Objective: 1)Describe how
segmented worms differ from flat
and roundworms. 2) Describe the
3 classes of segmented worms.
Segmented Worms (The Annelids)
Characteristics
1) Body is divided into segments.
2) Each segment contains digestive, excretory,
circulatory, and movement organs.
3) Some segments serve a specific function such as
reproduction.
4) Primitive brain called a cerebral ganglion, which is
connected to a nerve cord that runs along the ventral
part of the body.
5) Septa (internal body walls) separate the segment
internally.
6) Possess a fluid-filled coelom.
7) Complex organ systems.
8) Most annelids have bristles on their skin called
setae. Provides traction during crawling. Some
annelids have larger appendages called parapodia
which may have attached setae.
c. Some are free-swimming predators
Classes of Segmented Worms
#1 Polychaete
a. Largest group of annelids.
b. Possess fleshy, paddle-like parapodia on
most segments.
Types of polychaetes
a. Many polychaetes burrow in the ground.
b. Some secrete a tube-like structure.
#2 Oligochaeta
a. Earthworms and some freshwater
worms.
b. Possess few setae on each segment.
c. Have light, touch, and moisture sensitive
organs at each end of the body.
d. Digestion: Muscular pharynx pushes food
down esophagus into the crop where it is stored
before passing to the gizzard. The gizzard
crushes the soil to break it down before
releasing it to the intestine (through which
nutrients are absorbed.
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e. Hydrostatic skeleton: each segment is
composed of a fluid filled coelom.
Class Hirudinae
•One member: Leeches
•Flattened body plan
•No setae or parapodia.
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