Anatomy of Animals

Arthropods

Segmented animals have repeating units. This has led to specialization of parts over evolutionary time because the different segments could become specialized for different purposes. The ancestral arthropods probably had many segments with a pair of appendages on each segment. These animals belong to the classes Chilopoda (see Figure 15C) and Diplopoda (it has 2 segments fused into one - 2 pairs of legs for each segment). The segments of arthropods today (classes Arachnida and Insecta,

		Figure 15B and D) are often fused into regions such as the head, thorax, and abdomen. The head and thorax may be fused to form a cephalothorax (class Crustacea, Figure 15A). Since insects comprise one of the largest animal groups - both in number of species (about 1 million) and in number of individuals, the grasshopper will be used as the anatomical example (Figure 16e), also see a crayfish anatomy in Figure 16b. Figure 16a captures an image of
Figure 15 Arthropods [view large image]	Figure 16a Flight of the Bumblebee [view large image]	the flight of bumblebee.

Circulatory - They have an open circulatory system. Colorless blood is pumped dorsally from the heart and then enters sinuses where it comes in direct contact with the tissues. These sinuses are referred to as a hemocoel.
Digestive - The mouth contains grinding mouthparts. Food in the mouth is mixed with saliva, which contains enzymes that begin the process of digestion. The crop functions in storage. Upon leaving the crop, food enters the gizzard where it is ground into smaller particles. Most chemical digestion occurs in the stomach. Absorption of nutrients occurs in the stomach and pouches that are attached to the stomach called gastric caeca. Absorbed nutrients move into the hemocoel. The intestine functions mostly to absorb water.
Endocrine - Metamorphosis is controlled by hormones.

Excretory - Terrestrial forms have an excretory system that conserves water. The malpighian tubules collect wastes from the blood and reabsorb water so that only dry material is excreted. Marine Crustaceans such as grayfish have excretory system consists of a pair of green glands lying in the head region anterior to the esophagus. Each organ possesses a glandular region for waster removal - a bladder and a duct that opens ventrally at the base of the antennae (Figure 16b).

Figure 16b Crayfish Anatomy
[view large image]

Immune - Insects and other arthropods have several kinds of blood cells (hemocytes) two of which (the Granulocyte and/or Plasmatocyte) provide cellular immunity (e.g., phagocytosis and encapsulation) - the first line of defense. These immunocytes also synthesize and secrete into the blood (hemolymph) several humoral, antiviral, and other anti-bacterial factors; the hemolymph also contains neuropeptides and opioids, all of which often provide the second line of defense or assist in cellular immunity.
Musculo-skeletal - The external skeleton (exoskeleton) is composed of chitin, which is a strong, flexible polysaccharide. It serves to protect the animal as well as provide an attachment site for muscles, and prevention of desiccation on land. Because chitin is hard and nonexpandable; it requires periodic molting (shedding) for growth. The new skeleton must expand and harden when the old one is shed. The appendages are also covered by the skeletal material, but they are jointed. The Jointed appendages act like a system of levers and are ideal for any activity that requires movement. Arthropods have evolved a variety of different kinds of specialized appendages for activities such as walking, swimming, reproduction, or feeding.

Nervous and Sensory - Well-developed sense organs are needed for active animals. Arthropods have both compound and simple eyes. Insects and crustaceans have compound eyes (see Figure 16c, magnified 370X), each with many separate visual units called ommatidia. Each ommatidium contains a lens and a light-sensitive cell. Eyes of this design allow for the detection of small movements, but are weak in distinguishing detail. The tympanum is used for the reception of sound waves. The nervous system includes a dorsal brain and a double, ventral, solid nerve cord. Such well-developed nervous system is needed to integrate the sensory information and allow the animal to be active.

Figure 16c Compound Eyes of Mosquito [view large image]

Reproductive - Males deposit sperm directly into females where fertilization occurs. This type of breeding, called internal fertilization, allows animals to breed on dry land. Another advantage is that fewer gametes are needed than if both sexes deposit their gametes in water. The change in body form as an insect grows to adult is called metamorphosis (Figure 16d). In some species such as the grasshopper, the larvae look like the adult; the animal simply gets larger as it matures. This type of change is incomplete metamorphosis. Species that undergo a complete reorganization of the body tissues exhibit complete metamorphosis, in which case they have 3 stages of development: larval stage, pupal stage, and adult stage, for example, the three stages of caterpillar, cocoon, and

Figure 16d Metamorphosis

butterfly as shown in Figure 16d. Worm-like larvae form a cocoon called a pupa and then become transformed into the adult form. Metamorphosis occurs during the pupal stage, when the animal is enclosed within a hard covering. Complete metamorphosis allows the animal to take advantage

of two different ecological settings (niches). For example, caterpillars are specialized for feeding on plant leaves. The butterfly of many species is specialized for finding mates and reproduction.

Respiratory - Insects have a network of tubules called tracheae that bring oxygen directly to the tissues and allow carbon dioxide to escape. The openings to the outside, called spiracles, are located on the side of the abdomen. Air sacs attached to the tubules pump air through the system.