(a) Describe the different larval forms of Crustaceans with suitable diagrams. (20 marks) (b) What are coral reefs ? Describe the…

(a) Crustacean larvae Crustacean larvae show graded metamorphosis from a nauplius-like planktonic form to the adult. In a labelled diagram, the nauplius is a small unsegmented body with one median unpaired eye and three pairs of appendages: first antennules, second antennae and mandibles; it lacks thoracic appendages and occurs in Cirripedia and many branchiopods. Metanauplius is a modified nauplius in which, as in the typical Apus, the nauplius appendages are retained and only maxillules/maxillae are added; maxillipeds are not present at this stage. Protozoea is the early decapod larva, with a nauplius head and the first thoracic appendages beginning to differentiate. Zoea, characteristic of Decapoda, has a carapace, well-developed antennules, antennae, maxillipeds and several pairs of thoracic legs; it feeds on plankton. Mysis is a later larval stage in which abdominal segments and pleopods appear, uropods and a tail fan develop, and the body more closely resembles the adult. Megalopa is the transitional decapod stage with adult-like thoracic and abdominal appendages, while cypris is the barnacle larva with a bivalved carapace and long antennules used for attachment before metamorphosis. Metamorphosis to the benthic adult involves loss of larval setae and reorganization of appendages. The sequence is therefore nauplius, metanauplius, protozoea, zoea, mysis, megalopa/cypris, with appendage addition and metamorphic moults marking each transition.

(b) Coral reefs Coral reefs are massive calcareous structures built by colonial anthozoans, mainly scleractinian corals, in shallow tropical waters. Reef zonation reflects light, temperature and salinity. Darwin’s subsidence theory holds that fringing reefs, barrier reefs and atolls form as volcanic islands slowly sink while corals grow upward; evidence includes atolls on subsiding volcanic platforms, deep lagoons and forereefs. Its limitation is that some reefs occur on stable or uplifted platforms. Daly’s glacial-control theory explains reefs by Pleistocene eustatic changes: during glaciation sea level fell by about 60–70 fathoms (110–120 m), corals grew on exposed platforms, and later post-glacial rise drowned the reefs at various depths. It explains reefs at different depths but does not by itself account for all atoll structures. Murray’s offshore-bar theory proposes that reefs develop on offshore sand bars or coral rubble where currents and wave action favour accretion, rather than requiring uniform subsidence. Deep-core drilling supports the importance of basement and biogenic limestone: at Eniwetok basaltic basement was reached under thick biogenic limestone, whereas the Funafuti boring did not reach basement, showing that evidence is locality-specific.

(c) Evolution of kidney Excretory organs evolved from simple excretory cells to tubular organs with filtration and reabsorption. In Platyhelminthes, solenocytes perform ultrafiltration and discharge wastes; in rotifers and annelid larvae, protonephridia with flame cells increase filtration; in adult annelids and arthropods, metanephridia develop coelomic funnels and tubules with selective reabsorption. Insects use Malpighian tubules, and crustaceans use antennal/maxillary glands, often modified for osmoregulation. In vertebrates, the kidney evolves through three stages. The pronephros is the first kidney, seen in embryos and in some fish/amphibian larvae; pronephric tubules open to the coelom and remove nitrogenous wastes, but reabsorption is limited. The mesonephros is the functional kidney of most fish and amphibians; nephrons have a Bowman’s capsule, glomerulus and proximal/distal tubules, allowing ultrafiltration, reabsorption of water and ions, and osmoregulation in freshwater or marine habitats. Freshwater fish excrete dilute urine and reabsorb ions; marine fish conserve water and excrete salts. The metanephros is the permanent kidney of reptiles, birds and mammals; it arises from metanephric blastema and ureteric bud, and its nephrons, especially with the loop of Henle in mammals, produce concentrated urine and conserve water on land. Thus, the kidney’s evolution is a graded adaptation from simple excretion to integrated excretion and osmoregulation.