Biology 3B Laboratory Invertebrates II: Annelida, Nematoda ...

PHYLUM ANNELIDA
Members in the phylum Annelida are often referred to as segmented worms because of their
segmentation, a distinguishing characteristic that sets them apart form other animals. The most
recognizable members include the earthworms (terrestrial habitat), leeches (terrestrial and
freshwater), and marine worms. All annelids are triploblastic, bilaterally symmetrical, and
eucoelomate. In addition, annelids exhibit a body wall with both longitudinal and circular muscle
layers (which, along with segmentation mentioned above, allows these animals to be quite mobile).
They have a complete digestive tract. Their nervous system shows some degree of cephalization
with a “brain” and two ventral nerve cords that running the entire length of the body. They have a
closed circulatory system with aortic arches that act as the “heart” to pump blood through muscular
blood vessels. They also have a well developed excretory system which removes waste from the
blood and coelom.
There are three major classes within the phylum Annelida, described below.
Class Polychaeta - mostly marine worms, such as Nereis (the clamworm)
Class Hirudinea - the leeches (predominantly freshwater), such as Hirudo
Class Oligochaeta - mostly freshwater and terrestrial worms, such as Lumbricus (the
earthworms)
OBSERVATION OF POLYCHAETA
CLASS POLYCHAETA
Polychaete worms are mostly a marine group of worms characterized by many segments with a pair
of parapodia with numerous setae (figure 2). They have a distinct head with eyes, palps and
tentacles.
o Examine a clamworm (Nerius). These
are the “typical” polychaete worms that
can be found living in the mud and
debris of shallow coastal waters. Using
the dissecting scope, observe the head
region and find the following: eyes,
mouth on the ventral side, jaws, and
tentacles.
o Examine one of the segments. Locate a
parapodium on one side a body
segment. Parapodia function in
locomotion and respiration for
polychaetes. Each parapodium is
comprised of two lobes which bear
numerous setae (the reason for the
class name).
Figure 2: Structure of a clamworm (Nerius)
OBSERVATION OF OLIGOCHAETA
CLASS OLIGOCHAETA
Like polychaete worms, oligochaete worms are also segmented both outside and inside. However,
oligochaetes do not have parapodia, their head is less developed and they have fewer setae. The
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