Biology--Phylum Annelida
A)
Segmented worms: the phylum name literally means “little ring.”
1)
Characteristics
a)
Eucoelomate—true body cavity
b)
Serially segmented body—repeating segments with specialized segments; bilateral
symmetry
c)
Organ systems—circulatory, respiratory, digestive, nervous, excretory, muscular, and
reproductive
d)
Setae or parapodia—bristles or fleshy appendages with bristles used in locomotion;
classified on the basis of bristles
2)
Three (3) classes
a)
Polychaeta—“many bristles”—parapodia with setae (usually 1 pair per segment).
Includes marine worms
i. herbivores, omnivores or scavengers
ii. separate sexes; veliger and trochophore larval stage
iii. important in the ecosystem (clean up)
b)
Oligochaeta—“few bristles”—setae. Includes freshwater species and earthworms
(Lumbricus terrestris)
i. use their prostomium (“pre-mouth”) for scooping organic matter from the earth
ii. creates “casts” as inorganic matter passes through, aerating soil (good for farming
industry, gardening, fishing industry)
iii. hermaphrodites but with cross-fertilization (no veliger larva)
c)
Hirudinea—no bristles. Includes leeches. Named after the anticoagulant Hirudin.
(Hirudo medicinalis)
i. freshwater or marine species
ii. are ectoparasites
iii. useful in medicine (apparent in name)
iv. are also called blood suckers
v. also hermaphrodites with no veliger stage
Annelid Class Details
B)
Class Oligochaeta—Earthworm systems
1)
Digestive System—
a)
Prostomium—first segment, upper lip to push through soil
b)
Mouth—ventral—only an opening
c)
Pharynx—muscular throat that sucks in soil
d)
Esophagus—tube-like passage to crop
e)
Crop--thin-walled organ which stores soil
f)
Gizzard—thick-walled muscular organ for grinding soil
g)
Intestine
h)
Casting—indigestible minerals passed out through anus
2)
Circulatory System—
a)
Closed circulatory system—all blood stays inside a system of vessels
b)
Aortic arches—hearts—push blood from dorsal surface to anterior end and from ventral
surface to posterior end
c)
Dorsal and ventral blood vessels—maintwo vessels with smaller vessels branching to
each organtissuecell
3)
Respiration—O2 and CO2 gases diffuse through moist epidermis directly
4)
Excretory System—removal of cellular wastes
a)
Nephridia—ciliated, funnel-shaped tubes that filter out wastes from the blood and send
them to the excretory pores
i. There are 2 per segment except the first 3 and the last 1
ii. They function the same as our kidneys (remember homeostasis & diffusion??)
5)
Nervous System—control and coordination of activities
a)
Cerebral ganglion—a collection of nerve cell bodies and serves as a brain. located at
segment 3 above the pharynx
b)
Ventral nerve cord—on the ventral surface (floor) of the coelom from the cerebral
ganglion toward the posterior end. Sends nerve impulses to and from the cerebral ganglion
(very sensitive to vibrations)
c)
Receptors—specialized cells in epidermis (skin)
i. photoreceptors—at the two ends and are light sensitive. Why are worms negatively
phototropic??
ii. Other receptors sensitive to touch, chemicals, and temperature
6)
Muscular System
a)
Circular—both an outer and inner layer that contract to lengthen body
b)
Longitudinal—runs from tip to tail and shortens body
c)
Process:
plant anterior setaelongitudinals shortenplant posterior setaecirculars lengthen
7)
C)
Reproductive System—hermaphroditic but cross-fertilize
a)
two worms meet head to head, ventral to ventral and overlap and attach by clitella
b)
Sperm exchanged to each other and they separate for safety (seminal vesicle and
seminal receptacle)
c)
Clitellum—found on segments 32-37 when counting from the anterior end. Secretes a
mucus band that slips off the anterior end with sperm and eggs inside (cocoon)
d)
Fertilized eggs are protected and kept moist by the mucus and in about 2-3 weeks
gastrulization and development result in many tiny worms
Class Hirudinea—leeches
1)
Characteristics
a)
no setae
b)
have both anterior and posterior suckers
c)
found in freshwater streams and ponds (few terrestrial)
d)
ectoparasites (external)—live on fish, aquatic animals, and humans
i. makes a small wound with anterior sucker
ii. Hirudin—anticoagulant in saliva which prevents blood from clotting
iii. Attaches to host with posterior sucker
iv. Can eat up to 10 times its own weight in blood
f)
Medicine—how have leeches been used in medicine from the 1800’s to today?