Laboratory Notes for BIO 1003

© 30 August 1999, John H. Wahlert & Mary Jean Holland


Domain Eukarya
Kingdom Animalia

PHYLUM ARTHROPODA

Members of the Phylum Arthropoda may be aquatic, terrestrial (including flying), and parasitic. The body is coelomate and has bilateral symmetry. The skeleton is on the outside and is called an exoskeleton (an internal one, as in humans, is an endoskeleton); it is made of chitin. The appendages are jointed (this is the meaning of arthro-pod). The body is divided into three regions—head (cephalum), thorax, and abdomen. Portions of the body are segmented. In most species the sexes are separate. Hatchlings are larvae that undergo metamorphosis into the adult form. In order for growth to occur, the confining exoskeleton must be shed and a new one formed around the now larger individual; each growth stage is called an instar. A strong, hard skeleton and well developed organ systems permit some of these animals to attain considerable size. The invention of specialized respiratory structures for breathing air have permitted stunning success in colonizing land by a few groups. Use a dissecting microscope to observe details in the examples below.

Class Crustacea

The crustaceans include crabs, lobsters, shrimp, and barnacles. You will be examining the external features and internal anatomy of a crayfish, genus Cambarus. Identify the following features of the external anatomy:

Three body regions: Cephalothorax (the head and thorax have a continuous carapace, and there is no moveable neck), abdomen, tail (a fan consisting of telson in the middle and uropods lateral to it.

Head with compound eyes, antennules, and antennae

Mouth parts with mandibles, maxillae, and maxillipeds

Locomotory appendages: Chelipeds with claws, walking legs, and swimmerets on abdominal segments. Be sure you can tell the sexes apart. In males the first abdominal segment has stiff, trough-like swimmerets that transfer sperm to the female in mating. In females, the first pair of swimmerets is like the rest, but all of the swimmerets are much longer than in males; the fertilized eggs are fastened to them by a sticky secretion; as the female moves about, the water around the eggs is refreshed.

Dissection:

Cambarus lateral view, carapace removed

Make a dorsal, longitudinal cut from the posterior edge of the cephalothorax to the rostrum, and remove one side of the carapace. Identify the feathery gills and the dorsal heart in a pericardial sinus that is probably filled with colored stuff. The circulatory system is an open one with vessels that open into spaces among the cells. A short esophagus leads from the mouth to the stomach and foregut, which are surrounded by a digestive gland that supplies them with enzymes. Open the stomach and foregut; there are tooth-like hard structures at the boundary, and these grind up the food. You may even see the brain in the head and the connective branches of the ventral nerve cord around the esophagus. The gills are buoyant in water and the individual gill filaments float free; this creates a huge surface area for molecular exchange. When you remove a crayfish from water, the gill filaments are no longer supported and they collapse onto each other; the surface area for gas exchange is much less, and the crayfish suffocates. If the circulatory system of your crayfish has been injected with colored material, you may see evidence that blood vessels go into the gills.

Cambarus gill in water

Make a dorsal, longitudinal cut along the abdominal carapace segments to the tail and remove them; identify the large abdominal flexor muscles, abdominal aorta, and, beneath it, the intestine.

Class Insecta

Preserved specimens of the grasshopper Romalea is our example. Find the following external features:

Body regions: head, thorax, abdomen which terminates bluntly (male) or with a pair of digging claws, the ovipositor (female).

Mouth parts: maxilla with maxillary palps, labrum, labium with labial palps, mandibles

Compound eyes and antennae.

Legs for walking and jumping; tarsi have claws and pads for grasping.

Segments of the body are perforated by respiratory holes called spiracles.

Two pairs of wings, forewings protect fragile hind wings that are for flying.

The large, oval tympanic membrane is for hearing.

Dissection of a preserved specimen with dissecting microscope: make a mid-dorsal and mid-ventral longitudinal cut with the scissors. Remove the exoskeleton form one side of the specimen. Observe the digestive system with gastric ceca. The female will have a mass of rice-shaped eggs in the abdomen.

Class Chilopoda

The centipedes have a dorsoventrally flattened body and one pair of legs on each segment. The first body segment has poison claws.

Class Diplopoda

The millipedes have a cylindrical body and segments bear two pairs of legs. There are no poison claws. When disturbed they curl up, and some secrete a noxious fluid.

Class Arachnida

Spiders and scorpions have a body that consists of cephalothorax and abdomen. The two parts are fused in ticks and mites, which are parasitic. There are no true mandibles or antennae, and the eyes are simple. Arachnids have four pairs of walking legs in contrast to other arthropods.

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Last updated 22 September 2007 (JHW)