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

(a) Larval Forms of Crustaceans

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 large carapace (often with dorsal and rostral spines in crabs), stalked compound eyes and a segmented abdomen; it swims with the exopodites of its maxillipeds, while the thoracic legs are only rudimentary, and it feeds on plankton. Mysis (the schizopod stage of Penaeus and lobsters) is a later larval stage in which the thoracic legs bear swimming exopodites, 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. Other specialised larvae include the leaf-like phyllosoma of the spiny lobster Palinurus, the alima and erichthus of stomatopods such as Squilla, and the glaucothoe of hermit crabs. The sequence differs between groups: Penaeus passes through nauplius → metanauplius → protozoea → zoea → mysis → post-larva; crabs hatch as zoea and pass through megalopa; barnacles pass from nauplius to cypris. In each case appendages are added at successive moults.

(b) Coral Reefs and Theories of Their Formation

Coral reefs are massive calcareous structures built by colonial anthozoans, mainly scleractinian corals, in shallow tropical waters. Reef zonation reflects light, temperature and salinity. Three types are recognised: fringing reefs attached to the shore, barrier reefs separated from land by a lagoon, and atolls, ring-shaped reefs enclosing a lagoon without a central island. In India, fringing reefs occur in the Gulf of Mannar and Gulf of Kutch, atolls in Lakshadweep, and fringing and barrier reefs around the Andaman and Nicobar Islands. 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), wave erosion planed exposed islands into platforms, and during the post-glacial rise corals grew upward on these platforms keeping pace with sea level. It explains reefs at different depths but does not by itself account for all atoll structures. Murray’s standstill theory proposes that reefs develop on stable submarine platforms built up to suitable depth by sediment accumulation, with lagoons formed by solution, rather than requiring subsidence. Deep drilling settled much of the debate: the Funafuti boring (1896–98) went about 340 m without reaching basement, but the Enewetak drillings of 1952 reached volcanic basalt beneath about 1,300 m of shallow-water reef limestone, strongly supporting Darwin's subsidence. The current view combines subsidence of cooling oceanic crust, explained by plate tectonics, with Daly's glacial changes in sea level.

(c) Evolution of the Kidney as an Excretory Organ

Excretory organs evolved from simple excretory cells to tubular organs with filtration and reabsorption. In Platyhelminthes, flame cells (protonephridia) 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. All three develop from the nephrogenic intermediate mesoderm in an anterior-to-posterior sequence (the archinephros concept), and larval hagfish retain the most primitive, segmental condition. 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. Birds have some looped (mammalian-type) nephrons but mostly loopless reptilian-type ones and, like reptiles, excrete uric acid, which saves water. The nitrogenous waste changes with habitat: ammonia in aquatic animals, urea in amphibians and mammals, and uric acid in reptiles and birds. Thus, the kidney’s evolution is a graded adaptation from simple excretion to integrated excretion and osmoregulation.