Characteristics of Phylum Arthropoda (arthro = joint, poda = foot) include:
Characteristics of Classes of Phylum Arthropoda:
|Number of legs
|Number of body parts
|These were most common during the Cambrian and Ordovician periods of geological history, and can be found preserved in a number of rock formations in the Cincinnati area. They are now extinct.|
|0||4 prs, att. to cephalothorax, (chelicerae & pedipalps are m.p.)||cephalothorax & abdomen|
crayfish, crabs, pillbugs
|2 pair||5 prs incl. cheliped att. to cephalothorax, and swimmerets, m.p., incl. mandibles||cephalothorax & abdomen|
|1 pair||many, 2 pr per apparent segment bec of fused segm., m.p. incl. mandibles||head and “trunk” segments, every two segments fused into one apparent segment|
|1 pair||many, 1 pr per segment, m.p. incl. mandibles & poison claw on next segm.||head and “trunk” segments|
|1 pair||3 pr, 1 pr. per thoracic segment, m.p. incl mandibles, some abdominal||head, three-segmented thorax, segmented abdomen
(wings are not appendages)
Origins of Arthropod Appendages:
The theoretical origins of arthropod appendages from an earthworm-like ancestor are thought to include the following stages:
Note that, for example, the chelipeds (“pincers”) of crayfish and the chelicerae (“pincers”) of scorpions are not the same. The chelipeds of crayfish are appendages of segment #8, while the chelicerae of scorpions are appendages of segment #1. Note that in both the arachnids and the insects, segment #8 has no appendages at all. In crustaceans, segment #1 bears the antennae, and in insects, has no associated appendages.
Classification of Crayfish:
For many years, crayfish were classified as:
However, even though crustaceans have mandibles, many biologists now feel that this is due to convergent evolution rather than a close relationship to other mandibulates and question whether crustaceans should be included in Subphylum Mandibulata. In Crustaceans, the biting surface of the mandibles is thought to have evolved from an inner (mesal) bulge, or endite lobe of the protopodite (note that the gills come from a similar outer (lateral) bulge, the exite lobe) similar to the origins of the mouthparts in the Trilobita and the Chelicerata). In all other mandibulates (insects, centipedes, millipedes) the biting surface is modified from the tips (the distal portion) of the endopodite.
Unlike the other mandibulates and similar to the Chelicerata, crustaceans have a cephalothorax (cephalo = head), although the head and thorax are often somewhat delineated. They have a telson like trilobites had. Unlike any other arthropods, they have two pairs of antennae: their first pair, the antennules, are homologous with the antennae on insects and other mandibulates, while their second pair, the antennae, have no homologous structures in insects and other mandibulates. As mentioned above, crustacean mandibles are slightly different in construction/origin from those of other mandibulates, and they have more accessory mouthparts than any other class.
Thus, an alternative classification scheme for the arthropods is gaining acceptance, and more and more taxonomists are recognizing Crustacea as a totally separate subphylum, “unrelated” to the rest of the mandibulates.
Subphylum Trilobita — trilobites
Class Arachnida — spiders, scorpions, etc.
Subphylum Crustacea — crustaceans
Class Diplopoda — millipedes
Class Chilopoda — centipedes
Class Hexapoda — insects
Note the body regions: cephalothorax, covered by a
“shell-like” carapace (carapac = a covering, shield) on top,
and abdomen. Can you tell where the head and thorax join? Note the
abdomen, and see if you can find all seven abdominal segments.
The elongated, anterior portion
of the crayfish’s cephalothorax is called the rostrum.
On the head, note the stalked, compound eyes. These fit into grooves in the front of the head, and are often recessed into them.
In many decapods (deca = ten, poda = foot: referring to the walking legs), each segment of the abdomen bears a pair of small appendages called swimmerets with the exception of the last segment, called the telson. In crayfish, the penultimate segment bears a pair of two-parted appendages called uropods (uro = tail). The order gets its name from the ten walking legs (five pairs) located on the thorax, including the cheliped (the pincer; cheli =claw, ped =foot).
Observe all the appendages by carefully removing those from the left side one at a time and arranging in order on the desk top or a piece of paper for further study. It is easiest to do this starting from the rear.
