28.4A: Superphylum Ecdysozoa

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The superphylum Ecdysozoa includes the nematode worms and the arthropods, both of which have a tough external covering called a cuticle.

Learning Objectives

Discuss the phylogenetic position of Ecdysozoa

Key Points

The Ecdysozoans are the most diverse group of animals, containing the nematode worms and the arthropods.
These organisms have an external covering called a cuticle that protects their soft internal organs from water loss and the outside environment.
After they molt, or shed their cuticle, they grow in size and secrete a new shell; this is called ecdysis.
The phylogeny of the Ecdysozoans has been the cause of much scientific debate with no definitive consensus in the scientific community.

Key Terms

cuticle: a noncellular protective covering outside the epidermis of many invertebrates and plants
coelomate: any animal possessing a fluid-filled cavity within which the digestive system is suspended.
ecdysis: the shedding of an outer layer of skin in snakes, crustaceans and insects; moulting

Superphylum Ecdysozoa

The superphylum Ecdysozoa contains an incredibly large number of species. This is because it includes two of the most diverse animal groups: Phylum Nematoda (the roundworms) and Phylum Arthropoda (the arthropods). The most distinguishing and prominent feature of Ecdysozoans is their cuticle: a tough, but flexible exoskeleton that protects these animals from water loss, predators, and other aspects of the external environment. All members of this superphylum periodically molt or shed their cuticle as they grow. After molting, they secrete a new cuticle that will last until their next growth phase. The process of molting and replacing the cuticle is called ecdysis, which is the derivation of the superphylum’s name.

Figure \(\PageIndex{1}\): Molting in arthropods: This cicada is in the middle of the molting process. The old cuticle splits and the insect climbs out. At this time, the insect’s body is very soft. The cicada will then eat the old shell to replace nutrients that would otherwise be lost. This encourages the new shell to harden.

Phylogenetic Hypotheses

There are two main hypotheses about the phylogeny of the Ecdysozoans. The first is called the Articulata hypothesis. This grouping scheme is widely accepted, although some zoologists still hold to the original view that Panarthropoda should be classified with Annelida in a group called the Articulata, and that Ecdysozoa are polyphyletic. Others have suggested that a possible solution is to regard Ecdysozoa as a sister-group of Annelida, though many scientists consider them unrelated. Inclusion of the roundworms within the Ecdysozoa was initially contested, but since 2003, a broad consensus has formed supporting the Ecdysozoa, placing them in a new set of groupings that include the Ecdysozoa, the Lophotrochozoa, and the Deuterostomia.

The other idea about the phylogeny of the Ecdysozoa is called the coelomate hypothesis. Before Ecdysozoa, one of the prevailing theories for the evolution of the bilateral animals was based on the morphology of their body cavities. There were three types, or grades, of organization: the Acoelomata (no coelom), the Pseudocoelomata (partial coelom), and the Eucoelomata (true coelom). With the introduction of molecular phylogenetics, the coelomate hypothesis was abandoned, although some molecular, phylogenetic support for the Coelomata continued until 2005.