by ten branched arms (the feeding tentacles), while the other end bears the anus.

Crustacean Groups

Crustaceans: Crustaceans are characterized by a chitinous exoskeleton that protects the body; they periodically molt this external shell to grow.

• Branchiopods: This group also includes genera such as Artemia (brine shrimp). Many branchiopods, along with numerous copepods, are important components of plankton; plankton occur in vast quantities in the ocean and form the main food source for many schooling fish (for example, North Sea herring).
• Cirripedes (Barnacles): These sessile crustaceans have bodies completely enveloped by a calcareous shell. They inhabit all the seas and are among the most abundant intertidal animals. Examples include goose barnacles (Lepas) and other barnacle genera such as Mitella.
• Decapods: Decapods have five pairs of locomotor appendages. Their forms are highly varied and their sizes range widely; for example, some spider crabs living in the Pacific Ocean can reach up to three meters across.
• Anomura: Among the Anomura there are many unusual crustacean forms. One notable example is the hermit crab (Pagurus), whose abdomen is long and soft and is protected by occupying an empty shell rolled in a spiral.
• Brachyura (True Crabs): These have a shortened body form with a reduced (often folded) abdomen. This large group of decapod crustaceans includes familiar crabs from genera such as Callinectes and Cancer. They possess an atrophied tail and typically move sideways. In many crabs, walking legs are specialized for different roles; some of these legs are adapted as swimmerets and allow the animals to be outstanding swimmers.