Write short notes on each of the following in about 150 words: (a) Structure of visual organs of cockroach. (10 marks) (b) What…

(a) Structure of visual organs of cockroach

The visual apparatus of the cockroach (Periplaneta americana) comprises a pair of sessile, kidney-shaped compound eyes positioned dorsolaterally on the head capsule, complemented by a pair of simple eyes or ocellar spots (fenestrae) at the base of the antennae. Each compound eye is an aggregate of approximately 2,000 visual units termed ommatidia. An individual ommatidium consists of an outer dioptric (optical) apparatus and an inner receptive apparatus. The optical region includes a biconvex cuticular lens (cornea) secreted by corneagen cells, and a transparent crystalline cone enveloped by four vitrellae (cone cells) that focus light. The sensory unit beneath comprises seven to eight elongated retinular cells encircling a centrally located, light-sensitive rhabdome, which directly continues into nerve fibres entering the optic ganglion. Surrounding pigment sheaths optically insulate adjacent ommatidia. This structural configuration facilitates mosaic vision via the apposition image mechanism, granting high temporal resolution suited for motion detection in low light. The ocelli function non-visually as simple photoreceptors detecting illumination changes.

(b) What is meant by symmetry? Write about different types of symmetry in animals

Symmetry in animal biology refers to the balanced distribution of duplicate body parts across central axes or reference planes, determining the basic structural body plan.

Asymmetry denotes the absence of any plane of symmetry, characteristic of most Porifera (sponges) and adult gastropods, reflecting a sessile or irregular growth habit.

Radial symmetry occurs when any longitudinal plane passing through the central oral-aboral axis divides the animal into identical halves. It is characteristic of primary radiata like Cnidaria (Hydra, jellyfish) and secondary pentamerous forms like adult Echinodermata, enabling unhindered environmental interaction from all directions.

Biradial symmetry represents a specialized condition where radial arrangement is modified by paired structures, permitting division into equivalent halves through only two vertical planes, as seen in Ctenophora (comb jellies) and sea anemones.

Bilateral symmetry allows body division into mirrored left and right halves through a single median sagittal plane. Found across triploblastic Bilateria, it is evolutionarily linked with cephalization, active directional locomotion, and sophisticated central nervous system organization.

(c) Structure and functions of endocrine pancreas

The endocrine pancreas consists of the Islets of Langerhans, which are micro-endocrine aggregates interspersed throughout the exocrine acinar tissue, contributing approximately one to two percent of the total pancreatic mass. Histologically, each islet comprises four distinct, richly vascularized cell types: alpha (α) cells located predominantly at the periphery, beta (β) cells forming the predominant core (60–70%), delta (δ) cells, and F or pancreatic polypeptide (PP) cells.

Functionally, the endocrine pancreas is the central regulator of carbohydrate, lipid, and protein metabolism via antagonistic hormonal actions. Beta cells secrete insulin, a hypoglycemic hormone that promotes cellular glucose uptake via GLUT-4 translocation, accelerates glycogenesis, and stimulates lipogenesis and protein synthesis. Conversely, alpha cells produce glucagon, which raises blood glucose by stimulating hepatic glycogenolysis and gluconeogenesis during fasting states. Delta cells produce somatostatin, exerting a paracrine inhibitory effect on both insulin and glucagon secretion. Insufficient insulin production or peripheral receptor resistance causes chronic metabolic derangement manifested as diabetes mellitus.

(d) "Cnidoblasts as defensive organs in coelenterates". Justify

Cnidoblasts (cnidocytes) are highly specialized, single-use stinging cells unique to the phylum Cnidaria, serving as a defining synapomorphy vital for organismal survival. Structurally, each cnidoblast houses a fluid-filled collagenous capsule (nematocyst) containing a coiled, invaginated tubular thread, an opercular lid, and a rigid sensory trigger called the cnidocil.

Upon mechanical or chemical stimulation by potential predators, sensory signals trigger an explosive discharge driven by rapid alterations in capsule wall permeability. Under the osmotic hypothesis, a swift influx of water into the hypertonic capsule generates massive hydrostatic pressure (up to 150 atmospheres), causing the operculum to open and violently everting the coiled filament within microseconds. Specialized defensive nematocysts, such as penetrants (stenoteles), inject lethal neurotoxins (hypnotoxin) that paralyze or deter attacking organisms, while glutinants produce sticky threads to entangle adversaries. In soft-bodied, slow-moving polyps and pelagic medusae lacking protective exoskeletons, cnidoblasts concentrated on tentacles form an effective chemical and physical barrier, fully justifying their role as primary defensive organs.

(e) Evolutionary status of Sphenodon

Sphenodon (tuatara), restricted to offshore islands of New Zealand, is the sole surviving genus of the ancient Mesozoic reptilian order Rhynchocephalia (Sphenodontida). Morphologically conservative, it is classified as a classic 'living fossil' that has persisted virtually unchanged for over 200 million years since the Triassic period.

Phylogenetically, Sphenodon occupies a critical position as the sister group to Squamata (lizards and snakes) within the clade Lepidosauromorpha. It preserves numerous primitive diapsid characteristics, including a complete diapsid skull with unreduced upper and lower temporal arcades, amphicoelous (biconcave) vertebrae with persistent intercentra, abdominal ribs (gastralia), and uncinate processes on ribs. It uniquely retains a functional parietal (pineal) third eye equipped with a lens and retina, while lacking a copulatory organ and external auditory meatus. Alongside these ancestral traits, it exhibits derived specializations like acrodont dentition. Consequently, Sphenodon provides an indispensable morphological baseline for reconstructing primitive amniote anatomy and understanding the evolutionary divergence of early diapsids.