Question

1. What is countercurrent exchange? Give an invertebrate example, both naming the taxon and explaining how it works and what it achieves. Use a simple figure with labels as well as some text (in your own words, of course) to explain.
2. What is a coelom and what advantages does it provide in animals that have it? Name some phyla that have and do not have a full coelom. How is the status of having or not having a full coelom predicted to affect invertebrate design and function, including movement?
3. Give some examples of colonial animals where the individual units within a colony are specialized for different functions. What are the functions? What colonial animals do not show specialization within a colony, i.e., all the units are instead functionally equivalent. Be sure to consider this question at the level of phyla and do discuss ALL of the colonial phyla that we have discussed in this course. Note if all the members of the phylum share the features you mention or only a subset.
4. Describe the difference between ocelli and image-forming eyes. Give examples of phyla that have both types of light sensing abilities across different species within a phylum.

Answer 1

a. Countercurrent exchange mechanism is the process of exchange between two fluids flowing in opposite directions maintaining a concentration gradient between them with maximum efficiency. This means if one fluid flows from high to low then the other fluid must flow in reverse order to obtain maximum exchange of the desired substances or gases. In biological system, the countercurrent exchange is observed in fish gills where the dissolved oxygen is transported from the external water into the blood, loop of Henle in mammalian kidneys, etc. An example of invertebrate having countercurrent exchange is Spirula which is a squid like cephalopod belonging to phyla Mollusca. It has gas filled spaces in its shell and lives at a depth of 1200m in the ocean. It cannot rely simply on osmosis to remove the liquid from its shell. This is achieved by countercurrent mechanism through which the osmotic concentration of the blood in the siphuncle is elevated well above the osmotic concentration of the rest of the blood. The osmotic pressure of the fluid in the chambers is reduced by the epithelium of the siphuncle. The uppermost surface has numerous microvilli which faces the porous wall of the siphuncle tube. The cells pump salts out 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 series of spaces present inside the body. It generally refers to a large fluid-filled space (cavity) lying between the outer body wall and the inner gut wall (wall of digestive tube).

The animals possessing true coelom have certain advantages which are as follows: (i) Coelom or the body cavities help in the circulation of nutrients and gases and also efficiently removes waste products. (ii) The coelom is filled with a fluid which acts as a hydrostatic skeleton that helps in the better movement of an organism.

Animals that lacks coelom are acoelomates like phylum porifera, cnidaria. ctenophora and platyhelminthes. Animals having false or pseudocoelom is roundworms (Aschelminthes) and animals having true coelom 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 a social, polymorphic and colonial insect. It has three main castes- queen, drones and workers. Queen is the only fertile female of the colony which functions to lay eggs for 2-5 years. Drones are male honey bees and they develop from unfertilized eggs. Drones and virgin queens take part in nuptial flight. Workers are sterile females. Scout bees which are worker bees search food and intimate the same to worker bees by dances- round dance and tail wagging dance. Workers have a pollen collecting apparatus, honey storing mechanism and wax secreting glands. Young workers secrete royal jelly 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 eyes are the organs of sight. The difference between ocelli and image-forming eyes are as follows:

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

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

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

(iv) Simple or ocellar eyes are more prevalent in higher vertebrates while invertebrates like annelids, arthropods, mollusca have frequent presence of compound eyes.

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

The phyla having both type of light sensing abilities is Arthropoda, annelid and mollusca. As for example, arthropods have both simple and compound eyes. At the base of each antenna, on inner side, a small rounded and light colored area called fenestra or ocellar spot representing simple eye is present. Head bears a pair of large, dorsolaterally placed compound eyes which are the organs of sight.