Carefully cut away part of the carapace on the left side to expose the gill chamber. Note the gills and the inner wall of the gill chamber (the wall of the thorax). Note how some of the gills are attached to the wall of the thorax and others to the bases of the appendages. In the crayfish, the gills resemble feathers in shape. Count the number of gills.
Beginning at the back edge of the carapace, cut two lines forward, about ¼ to ½ inch (about 1 cm, or so) to each side of the middle. Note: cut very shallowly because there are body organs located right below where you will be cutting. Gently lift off the flap, trying to remove only the chitinous exoskeleton. The underlying epidermis may adhere to the exoskeleton or it may remain covering the internal organs. If it remains, you will need to gently remove it also. You may need to widen the opening further to view all the organs. Also, continue these slits into the abdomen as far as feasible and carefully remove the flap of exoskeleton.
In the thorax, the first thing you should see dorsally is the
heart. You may be able to see small openings, ostia (os
= mouth), through which the blood enters the heart. At several points, you
may be able to see arteries leaving the heart. The circulatory
system is an open one, thus once the blood leaves the arteries, it
circulates throughout the body cavity. The membranous chamber in which the
heart lies is the pericardial sinus (peri = around;
cardio = heart).
If your crayfish is a female, much of the thorax will probably be filled with eggs (brownish). Below the heart, behind the stomach are the reproductive organs. As mentioned, if the ovaries (ova, ovo = egg) of the female are filled with eggs, they will be easily recognizable, although the ovary walls will be “invisible.” Those of a female without eggs as well as the testes of the male may be difficult to distinguish from the digestive glands and may even be partially imbedded in the glands. Generally, the reproductive organs (the ovaries at least) are a darker, orange-brown color and tougher than the digestive glands. The female’s oviducts extend toward the bases of the third pair of walking legs while the male’s vasa deferentia open between the fifth walking legs. Check a crab/crayfish of the opposite sex to see the differences.
Intact, Unopened Stomach/Gizzard
Opened Gizzard with One
Grinding Structure Removed Forward of (and perhaps slightly below) the heart is the stomach. To see this, you may gently need to remove some of the eggs and the heart. You may be able to insert a blunt probe through the mouth and short esophagus (eso = within, inward; phago = eat) into the stomach. In the crayfish, the stomach is divided into anterior and posterior sections (crop and gizzard) by a constriction bearing grinding structures to shred the food as it passes. From the stomach, the intestine continues to the end of the abdomen, but may be hidden in the thorax by other organs. A pair of large, soft, yellowish structures around the stomach (under the eggs) are the digestive glands. You may remove the heart (and eggs), if not already removed, to see the digestive glands better. They are connected to the intestine and aid in digestion and absorption of food.
Side View of Nerve Cord Going Around Esophagus
Rear View of Brain and Nerve Cord Going Around Esophagus The nerve cord is located ventrally. It is a pair of thin, whitish, fairly tough “strings” that can best be located by first looking for the two halves that go around the esophagus. To follow the nerve cord posteriorly, you may have to gently and carefully remove abdominal muscles and/or push aside or remove the digestive organs in the thorax. Note that there is a nerve ganglion in each segment and that they are connected by the double-stranded nerve cord, resulting in the “ladder-like” appearance of the nerve cord. In the thorax, there are chitinous structures over the cord which will need to be cut away to see it. Each of the posterior thoracic segments bears its own ganglion, but those of the anterior thoracic and posterior head segments (segments 3 through 7) are fused to form one subesophageal (“under the esophagus”; sub = under, beneath) ganglion (gangli = a knot on a string, swelling). The two halves of the nerve cord continue anteriorly around each side of the esophagus to the supraesophageal (“over the esophagus”; supra = above, over, beyond) ganglion or “brain,” from which nerves radiate to the eyes, antennae, etc.
Examine the various thoracic and abdominal nerve ganglia to
determine whether the those ganglia are the same or different in size. How
do you think the size of a ganglion correlates to the functions being
performed in/by that segment of the body?
In the anterior part of the body cavity, between the esophagus,
supraesophageal ganglion, and attachment of the antennae, are
the green glands. These excretory organs function similarly to our
kidneys in that they remove waste from the blood and discharge
it to the outside.