Evolution of vertebrate immune system, Intra cellular and extra cellular digestion and feeding strategy in animals
Evolution of vertebrate immune system:
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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