Comparative Anatomy: Digestion, Circulation, and Homeostasis
Primitive Digestion: Intracellular Systems
The Porifera (sponges) are primitive organisms that lack true tissues or organs. They perform the digestion of food particles in specialized cells called choanocytes. Food, carried by water, penetrates through pores. The beating of the flagellum pushes water toward the collar, and food is captured by endocytosis. The resulting vacuole fuses with digestive enzymes from lysosomes. Undigested waste is expelled by exocytosis (intracellular digestion).
The primary disadvantage of this system is that it is limited to single, microscopic particles, and every cell must possess the necessary machinery for processing. For larger food sources, evolution favored a new type of digestion:
Evolution of the Gastrovascular Cavity
The gastrovascular cavity has only one opening, functioning as both the mouth and the anus. Food is broken into smaller pieces by the action of digestive enzymes discharged directly into the cavity (extracellular digestion). However, this breakdown is not complete; the pre-digested food then enters the cells lining the cavity where catalysis is finalized.
Specialized Feeding Structures and Initial Processing
The digestive tract often begins with a mouth, pharynx, and specialized mouthparts. Animals that feed on liquids have developed structures to absorb food:
- Insects: The proboscis (e.g., butterflies and moths) or piercing-sucking mouthparts (e.g., mosquitoes).
- Cephalopods: Catch fish using tentacles and tear them with a horny beak.
- Starfish: Use their ambulacral feet to open the valves of oysters and clams, projecting their stomach to consume the soft body of the bivalve.
Other specializations include:
- Gastropods (Herbivores): Use the radula, a tongue with tiny teeth, to gnaw leaves.
- Sea Urchins: Use Aristotle’s lantern, limestone pieces acting as teeth, to scrape algae embedded in rock.
Among vertebrates, teeth and a tongue first appeared. In birds and turtles, teeth are replaced by a beak. The combined action of the tongue and teeth grinds the food and mixes it with saliva, which contains the enzyme salivary amylase (ptyalin). This enzyme begins breaking down starch molecules, releasing maltose.
Food Storage
Worms, insects, and birds ingest and store food in a specialized pouch called the crop.
The Stomach and Mechanical Grinding
This region corresponds to the stomach, a pouch where food is stored and mixed with gastric juices for digestion. In animals that lack teeth or do not use them for grinding, a muscular gizzard is present. This structure grinds food:
- Annelids, Crocodiles, and Birds: The gizzard often uses ingested stones (gastroliths) to grind food.
- Crustaceans: Calcareous pieces form the gastric mill.
- Insects: Use chitinous teeth.
Final Digestion and Nutrient Absorption
The small intestine is the site of most chemical digestion and absorption. Nutrients traverse the wall and reach the circulatory stream, which transports them to all cells. To make absorption more efficient, the intestine has developed specializations to increase surface area:
- Annelids: A fold in the dorsal wall called the typhlosole.
- Sharks: Mucus folds forming the spiral valve.
- Vertebrates: The intestine folds into fingerlike projections called villi. The membrane of each cell is further folded into microvilli.
Water Absorption and Waste Elimination
In the large intestine, water is reabsorbed. Undigested materials form stool (feces), which is expelled. Fermentation by bacteria also occurs in this region.
Excretion: Eliminating Metabolic Waste
Excretion is the elimination of metabolic waste continually produced by cells.
The Circulatory System: Vessels and Flow
The circulatory system relies on specialized vessels for transport:
- Arteries: Have thick, muscular walls and elastic connective tissue, allowing them to vary their diameter.
- Arterioles: Lack elastic fibers but possess a thin layer of smooth muscle. The ring of muscle at the junction between an arteriole and a capillary is called the precapillary sphincter.
- Capillaries: Vessels where the exchange of nutrients and wastes occurs by diffusion between the blood and the body’s cells.
- Venules: Collect blood from the capillaries and drain into larger veins.
Types of Circulation
- Pulmonary Circulation: Deoxygenated blood leaves the heart via the pulmonary arteries to the lungs, where it is oxygenated, and returns to the heart via the pulmonary veins.
- Systemic Circulation: Oxygenated blood leaves the heart through the aorta, is distributed throughout the body, and returns to the heart via the vena cava.
Osmoregulation
Osmoregulation is the exchange of salt and water between the extracellular environment and the outside world to compensate for gains and losses of these substances, maintaining internal balance.