Zoology Terms Starting With R

Radial Symmetry is a body plan in which multiple planes passing through a central axis divide the body into similar halves or repeated sectors.

Radial symmetry occurs in sessile, drifting, or slow-moving animals that interact with the environment from many directions rather than mainly from a head-first axis. Cnidarians such as jellyfish, corals, hydras, and sea anemones show radial or biradial organization, with tentacles and sensory structures arranged around the mouth. Adult echinoderms display pentaradial symmetry, usually organized into five sectors, even though their larvae are bilaterally symmetrical and their ancestors were bilaterians.

This body plan suits feeding, sensing, and defense around a central point, but it is less suited to fast directional locomotion than bilateral symmetry.

Reptile is a scaly amniote vertebrate traditionally grouped with lizards, snakes, turtles, crocodilians, and the tuatara; in modern cladistics, birds are nested within the broader reptile lineage, making the traditional grouping paraphyletic when birds are excluded.

Modern phylogenetics shows that crocodilians are more closely related to birds than to lizards, a relationship supported by molecular clock estimates placing the crocodilian-bird divergence at roughly 240 million years ago. Reptiles and their relatives dominated terrestrial ecosystems during the Mesozoic, with dinosaurs, pterosaurs, and marine reptiles occupying nearly every major ecological niche. Reptile scales are keratinous epidermal structures, distinct from fish scales, which incorporate dermal bone; most living non-bird reptiles are ectothermic and regulate body temperature through behavioral means such as basking.

Rostrum is an elongated anterior projection of the head, snout, or mouthparts in multiple animals, used for piercing, probing, feeding, or defense depending on the lineage.

In plant-feeding true bugs (order Hemiptera), the rostrum is a tubular piercing structure containing stylets that penetrate plant or animal tissues to extract sap or blood. The rostrum of crustaceans such as crayfish and shrimp is a rigid anterior projection of the carapace positioned between the eyes, used in defense and species recognition. Among vertebrates, the elongated upper jaw of the sawfish (family Pristidae) bears tooth-like denticles along its margins and can reach more than 1.8 meters in length, functioning both to detect prey through electroreceptors and to slash through schools of fish.

Ruminant is a hoofed mammal that digests plant material through a specialized multi-chambered stomach, including a rumen where microbial fermentation breaks down cellulose before further digestion.

The ruminant stomach has four compartments: rumen, reticulum, omasum, and abomasum. Inside the rumen, dense microbial communities of bacteria, archaea, protozoa, and fungi ferment cellulose into volatile fatty acids, which the animal absorbs directly through the rumen wall as its primary energy source. Rumination involves regurgitating rumen contents as a bolus for further mechanical breakdown by chewing cud, increasing microbial access to plant cell wall polysaccharides and improving fermentation efficiency.