৪৯তম বিসিএস ⎯ প্রাণিবিদ্যা [৫৯১]

​​​​​​​​​​​​​​They are bilaterally symmetrical, triploblastic, metamerically segmented animals.
Body organization is of an organ-system level.
The body is covered with a thick chitinous cuticle forming an exoskeleton.
Body segments usually bear lateral and jointed appendages with varied functions as jaws, gills, legs, etc.
Body divisible into head, thorax, and abdomen. Head and thorax often fused to form a cephalothorax.
The musculature is not continuous but comprises separate striped muscles capable of rapid contraction.
The body cavity is hemocoel. The true coelom is reduced to the spaces of the genital and excretory organs.
The complete digestive system with mouth and anus. Mouthparts adapted for various modes of feeding.
Open circulatory system with dorsal heart and arteries but without capillaries.​​​

Urochordata - Sea squirt.
Cephalochordata - Lanceolates.
Chondrichthyes - Scoliodon laticaudus. 

​​​​​​Pila  globosa - Asymmetrical. 
Metaphire posthuma - bilaterally symmetrical.
Hirudinaria manillensis ​- bilaterally symmetrical.

Musca domestica - Arthropoda.
Apis indica - Arthropoda.
Macrobrachium  rosenbergii - Arthropoda.
Homo sapiens - Chordata.

Annelids, or segmented worms, utilize various methods for respiration, primarily through their skin (cutaneous respiration) or gills (branchial respiration), depending on the species and habitat. They lack specialized respiratory organs like lungs.

​Retrogressive metamorphosis in Urochordata, also known as ascidians, is a process where a free-swimming, tadpole-like larva with advanced chordate features (notochord, dorsal nerve cord, gill slits) transforms into a sedentary, often simpler, adult form. This metamorphosis is called "retrogressive" because the adult loses or degenerates many of the larval features that were present during the active, mobile stage. ​​​

Mammalian characteristics:
• Mammary Glands: These glands produce milk to nourish their young. 
• Hair or Fur: A covering of hair or fur provides insulation and protection. 
• Four-Chambered Heart: This ensures efficient separation of oxygenated and deoxygenated blood.
• Warm-Blooded (Endothermic): Mammals can regulate their own body temperature, maintaining it at a relatively constant level.
• Diaphragm: A muscular sheet that aids in breathing by expanding and contracting the chest cavity.
• Specialized Teeth: Different types of teeth (incisors, canines, premolars, molars) allow for diverse diets.
• Three Middle Ear Bones: These bones (malleus, incus, stapes) transmit sound vibrations from the eardrum to the inner ear. ​​​​​​​​​​​​​​​​​​

The horseshoe crab, specifically Limulus, is a living fossil within the phylum Arthropoda. It's a marine arthropod that has remained relatively unchanged for millions of years, with some estimates suggesting it has persisted for 190 million years with minimal evolutionary change. 

Poekilocerus pictus is a grasshopper.
Solependra latea is from classv diplopoda.

​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​Pedicellariae are small, pincer-like structures found on the surface of some echinoderms, like sea stars and sea urchins act as exoskeleton. They are thought to play a role in keeping the echinoderm's body surface clean of debris and can also be involved in defense against predators and, in some cases, even prey capture. These structures are composed of specialized spines (valves) that articulate on a baseplate, and they can move independently, exhibiting reflex responses to stimuli. ​​​​​

​Source: Britannica

Scorpions utilize book lungs for respiration. These are specialized respiratory organs located on the underside of the scorpion's abdomen, consisting of stacked, plate-like structures resembling the pages of a book.​​​
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​Source: Britannica

The endostyle, a structure found in certain chordates, is considered homologous to the vertebrate thyroid gland. This means that both structures share a common evolutionary origin, despite potentially having different functions in different organisms.​​​​

​​​​​Holothuria tubulosa, the cotton-spinner or tubular sea cucumber, is a species of sea cucumber in the family Holothuriidae. It is the type species of the genus Holothuria and is placed in the subgenus Holothuria, making its full name Holothuria (Holothuria) tubulosa and its phylum is Echinodermata​​​​.
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​Source: Britannica​

Parapodia and setae are structures found in certain types of segmented worms (annelids), particularly in the class Polychaeta. Parapodia are paired, unjointed, lobe-like appendages that extend from the body segments, while setae are bristle-like structures located on or near the parapodia, or directly on the body wall. They are primarily used for locomotion, but also play roles in respiration and sensing. ​​​​​​​​

In the context of Cephalochordata, "lanceolate" refers to the body shape of lancelets, which are small, fish-like marine animals also known as amphioxus. Their body is elongated and laterally flattened, resembling a lancet or needle, hence the name "lanceolate" or "lancelet". This body shape is a key characteristic of the subphylum Cephalochordata​​​.

