(a) Describe the general arrangement of the viscera of a named mammal. (b) Describe the process of feeding in (i) Hydra; (ii) Amoeba.

(a) General arrangement of the viscera of a named mammal (the rabbit)

The viscera are the internal organs contained in the body cavity (coelom). In a mammal such as the rabbit the coelom is divided by a muscular sheet, the diaphragm, into a front thoracic cavity and a hind abdominal cavity.

Thoracic cavity (chest):

• The heart lies centrally, enclosed in the pericardium.
• A pair of lungs lies on either side of the heart.
• The trachea runs down and divides into two bronchi entering the lungs, and the oesophagus passes behind them through the diaphragm.

Abdominal cavity (belly):

• The stomach lies just behind the diaphragm on the left.
• The large liver lies behind the diaphragm, with the gall bladder associated with it.
• The coiled small intestine and the large intestine (with a large caecum in the rabbit) fill much of the cavity, held by the mesentery.
• The pancreas lies in the loop of the duodenum and the spleen near the stomach.
• A pair of kidneys lies against the dorsal body wall, leading by ureters to the bladder.
• The reproductive organs (ovaries and uterus in the female; testes in the male) lie in the lower part of the abdomen.

(b) Process of feeding

(i) Hydra: Hydra is carnivorous. When a small animal (e.g. a water flea) touches the tentacles surrounding its mouth, the stinging cells (cnidoblasts/nematocysts) discharge threads that paralyse and hold the prey. The tentacles then push the prey through the mouth into the enteron (gastrovascular cavity), where enzymes secreted by gland cells digest it (extracellular digestion); the products are then taken up by the endodermal cells and digested further inside them (intracellular digestion). Undigested remains are egested through the mouth.

(ii) Amoeba: Amoeba feeds by phagocytosis (holozoic nutrition). When it meets a food particle such as a smaller organism, it flows around it by pushing out pseudopodia which surround and enclose the food together with a little water, forming a food vacuole. Enzymes are secreted into the vacuole to digest the food; the soluble products are absorbed into the cytoplasm, and the undigested matter is left behind (egested) as the Amoeba moves on.