CAMBARUS
1.
CLASSIFICATION
PHYLUM ARTHROPODA
CLASS CRUSTACEA
SUBCLASS MALACOSTRACA
ORDER DECAPODA
FAMILY ASTACIDAE
Genus cambarus
(in western united states, the pacific
form, pacifastacus, may be used).
2.
General observations. the crayfish,
a common inhabitant of freshwater
streams and ponds,
often emerges at night and makes characteristic burrows and mounds in wet fields or gardens.
along with its marine relatives, lobsters and crabs, the crayfish feeds chiefly
on decaying organic matter, (it is also locally known as crawfish) observe the
living crayfish in an aquarium.
Drawing
·
How
does the animal right itself when turned over?
·
Pick
your crayfish up by the dorsal shield (carapace). Hold it firmly!
·
Why
does the animal buck and flip its tail down so strongly?
·
What
is the function of this movement when the animal is in water?
·
How
do you suppose crayfish dig in soft
Observation
respiratory currents of a crayfish
resting in a shallow pan of water and note how they are produced. Add a few
drops of India ink near the animal and watch the particle flow into the respiratory
stream.
SUGGESTION
In
many areas of our southern and prairie state, it is possible to watch foraging and digging
crayfishes (“crawdads”) at night, seek them out with a flash light in
marshy or moist open fields and wach their
pattern of movement, digging, and rapid escape. Dig a few from their tunnels and observe their action in a laboratory
or home aquarium. Baked or boiled
crayfishes represent another biological aspect of these organisms well worth investigating,
3.
External anatomy (FIGURES
12.12 and 12.13 ) if you Specimen is preserved in formalin,
avoid fumes by rinsing it overnight in
tap water before dissection.
a.
Segmentation. Observe the tagmata, or grouping
of segments, into three general regions (1)
head (somites i-v , with
antennules, antennae, mandibles, and two pairs of maxillae) (2) thorax (pereion) (somites vi-xiii, with three
pairs of maxillipeds, one pair of
chelipeds, and four pairs of walking
legs or pereiopods) and (3) abdomen (pleon) (somites XIV-XI, with five pairs of swimmerets or pleopods and a
uropod and telson (Tail)
Drawing
·
Are
all appendages jointed?
·
How
do the pincers operate? (Better use
a pickled specimen)
·
What
are the advantages of a hollow exoskeleton?
·
How
does the surface area for muscle at tachement compare between a hollow
exoskeleton and a solid endoskeleton?
·
What
are the disadvantages of an exoskeleton?
Observe
the pag and socket joints between
abdominal segments. The exoskeleton of
each body segment is divided into a dorsal tergum, a lateral
pleuron, ad a ventral sternum.
Find these parts on the abdominal and
thoracic someites of your specimen
b.
Body parts.
Before removing the appendages and studying their relationship, identify
the major body parts visible dorsally, (FGURE
12. 11) head, antennule, antenna,
cephalothorax, carapace, cervical grovve, rostrum, eyes, cheliped, pereiopods, uropod, and telson,
then observe the crayfish from the lateral view and located the fill chambers under the pleura of
the caracpace (BRANCHIOSTEGITES). Study
the animal ventrally and observe its
19 paired appendage. First, find
the anteriormost sensory antennules, biramous (two branched ) structures with
many joints and a balancing organ or
statocyst n the flatteened dorsal surface of the basal joint. Next are theantennae, a pair of long,
slender, many jointed appendages with excretory openings at the basal segment of each. Then come the mandibles or chewing jaws, followed by
two pairs of maxillae for food handling.
