What is countercurrent exchang

a. Countercurrent exchange mechanism is the process of exchangebetween two fluids flowing in opposite directions maintaining aconcentration gradient between them with maximum efficiency. Thismeans if one fluid flows from high to low then the other fluid mustflow in reverse order to obtain maximum exchange of the desiredsubstances or gases. In biological system, the countercurrentexchange is observed in fish gills where the dissolved oxygen istransported from the external water into the blood, loop of Henlein mammalian kidneys, etc. An example of invertebrate havingcountercurrent exchange is Spirula which is a squid like cephalopodbelonging to phyla Mollusca. It has gas filled spaces in its shelland lives at a depth of 1200m in the ocean. It cannot rely simplyon osmosis to remove the liquid from its shell. This is achieved bycountercurrent mechanism through which the osmotic concentration ofthe blood in the siphuncle is elevated well above the osmoticconcentration of the rest of the blood. The osmotic pressure of thefluid in the chambers is reduced by the epithelium of thesiphuncle. The uppermost surface has numerous microvilli whichfaces the porous wall of the siphuncle tube. The cells pump saltsout into the spaces reducing the cellular concentration of salts.This enables water to flow by osmosis.

b. A coelom or body cavity means any internal space or a seriesof spaces present inside the body. It generally refers to a largefluid-filled space (cavity) lying between the outer body wall andthe inner gut wall (wall of digestive tube).

The animals possessing true coelom have certain advantages whichare as follows: (i) Coelom or the body cavities help in thecirculation of nutrients and gases and also efficiently removeswaste products. (ii) The coelom is filled with a fluid which actsas a hydrostatic skeleton that helps in the better movement of anorganism.

Animals that lacks coelom are acoelomates like phylum porifera,cnidaria. ctenophora and platyhelminthes. Animals having false orpseudocoelom is roundworms (Aschelminthes) and animals having truecoelom are Eucoelomate like phylum annelids, echinoderms,chordates.

c. Some examples of colonial animals are Hydrozoans(Siphonophorae), Honey bees, etc. As for example, honey bee are asocial, polymorphic and colonial insect. It has three main castes-queen, drones and workers. Queen is the only fertile female of thecolony which functions to lay eggs for 2-5 years. Drones are malehoney bees and they develop from unfertilized eggs. Drones andvirgin queens take part in nuptial flight. Workers are sterilefemales. Scout bees which are worker bees search food and intimatethe same to worker bees by dances- round dance and tail waggingdance. Workers have a pollen collecting apparatus, honey storingmechanism and wax secreting glands. Young workers secrete royaljelly which is given to potential queens.

d. Ocelli are simple photo-receptors (light detecting organs).They have a single lens and several sensory cells. Compound eyesare the organs of sight. The difference between ocelli andimage-forming eyes are as follows:

(i) Image forming eyes are also can be categorized as compoundeyes which are formed of thousands of ommatidia whereas simple eyeis formed of a single lens with sensory cells.

(ii) The image formed in compound eyes is complex while thatformed by ocelli is simple and used to detect movement.

(iii) Compound eyes cover a wide range of the visual field ascompared to simple eye.

(iv) Simple or ocellar eyes are more prevalent in highervertebrates while invertebrates like annelids, arthropods, molluscahave frequent presence of compound eyes.

(v) The types of simple eyes are more diversified as compared tocompound eyes.

The phyla having both type of light sensing abilities isArthropoda, annelid and mollusca. As for example, arthropods haveboth simple and compound eyes. At the base of each antenna, oninner side, a small rounded and light colored area called fenestraor ocellar spot representing simple eye is present. Head bears apair of large, dorsolaterally placed compound eyes which are theorgans of sight.