Intracellular digestion:
In simple animals (protitsts and sponges) some cells take in whole food particles directly from the environment by diffusion, active transport and endocytosis and break them down with enzymes to obtain nutrients. It is called intracellular (within the cell) digestion. It circumstances the need for the mechanical breakdown of food or for a gut or other cavity in which to chemically digest food. It limits animal’s size and complexity only very small pieces of food can be used. It provides all or some of the nutrients in protozoa, sponges, cnidarians, platyheliminthes, rotifers, bivalve molluscs and primitive chordates.
Extra cellular digestion:
Larger animals have evolved structures and mechanisms for extra cellular digestion; the enzymatic breakdown of larger pieces of food into small molecules usually in special organ or cavity. Digested food then passes into body cells lining the organ or cavity and can take part in energy metabolism or biosynthesis.
Suspension feeders:
It is removal of suspended food particles from the surrounding water by capturing trapping or filtration structure. It involves transport of water past the feeding structure. It involves in removal of nutrients from water and transport of nutrients to the mouth of the digestive system.
Sponges, ascidians, branchiopods, entoprocts, phoronids, most bivalves and many crustaceans, are suspension feeders.
Deposit feeders:
It involves primarily omnivorous animals. These animals obtain their nutrients from sediments of soft bottom habitats (mud and sand) or terrestrial soils. Direct deposit feeders simply swallow large quantity of sediment mud, sand, organic matter; the usable nutrients and digested and the remains pass out the anus. E.g. polychaete annelids, some snails, some sea urchins and in most earthworms. Other direct deposit feeders utilize tentacle like structures to consume sediment e.g. sea cucumbers, most sipanculans, curlain clams and several types of polychaetes.
Herbivory:
It is consumption of macroscopic plants. It requires the ability to bite and chew large pieces of plant matter (macroherbivory). Biting and chewing mechanisms evolved in a number of invertebrate lineages are often characterized by the development of hard surface (e.g. teeth) that powerful muscles manipulate e.g. Molluscs, polychaete worms, arthropods and sea urchins.
Many molluscs have a radula. A radula is a muscularized belt like rasp armed with chitinous teeth. Molluscs use the radula to scrape algae of rocks or to tear the leaves of terrestrial plants. Polychaets have sets of large chitnous teeth on eversible proboscis on pharynx that is used to scrape off algae. This toothed pharynx is also suitable or carnivory when plants material is scarce.
Macroherbivory is found in almost every group of Arthropods e.g. insects and crustaceans have large powerful, mandibles of biting off plant material and grinding and chewing it.
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