Roundworms, phylum Nematoda, are characterized by elongated cylindrical bodies that are tapered at both ends and covered by a tough cuticle.
Nematodes are bilaterally symmetrical and are the simplest animals with a complete digestive tract with two openings: a mouth and an anus, thus they have a tube-within-a-tube body plan.
These animals are pseudocoelomate, possessing a pseudocoel located between the endoderm and mesoderm, enabling them to move freely than more primitive forms and it provides more space for organs.
Core Concept (Annelida)
The segmented worms, phylum Annelida, are distinguished by bodies that are divided into segments.
Annelids have a tube-within-a-tube body plan. The body wall, which is covered with ectoderm, is the outer tube, while the digestive tract, which is lined with endoderm, makes up the inner tube.
A fluid-filled cavity, the coelom, is found between the inner and outer tubes. The coelom is lined with mesoderm and outgrowths from this hold the body organs in place, allowing the development of a more complex organization.
The phylum contains three classes: Polychaeta(marine sandworm), Oligochaeta (earthworm) and Hirudinea (leech).
tube-within-a-tube body plan
closed circulatory system
the name: NEMA (thread) + ODA (like)
smooth narrow cylindrical body tapered at both ends and covered by a protective layer called a cuticle
Eyeworm ( Loa loa ) – infects tissues just below the skin
Pinworm ( Enterobius ) – deposits eggs in anal region
Hookworm ( Ancylostoma and Necator ) – infect people walking barefoot on contaminated soil
The name: ANNELLUS (ring) ringed/segmented worms
The body is divided externally and internally into segments separated internally by a membrane (septum), wherein most segments are identical except for some that are modified to perform specific functions; while segmentation is obvious externally as a series of rings separating the segments.
Annelids have elongated bodies that are bilaterally symmetrical with a tube-within-a-tube body plan, possessing a complete digestive tract/gut.
Between the gut and other body organs there is a fluid-filled cavity called the coelom which acts as a hydrostatic skeleton, while the lining of the coelom holds the organs in place.
Annelids thrive in marine, freshwater & terrestrial habitats; most are free-living, while a few species are parasitic.
Structure and Function
digestive system – complete gut; may be carnivorous, herbivorous or detritus feeders
respiratory system – through the skin or gills
circulatory system – closed type (blood travels through vessels)
excretory system – metabolic waste via nephridia; solid wastes through anus