Bio202 Lab 10:
Nematoda, Mollusca,
and Annelida
Chapters 37 & 38
Phylum Nematoda
Nematodes, commonly called roundworms
 Ubiquitous--found everywhere, span all
habitable environments
 bilaterally symmetrical
 Cylindrical
 Unsegmented (unlike most tapeworms)

Phylum Nematoda

Pseudocoelomate – body cavity between
mesoderm and endoderm
 Contrast

with Acoelomate, Coelomate (Fig 38.1)
advanced animal characteristics
 complete
digestive tract and other internal body
cavities

Most species of nematodes are free living,
though the most infamous species cause
disease.
Phylum Mollusca

Mollusks are far more diverse than most people
realize.
 Snails,
Clams, Octopuses, Squid, Chitons
 Occur in marine, freshwater, and terrestrial habitats.


Bilaterally symmetrical
Coelomate – body cavity bounded by mesoderm

Contrast with Acoelomate, Pseudocoelomate (Fig
38.1)
Phylum Mollusca

Protostomes
 Bilateral
 Metazoan
 Determinate,
spiral cleavage
Cleavage
Spiral
Cleavage
In
Mollusk
Embryo
Spiral
Cleavage
In
Mollusk
Embryo
Phylum Mollusca

Protostomes
 formation
of a mouth and anus directly from a
blastopore (opening in blastula)
 formation of the coelom by splitting of the
embryonic mesoderm
Blastula and blastospores


Solid ball of cells (morula) develops fluid-filled center (blastocoel).
Blastospores are produced by budding of blastula
Phylum Mollusca

Mollusks are far more diverse than most people
realize.
 Snails,




Clams, Octopuses, Squid, Chitons
Bilaterally symmetrical
Protostome
Coelomate
Occur in marine, freshwater, and terrestrial
habitats.
Phylum Mollusca





Visceral mass – tissue forming center of body
muscular foot used in locomotion
many mollusks have a head
most mollusks form a shell, and are classified
by shell type
Radula
 organ
used for scraping, drilling, or capturing prey.
 Not found in Bivalves
Phylum Mollusca

circulatory system
 Heart
 usually,
an open system through which the
blood circulates freely.
Class Polyplacophora (chitons)

Shell
 eight

dorsal overlapping plates
Foot
 ovaly
flattened
 extends from back to front (dorsoventral)

Mouth
 scraping

Chiton sp.
radula.
Class Gastropoda (snails)
Gastropods are terrestrial as well as
aquatic.
 shell

 single

and often coiled, sometimes absent.
Mouth
 has
a radula.
Class Bivalvia (clams, oysters)

Shell


Foot






flattened on the sides (laterally)
often used to burrow
Mouth


in two dorsally hinged halves (valves).
no radula.
Cilliated gills used for filter feeding
can filter particles as small as a virus; oyster harvesting is
quickly shut down following a pollution alert.
Anodonta sp.
Pinctada sp.
Class Cephalopoda (octopus, squid)

Shell
 Usually
absent
 single and coiled if present (very rarely).

Foot
 modified
into tentacles
 image-forming eyes
 mouth usually with pinching beak.

Loligo sp.
Phylum Annelida (Segmented Worms)

Segmented
 segments






are divided internally by septa
bilaterally symmetrical
Protostome
coelomates.
Cerebral ganglion (brain) is well developed
circulatory system is closed
digestive tract is complete.
Class Polychaeta (marine worms)

Body is distinctly segmented (believed to
be an adaptation to burrowing).

Segmentation allows fine control of
annelids’ hydrostatic skeleton in concert
with circular and longitudinal muscles to
alternately expand and contract segments
to aid burrowing.
Class Polychaeta (marine worms)

paired appendages of each body segment
are apparent in polychaetes. This
arrangement of appendages indicates a
common ancestry with arthropods (next
chapter)

Leeches are the most modern and highly
modified of the annelids, as is true for
parasitic groups of other phyla. Why is
that a common theme?
Class Polychaeta (marine worms)

body is distinctly segmentedThe most anterior
(toward the rear) segment (prostomium) has
sense organs as a head.
 Parapodia:
paired, lateral appendages extending from
the body segments
 Setae: small, bristlelike appendages often occuring in
pairs on lateral and ventral (frontal) surfaces



Nereis sp.
Aphrodita sp.
Chaetopterus sp.
Class Oligochaeta (oligochaetes)

Segmentation
 Distinct
 No
parapodia
 Prostomium (38.12)

Fleshy, small lobe before mouth.
 Setae


present but small
Lumbricus terrestris
Class Hirudinea (leeches)

Body
 Flattened
 anterior
(frontal) and posterior suckers
 no head development.

Segmentation
 greatly
reduced
 No setae

Hirudo medicinalis