Monotremes are mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young.​​​

​​​​​​​Hemerythrin is primarily found in certain marine invertebrates, specifically in the body fluids and tissues of some sipuculids (peanut worms), brachiopods, and annelids (segmented worms). It also appears in some bacteria and archaea. In these organisms, hemerythrin functions as an oxygen-binding protein, particularly as an oxygen storage reservoir.​
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​Source: Britannica

Incomplete metamorphosis, also known as hemimetabolism or partial metamorphosis, is a type of insect development that involves three stages: egg, nymph, and adult. Unlike complete metamorphosis, it does not include a pupal stage. The nymph, which hatches from the egg, gradually develops into the adult form through a series of molts. ​​​​
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​Source: Britannica​

The green gland in crustaceans is a specialized excretory organ, also known as the antennal gland or maxillary gland, that functions similarly to a vertebrate kidney. These paired glands are located in the head region of crustaceans and play a crucial role in maintaining homeostasis by regulating water and ion balance, as well as excreting nitrogenous waste, primarily ammonia. They also contribute to chemical signaling and, surprisingly, can serve as an entry point for viruses and bacteria. 

https://www.britannica.com/animal/echinoderm/Distribution-and-abundance
Echinoderms are exclusively marine animals, found in all oceans, from shallow waters to the deep sea. They inhabit a wide range of marine environments, including coral reefs, sandy and muddy bottoms, and rocky substrates. Some echinoderms, like certain sea cucumbers, can even be found at depths exceeding 5,000 meters. ​​​

https://www.britannica.com/animal/cephalochordate/External-features​

Oral cirri in cephalochordates are hair-like projections around the mouth that act as sensory devices and filters, helping to screen out large particles from entering the mouth while drawing in water for filter feeding. 

Ectothermic vertebrates, often called "cold-blooded" animals, rely on external sources to regulate their body temperature. This group includes fish, amphibians, and reptiles. Unlike endothermic animals (birds and mammals), ectotherms don't generate their own heat and their body temperature fluctuates with the environment. 

https://www.britannica.com/animal/mollusk
A U-shaped alimentary canal is characteristic of some members of the phylum Hemichordata. Specifically, certain hemichordates, like Balanoglossus Saccoglossus​​​​​​​​

Ecdysis, in biology, refers to the periodic shedding of an outer layer (cuticle or skin) by arthropods and reptiles. This process is essential for growth and development, as the outer layer is typically rigid and cannot grow with the animal. It's commonly known as molting. 

Placoid scales are a type of scale found on cartilaginous fish, such as sharks and rays. They are also known as dermal denticles and are characterized by their tooth-like structure. Unlike the scales of bony fish, placoid scales do not grow in size as the fish matures; instead, new scales are added between existing ones. 

https://www.britannica.com/animal/cnidarian
An Obelia colony is a marine hydroid colony characterized by a branching structure composed of polyps (zooids) attached to a root-like base called the hydrorhiza. These colonies are typically found in shallow, coastal waters, attached to submerged rocks or seaweed. The colony's structure is made up of a network of interconnected tubes, with vertical branches (hydrocauli) arising from the hydrorhiza. These branches bear different types of zooids, including gastrozooids for feeding and gonozooids for asexual reproduction. ​​​​

https://www.britannica.com/animal/stony-coral
Scleractinia, also called stony corals or hard corals, are marine animals in the phylum Cnidaria that build themselves a hard skeleton. The individual animals are known as polyps and have a cylindrical body crowned by an oral disc in which a mouth is fringed with tentacles and they are from anthozoa .​​​​

https://www.britannica.com/animal/platypus
he platypus (Ornithorhynchus anatinus), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal​​​​

https://www.britannica.com/science/radula
The radula is a unique anatomical structure found in most molluscs, acting as a "rasping tongue" for feeding. It's a ribbon-like membrane covered in rows of tiny, chitinous teeth ​​​

https://www.merriam-webster.com/dictionary/asymmetrical
In biology, asymmetrical symmetry refers to the absence of symmetry, where an organism's body cannot be divided into equal halves along any plane. This means there's no mirror image or balanced arrangement of body parts around a central point. Sponges are a classic example of asymmetry, lacking true tissues and organs, and their bodies don't have a defined midline.
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