Digestion is carried out with the help of certain digestive enzymes. It is still not clear just how the digestive enzymes enter the food vacuoles (Barrington, 1962).
According to De Duve 1963, the enzymes are hydrolytic in nature and are contained in special packages called lysosomes.
It is believed that during the process of digestion the lysosomes arc discharged in some manner into the food vacuoles and thus contained food is broken down into its constituents whom then pass into the cytoplasm and be assimilated.
The parasitic protozoans either absorb the predigested food of their host or digest their food with the help of hydrolytic enzymes outside the body.
2. Digestion in Porifera:
In porifers or sponges the digestion is intracellular, therefore, it occurs in food vacuoles of collar cells much as in protozoans (Barrington, 1962), but these animals with their multicellular organisation require a distributing system.
Apparently this function is performed by amoebocytes which transfer the food materials from the collar cells to other cells of the body.
3. Digestion in Coelenterata:
In coelenterates both intracellular and extracellular digestions are met. During intracellular digestion the food or prey is engulfed and digested in a food vacuole with the help of digestive enzymes as in protozoans.
During extracellular digestion the food is digested in the gastrovascular cavity, the wall of which contains numerous glandular cells which secrete digestive enzymes. These enzymes help in the digestion of food materials.
4. Digestion in parasitic worms:
In most turbellarians the digestion is extracellular but in some it is carried out in the meshes of a temporary syncytium formed by processes from amoeboid cells.
In some of the Rhabdocoelida digestion is mainly or perhaps entirely extracellular. The pharyngeal glands of the turbellarians produce mucus while in some species of Rhabdocoelida their secretions contain digestive enzymes which are used in the digestion of food materials outside the body.
In liver-fluke, the so called Faiciola hepaticq, the digestion is extracellular and it feeds on bile, blood, lvmph, and cell debris.
It can also absorb glucose, fructose, galactose and maltose but not lactose or sucrose, through the general body surface.
It is believed that an enzyme called proteinase is secreted in the digestive tract which digests the protein of the solid tissues.
In parasitic tapeworms such as Taenia solium the digestion is supposed to be intracellular as they absorb digested food in the form of nitrogenous substances from the mucous membrance of the host. They also absorb glucose and galactose but they do not secrete any carbohydrases.
In some nematodes including AscariS the digestion is extracellular and it is believed that amylase, maltase, protease, peptidase and lipase enzymes are secreted in the alimentary canal which digests the starch, maltose, proteins and fats respectively.
5. Digestion in Annelida:
In most of the annelids the digestion is extracellular. Annelids take all types of food and their intestinal walls produce several enzymes such as proteolytic enzyme, diastase, glycogen hydrolyzing enzyme, lipase and amylase, these en- zvmes digest the food materials in the intestine.
In earthworm, the salivary glands of pharynx produce mucin and proteolytic enzyme, the former lubricates the food and passes it down and the latter starts the digestion of protein.
6. Digestion in Arthropoda:
In Crustacea the digestion takes place outside the body (extracellular). In decapods the food is broken down into its constituents in the gastric mill and at the sametime attacked by a proteinase which is sent forward from the digestive diverticula.
At the entrance to the latter there is a complicated filter which permits only fine food particles to pass.
The food particles are taken into the diverticula by the contraction of longitudinal muscles and expelled by the contraction of circular muscles.
In some insects the digestion is extracellular. The labial glands of these insects secrete an enzyme called amylase in cockroach, sucrase in honeybees and moths.
These enzymes digest the starch and sucrase respectively. The labial glands open immediately into the crop where digestion may take place by enzymes.
The crop is the chief site of digestion of these insects. Yeast and bacteria may also carry out the digestion of food materials.
The insects which feed on solid food, the bolus on leaving the foregut is enclosed in a thin sac of chitin called the peritrophic membrane which is permeable to both enzymes and digested food.
Absorption of digested food materials also takes place through it. This is completely lacking in fluid-feeders such as bugs, fleas, lice, etc.
Many adult butterflies and moths have no digestive enzymes except sucrase which digests the sucrose. In these insects the digestion is extracellular.
7. Digestion in Mollusca:
In molluscs both types of digestion, e.g., extracellular and intracellular, are found. Two specializations are of particular interest.
The first of these is the digestive gland made up of branched glandular follicles communicating with the stomach by a system of ciliated ducts.
In some groups the epithelial cells are phagocytic, and digestion is intracellular. In nuclidae the digestion is entirely extracellular and is carried out with the help of digestive enzymes (Owen, 1956).
In many forms there is a combination of the two processes (extracellular and intracellular).
A curious process in connection with the intracellular digestion taking place in the epithelial cells is a fragmentation of the outer wall of the cell to form spherical bodies containing the food vacuoles together with waste products and enzymes.
These bodies than pass into the stomach and may be sources of some of the enzymes there (Owen, 1955, 56).
The second of these special features is the crystalline style which is generally found in lamellibranchs but rarely in the more advanced rbivorous gastropods.
The gradual development of the style in connection with ciliary feeding is discussed by Morton (1958, 1960).
It is a thick gelatinous rod loaded with enzymes. It is rotated by strong cilia which force it gradually into the stomach where it rubs against the horny gastric shield to release its enzymes and to stir up the stomach contents.
It is believed that amylases are the most abundant enzymes of the crystalline style, however, lipases seem also to be present.
In lamellibranchs and in some echinoderms their intracellular ingestion is partially dependent on an extensive phagocytosis as the amoebocytes or phagocytes procure food particles, digest them and pass their products to the other cells of the body (Yonge, 1937, 1960 a).
In most gastropods the digestion is extracellular and digestion and absorption take place in the digestive diverticula.
In these animals radula is an important feeding organ which is lubricated by the secretion of buccal glands. In cephalopods the digestion is entirely Ultracellular.
The digestive diverticula of Sepia and Octopus but not or Loligo are absorptive as well as secretory in function.
8. Digestion in Echinodermata:
In echinoderms the digestion is extracellular as well as intracellular. In starfish, digestion takes place outside the body with the help of digestive enzymes.
The pyloric caeca contain many phagocytes which probably take up small food particles and migrate through the gutwall.
It is believed that in sea urchin, brittle star and holothuria the digestion occurs partially in the gut and partially in the phagocytes.