(a) Structure of visual organs of cockroach. (10 marks) (b) What is meant by symmetry? Write about different types of symmetry in…

(a) Structure of visual organs of cockroach

The visual organs of the cockroach (Periplaneta americana) are a pair of compound eyes and two ocelli.

• Compound eyes: kidney-shaped, one on each side of the head, each made of about 2,000 units, the ommatidia, of the fused-rhabdom, apposition type.
• Each ommatidium has an outer dioptric part, a transparent corneal lens (facet) secreted by corneagen cells and a crystalline cone formed by four cone cells, and an inner receptor part, a ring of seven retinula cells (an eighth is rudimentary) that form the central rhabdom. Their axons pass through the basement membrane to the optic ganglia.
• Pigment around the ommatidia isolates them, so each contributes one spot to a mosaic image, good for detecting movement.
• Ocelli (fenestrae): two whitish patches on the frons near the antennal bases, each with a large flat lens and about 10,000 photoreceptors. They form no image and only detect changes in light intensity.

(b) Symmetry and its types in animals

Symmetry is the arrangement of body parts around a central point or axis so that a plane through the body divides it into similar halves.

• Asymmetry: no plane gives equal halves, as in most sponges, Amoeba and adult gastropods (after torsion and coiling).
• Spherical: any plane through the centre gives equal halves, as in Volvox, Heliozoa and Radiolaria.
• Radial: any plane through the oral-aboral axis gives equal halves, as in Hydra, Obelia and jellyfishes. Adult echinoderms are secondarily radial, with a five-part (pentamerous) plan.
• Biradial: only two planes through the axis give equal halves because of paired structures, as in ctenophores (Pleurobrachia) and sea anemones.
• Bilateral: only the median sagittal plane gives right and left mirror halves, as in flatworms, arthropods, annelids and vertebrates. It goes with a head (cephalisation), directional movement and distinct dorsal, ventral, anterior and posterior surfaces.

(c) Structure and functions of endocrine pancreas

The endocrine pancreas is formed by the islets of Langerhans, about a million clusters, each about 0.2 mm across, scattered among the exocrine acini. They make up only 1–2% of the pancreas but have a rich supply of fenestrated capillaries. Four main cell types occur:

• β cells, the most numerous, secrete insulin (with amylin);
• α cells (about 20–25%) secrete glucagon;
• δ cells (under 10%) secrete somatostatin;
• PP (F) cells (under 5%) secrete pancreatic polypeptide.

Functions:

• Insulin lowers blood glucose: it increases glucose uptake by muscle and fat cells through GLUT4, glycogen synthesis, lipogenesis and protein synthesis, and inhibits gluconeogenesis.
• Glucagon raises blood glucose through hepatic glycogenolysis and gluconeogenesis and promotes ketogenesis in fasting.
• Somatostatin inhibits the release of insulin and glucagon and slows nutrient absorption.
• Pancreatic polypeptide inhibits pancreatic enzyme secretion and gall-bladder contraction.

Insulin deficiency or resistance causes diabetes mellitus.

(d) Cnidoblasts as defensive organs in coelenterates

Cnidoblasts (cnidocytes) are stinging cells found only in Cnidaria, most numerous on the tentacles. Each holds a nematocyst, a capsule closed by an operculum and enclosing a coiled hollow thread, with a trigger (cnidocil) outside.

Justification:

• Touch or chemicals from a predator trigger discharge: water enters the capsule by osmosis, the pressure rises, the operculum opens and the thread everts explosively within microseconds.
• Penetrants (stenoteles) pierce the attacker and inject a paralysing toxin; volvents (desmonemes) entangle it; glutinants (isorhizas) stick to it.
• Hydra, sea anemones and jellyfishes are soft-bodied and slow or sessile, so nematocysts are their main protection. The stings of Physalia and the sea wasp (Chironex) are painful or dangerous even to humans.
• A nematocyst fires only once, but is replaced from interstitial cells (as in Hydra).

Nematocysts also capture prey, so defence is one of their functions.

(e) Evolutionary status of Sphenodon

Sphenodon punctatus (tuatara) is the only living member of the reptilian order Rhynchocephalia, confined to about 30 small offshore islands of New Zealand. Rhynchocephalians appear in the Middle Triassic, about 240 million years ago, and were diverse in the Mesozoic. Hence Sphenodon is called a "living fossil", though the living species itself has not remained unchanged. Its nearest living relatives are the Squamata (lizards and snakes); together they form the Lepidosauria.

It shows a mosaic of characters.

• Primitive: diapsid skull with both temporal arches complete (the lower arch is lost in squamates); amphicoelous vertebrae; gastralia and uncinate processes on the ribs; a parietal (pineal) eye with lens and retina.
• Specialised or lost: acrodont teeth fused to the jaw, and no copulatory organ or eardrum.

Sphenodon thus gives a baseline for the early evolution of lepidosaurs.