Animal tissues are divided into four types:
- Epithelial tissue: This tissue has a free surface, which faces either a body fluid or the outside environment and thus provides a covering or a lining for some part of the body. There are two types of epithelial tissues: simple epithelium and compound epithelium.
- Connective tissue: Connective tissues are most abundantly and widely distributed in the body of complex animals. They are named connective tissues because of their function of linking and supporting other tissues/organs of the body. These include: cartilage, bone, adipose and blood.
- Muscle tissue: Each muscle is made of many long, cylindrical fibres arranged in parallel arrays. There are three types of muscles: skeletal, smooth and cardiac.
- Neural tissue: Neural tissue exerts the greatest control over the body’s responsiveness to changing conditions. The units of the neural system are neurons.
In earthworm, the body is covered by cuticle. All segments of its body are alike except the 14th, 15th and 16th segment. A ring of S-shaped chitinous satae is found around each segment. These help in locomotion. On the ventral side, spermathecal openings are present. The alimentary canal is a narrow tube made of mouth, buccal cavity, pharynx, gizzard, stomach, intestine and anus. The blood vascular system is of closed type with heart and valves. Nervous system is represented by ventral nerve cord. Earthworm is a hermaphrodite.
The body of a cockroach is covered by chitinous exoskeleton. It is divided into head, thorax and abdomen. Segments bear jointed appendages. There are three segments of thorax. Two pairs of wings are present. There are ten segments in the abdomen. Alimentary canal is well developed with a mouth surrounded by mouth parts, a pharynx, oesophagus, crop, gizzard, midgut, hindgut and anus. The blood vascular system is of open type. Respiration takes place by network of thrachae.
The body of the frog is covered by skin. Mucous glands are present in the skin which helps in respiration in water and on land. Body is divisible into head and trunk. A muscular tongue is present, which is bilobed at the tip and is used in capturing the prey. The alimentary canal consists of oesophagus, stomach, intestine and the rectum. The main digestive glands are the liver and pancreas. Circulatory system is closed with single circulation. Nervous system is organized into central, peripheral and autonomic.