It parallels that of lymphatic system. Jaw less vertebrates (cyclostomes), cartilaginous fishes (chondrichthuns) and bony fishes (osteichthians) have network of small blind vessels that accompany the veins and help the capillaries to drain the tissues. These vessels are developed from veins and empty into them at frequent intervals. These represent the earliest stage in the evolution of lymphatic system; however no lymph nodes or lymphocytes are present. Teleosts and Tetrapods have independently evolved lymphatic system in which lymphatic capillaries help to drain most of tissues of the body. Lymph nodes and lymphocytes occur in few aquatic birds and are abundant in mammals.
Intra cellular and extra cellular digestion:
Intra cellular 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 circumvents 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.
Feeding strategy in animals:
(1) Suspension Feeders: It is the removal of suspended food particles from the surrounding water by capturing trapping or filtration structure. It has three steps (i) Transport of water past the feeding structure (ii) Removal of nutrients from water (iii) Transport of nutrients to the mouth of digestive system e.g. sponges, ascidians, bivalves, crustaceans, polychaetes, gastropods and some chordates.
Deposit feeders: It involves primarily omnivorous animals. These animals obtain their nutrients from sediments of soft bottom habitats or terrestrial soils. Direct deposit feeders simply swallow large quantity of sediment. Usable nutrients and digested and the remains pass out the anus. E.g. polychaete annelids, some snails etc.
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 (teeth) that powerful muscles manipulate e.g. Molluscs, polychaete worms, arthropods and sea urchins.
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