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Lab 6 - Phylum Annelida.docx

Biology (Sci)
Course Code
BIOL 111
Suzanne Gray

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Phylum Annelida
Annelida means “little rings”
Phylum contains segmented worms:
o Earthworms
o Tubeworms
o Leeches
Bodies are composed of many essentially similar segments
Phylum Annelida Common Earthworm
Daytime, earthworms remain below the surface to avoid drying out
Nighttime, earthworms come to the surface to collect plant debris for food
and to mate
External Anatomy:
Covered in a thin transparent cuticle, secreted by the epidermis
o Protects the animal from desiccation
o Glandular cells of the epidermis secrete mucus, which also
adds in protection, locomotion and respiration
Bilaterally symmetrical
Anterior end is more cylindrical and thicker than the posterior end
o The Clitellum: thickened band around the middle of the
earthworm that is located closer to the anterior end
The ventral surface of the earthworm is light and flatter than the dorsal
Which end elicits a stronger response? The anterior end should
elicit a stronger response because of the large concentration of
neurons located there.
Earthworms emerge from their burrows anterior head first,
why? The majority of neurons, including sensory cells, are located
in the anterior end. It can use these receptors to determine the
relative condition and safety of the environment it is emerging into.
Evolutionarily speaking, those earthworm that did not show this
behavior (i.e., they emerged posterior end first) were removed from
the gene pool. Thus their genes for this behavior were not passed
on to the next generation.
Segments: on each segment (except the first and last) there are four pairs of
posterior-facing setate (bristles) which anchor the earthworm to the substrate
while the rest of the body is moving forward
Segment 1: the mouth is located here (small ventral slit)
o Prostomium: fleshy lobe that precedes the mouth
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o Peristomium: portion of the segment that surronds the mouth
Segment 9 and 10: located on the ventral surface
o Two pores on each segment which lead to internal sacs,
seminal receptacles, used to store sperm
Segment 14: located on the ventral surface
o Two pores are the female genital pores from where the eggs are
Segment 15: located on the ventral surface
o Two pores are the male genital pores from where the sperm is
o Sperm travels down the seminal grooves running from
segment 15 to the clitellum
Segment 26: located on the ventral surface
o Genital setae are contained within the swollen tissue help
hold the ventral surfaces of the worm together, in opposing
directions for copulation
Segment 32 37: thickened band of epidermis
o Clitellum secretes a mucus cocoon in which the eggs and
sperm are deposited
Last segment: location of the anus
Locomotion: two muscle groups: circular and longitudinal (shortening and
Antagonistic muscle types and hydrostatic skeleton enable
1. Circular muscles contract, pressure is then placed on the fluid
in the coelom
2. Pressure causes segments to elongate, pushing the segments
anterior to it forward
3. Upon contraction of the longitudinal muscles, the segment
shortens, pulling the segments posterior to it forward
4. Setae help anchor some of the segments during this process
Blood Circulation:
Thin purple line is the dorsal blood vessel
Ventral blood vessel can be seen on the other side
What direction does the blood travel in each vessel? Dorsal
blood vessel: anteriorly and the ventral blood vessel: posteriorly
Internal Anatomy:
Coelom: the body cavity filled with internal organs which is enclosed
within the body wall
o Acts as a hydrostatic skeleton with the surrounding
muscles contracting against the non-compressible fluid
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