What is evolution in digestive structures in invertebrates


Protozoa:
Protozoa may be autotrophic, saprozoic or heterotrophic (ingest food pasticles). Ciliated Protozoa are good example for protests that utilize heterotrophic nutrition. Ciliary action directs food from the environment into the buccal cavity and cytostome. The cytostome opens into cytopharynx which enlarges as food enters and pinches off food containing vacuole. The detached food vacuole then moves through the cytoplasm. During this movement excess water is removed from the vacuole; the contents are acidified and then made alkaline and a lysosome adds digestive enzymes. The food particles are then digested within the vacuole and the nutrients are absorbed into the cytoplasm. The residual vacuole excretes its waste products via cytopyge.

Bivalve Molluscs:
Many bivalve molluscs feed and ingest small food particles. The digestive tract has a short oesophagus opening into stomach, midgut and hindgut. The stomach contains a crystalline style gastric shield and diverticulated region. These diverticulate are blind ending sacs that increase the surface area for absorption and intercellular digestion. The midgut, hindgut and rectum function in extra cellular digestion and absorption.
Digestion is a coordination of three cycles (1) feeding (2) extra cellular digestion and (3) intracellular digestion. The mechanical and enzymatic breakdown of food during feeding provides small particles for intracellular digestion. Intracellular digestion releases nutrients into the blood and produces fragmentation spherules that both excrete wastes and lower the p for optimal extra cellular digestion. These three cycles are linked to the tidal immersion and emersion of molluscs.

Insects:
Grass hopper is an insect with complete digestive and extra cellular digestion. During feeding the mandibles and maxillae first break up (masticate) the food which is then taken into mouth and passed to the crop via oesophagus. During mastication, the salivary gland, add saliva to the food to lubricate it. Saliva also contains the enzyme amylase which digests carbohydrates. This digestion continues during food storage in the crop. The midgut secretes other enzymes (carbohydrases, lipases, proteases) that enter the crop. Food passes slowly from the crop to the stomach, where it is mechanically reduced and the nutrient particles sorted. 

Large particles are returned to the crop for further processing; small particles enter the gastric caecae where extra cellular digestion is completed. Most nutrient absorption then occurs in the intestine. Undigested food is moved along the intestine and passes into the rectum where water and ions are absorbed. Solid fecal pallets that form the pass out of the animal via anus.

During this entire feeding process, the nervous system, the endocrine system and the presence of food extent considerable control over enzyme production at various points in the digestive tract.

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