Three pairs of mxillipeds follow,. These serve for manipulation and
sensory perception f food. Then, completing the largest claws, the first pair of walking legs, followed by the remaining
four pairs of walking legs (pereiopods,
a term taken from pereion, or thorax,
the thoracic legs), five pairs of
swimmerets, and , finally, the uropods, the terminal pair
of appendages, which with the central
telson provide a broad propulsive paddle, also used for egg protection
Next review the location of the following body
openings: moth , anus (one the ventral surface of the telsion, the central
portion of the tail) and excretory pore
on each antenna, find the external sex
openings. the male genital ducts
or onoducts open at the base of the last leg. A trough for the transfer of
spermatophores (SPREERM CAPSULES) into the female seminar receptacle is formed
from the fused tubular first and second swimmerets (figure 12.150.
the oviducts open at the base of the second walking legs , another pore at the base
between the fourth walking legs serves to receive the spermatophores, the first swimmeret in feels is Small lor absent,
the second is a typical swimmeret
rather than a specialized organ as in
the male. The
female telson and filamentous swimmerets hold the egg cluster to form an external brood
pouch for the eggs and young. Females
carrying such a mass of eggs are said
to be “in berry”
c.
Appendages (Figures 12.14, 12.16). after this initial examination of your specimen,
you should be ready for a detailed
review of the appendages
Learn not only functional and structural
differences between appendages but also their relationship to a common prototype
which is through to be a simple,
biramous (two-branched) appendage such as the swimmeret, species thought to be primitives shows a repetition
of parts
with little specialization as seen in
the brine shrimp. In more highly evolved animals, the specialization as seen in the
brine shrimp. In more highly
evolved animals, the specialization of appendages , especially on the head,
is apparent. There are striking differences between a
brine shrmp with its many
similar biramous appendages (homologous matamerism) and a crab, shrimp, or crayfish with highly
modified appendages (heretonomous metamerism),
apparently derviced from more simple appendages,. structrueal changes in serially
homoloous parts can often be traced by
comparing them with a unspecialised
segment or appendage.
Examine
a sample dissection of crayfish appendages if one is available. Read the following instructions carefully before starting your
dissection and study figure 12.14
Each
limb consists of three basic portions: the stem or attachment, the
protopod, and two branches (endopod and exopod ) are
attached. Each appendage fundamentally follows this structural pattern, though with
considerable modification in the head region. In some cases , homology can be determined only
by embryological study, owing to the loss of some
portions during growth and to the
development of more specialized appendages, in certain cases, as in
antennules, the precise homology of the segments is still uncertain
Drawing
Carefully cut away the lateral extension of the
carapace covering the fills, the branches
tegite, from one side to expose the internal sheet of gills (figure
12,17)
Begin
your dissection with uropod and work
forward, the simple structure of the
abdominal appendages more closely resembles what is presumed to have been the primitive crustacean biramous
appendage, and makes a more suitable starting point, each appendage should be examined critically with a
hand lens or under a dissecting microscope.
Observe the structure, check it against
its stated function, and
note the increased degree of structural specialization as you proceed
toward the head. You
may find it more instructive to pin the appendages alongside the animal
under water in a waxbottom dissecting pan,
in order to view them from
the same orientation, afterwards
the appendages can be dried and glued onto a cardboard sheet and
labeled for later reference (figure 12.16). be sure to remove a bit of
attachment membrane along with each
appendage to ensure having the intact structure. Refer to figure
12.14 for detailed structure and terminology
Beginning
with the uropod, remove one appendage at a time
the side on which you have exposed the fills, identify the three basic
parts (if all are present ) and pin out the appendage, carefully orienting the
protopod to the left, the endopod
anteriorly, and the exopod posteriorly. Compare the normal
plane of movement of each part and of each
limb with the others to get an idea of how the appendage functions.
From the uropod proceed anteriorly
through the simple pleopods, noting
again the sexual differences and water circulating role of these organs. The walking
legs, removed next, are of interest for their endopod specialization (the
exopod disappears entirely during development )
and for their respiratory role, special gills ,
podobranchlea, are attached to certain legs (which ones?) the coxopod has a specialized extension or
epiopod to which the gill filaments attach
Drawing