Samacheer Class 11 Bio Zoology - Kingdom Animalia

Cartilaginous fishes (Chondrichthyes) and living jawless fishes (Cyclostomata) exhibit several distinct characteristic features. Cyclostomes, which include lampreys and hagfishes, are often ectoparasites on other fishes or scavengers, whereas chondrichthyes, such as sharks and rays, are typically free-living predators. The body of cyclostomes is slender and eel-like, lacking scales, while chondrichthyes have tough skin covered with minute, placoid scales. Respiration in cyclostomes occurs through 6 to 15 pairs of gill slits, whereas cartilaginous fishes respire using 5 to 7 pairs of lamelliform gills. A major distinguishing feature is the mouth: cyclostomes possess a circular, suctorial mouth without jaws, while chondrichthyes have well-developed upper and lower jaws. Regarding reproduction, cyclostomes often migrate to freshwater for spawning, after which they typically die. In contrast, cartilaginous fishes do not generally undertake such migrations for breeding and do not die after spawning; their reproductive strategies can be oviparous, viviparous, or ovoviviparous, offering a greater diversity compared to the primarily oviparous cyclostomes.

• These fishes have a bony endoskeleton.
• The skin is covered by ganoid, cycloid or ctenoid scales.
• Gills are covered by an operculum.
• They are ammonotelic.
• They have mesonephric kidneys.
• External fertilization is seen.

• The characteristics that contribute to the success of reptiles on land are as follows:
• The presence of dry and cornified skin with epidermal scales or scutes prevents the loss of water.
• The presence of metanephric kidney.
• They are uricotelic (they excrete uric acid to prevent the loss of water).

No, the number of eggs or young ones produced by an oviparous and viviparous female cannot be equal, and there are fundamental biological reasons for this difference. Oviparous animals, which lay eggs that develop externally, face significant challenges to the survival of their offspring. When eggs are laid in the external environment, whether on land or in water, they are highly vulnerable to predation, environmental fluctuations like temperature changes or desiccation, and lack of direct parental protection. To compensate for these high mortality rates and ensure that at least some offspring survive to adulthood, oviparous animals typically produce a very large number of eggs. In contrast, viviparous animals give birth to live young that develop internally within the mother's body. This internal development provides a protected, stable environment and often involves direct maternal nourishment, significantly increasing the chances of survival for each individual offspring. Consequently, viviparous animals invest more energy and resources into fewer, larger young, as the survival rate per offspring is much higher due to prolonged parental care and protection from external threats.

All vertebrates, at some point in their development, exhibit a set of characteristic features that define them as chordates and then as vertebrates. These include the presence of a notochord, a dorsal hollow nerve cord, pharyngeal gill slits, and a post-anal tail. In most adult vertebrates, the notochord is replaced by a vertebral column, which forms the main axial skeleton. All vertebrates also possess paired appendages, such as fins in aquatic forms or limbs in terrestrial forms, which are crucial for locomotion. Their skin is typically covered by a protective outer skeleton, which can manifest as scales in fishes and reptiles, feathers in birds, or hair, claws, and nails in mammals. Respiration in vertebrates is aerobic and occurs through various specialized organs, including gills in aquatic species, skin, buccopharyngeal cavity, and highly developed lungs in terrestrial forms. Furthermore, all vertebrates possess a muscular heart, which can have two, three, or four chambers, facilitating efficient circulation. They also have kidneys, which are vital organs for excretion of metabolic wastes and osmoregulation, maintaining the body's internal fluid balance.

Circulatory systems in animals are broadly categorized into two main types: open and closed, differing primarily in how blood or hemolymph is contained and distributed throughout the body. In an open circulatory system, blood, often referred to as hemolymph, is not always confined within blood vessels. Instead, it is pumped by a heart into open spaces or sinuses, called hemocoel, where it directly bathes the tissues and organs. There are no distinct capillaries connecting arteries to veins, and the hemolymph eventually returns to the heart through ostia. This system typically operates at lower pressure and is less efficient for rapid transport of oxygen and nutrients. Examples of animals with open circulation include arthropods like insects and crustaceans, most molluscs, and echinoderms. Conversely, a closed circulatory system ensures that blood is always contained within a network of vessels, including arteries, veins, and capillaries. The heart pumps blood through arteries to capillaries, where exchange of substances occurs with the surrounding tissues, and then blood returns to the heart via veins. This system allows for higher blood pressure and more efficient and rapid transport of oxygen, nutrients, and waste products. Animals such as earthworms, cephalochordates, and all vertebrates possess a closed circulatory system.

Schizocoelom and enterocoelom are two distinct embryonic mechanisms by which a true coelom, or body cavity, forms in triploblastic animals. The schizocoelomate condition is characteristic of protostomes, such as annelids and arthropods. In these animals, the coelom develops by the splitting of the mesoderm. During gastrulation, a solid mass of mesodermal cells forms between the ectoderm and endoderm. Subsequently, this mesodermal mass splits internally, creating a fluid-filled cavity that becomes the coelom. This process essentially carves out the body cavity from within the mesodermal tissue. In contrast, the enterocoelomate condition is typical of deuterostomes, including echinoderms and chordates. Here, the coelom originates as outpocketings or pouches from the archenteron, which is the primitive gut formed during gastrulation. These mesodermal pouches pinch off from the archenteron, expand, and eventually fuse to form the coelom. This method of coelom formation is considered a more derived evolutionary trait, distinguishing deuterostomes from protostomes based on their early embryonic development.

• Notochord, cephalization, dorsal nerve cord, and radial symmetry.
• Notochord, cephalization, and dorsal nerve cord are the characteristic features of chordates. The radial symmetry is not a characteristic feature of chordate. It is the feature of cnidarian and adult echinoderms. Hence it does not belong to the group.

Invertebrates and chordates represent two major groups of animals that differ fundamentally in their anatomical structure and evolutionary position. Invertebrates are the major group of animals that lack a notochord or vertebral column, which are characteristic features of more advanced animals. These are considered lower animals in the evolutionary hierarchy and comprise the vast majority of animal species on Earth, including insects, mollusks, crustaceans, and many other groups. They display tremendous diversity in body structure, size, and habitat adaptation. In contrast, chordates are the major group of animals that possess a notochord during at least some stage of their development, and in most chordates, this notochord is replaced by a vertebral column or backbone during development. Chordates are considered higher animals in the evolutionary hierarchy and include vertebrates such as fish, amphibians, reptiles, birds, and mammals. The presence of a notochord or vertebral column is a defining characteristic that distinguishes chordates from invertebrates and provides structural support and protection for the nervous system, allowing for greater complexity in body organization and function.

Choanocytes, commonly referred to as collar cells, are highly specialized and distinctive cells that form the inner lining of the spongocoel and the radial canals within sponges, belonging to the phylum Porifera. Each choanocyte is uniquely structured with a single flagellum encircled by a delicate, net-like collar composed of numerous microvilli. The primary function of these flagellated cells is to generate and maintain a constant unidirectional flow of water through the sponge's intricate canal system. The coordinated beating of millions of flagella creates a negative pressure that draws water in through the tiny pores (ostia) on the sponge's surface and expels it through the larger excurrent opening (osculum). This continuous water current is vital for the sponge's survival, as the collar of microvilli acts as a sophisticated filter. As water passes through, food particles like bacteria, plankton, and detritus are trapped by the microvilli, then engulfed by the choanocyte through phagocytosis and subsequently digested or passed to other cells. Beyond feeding, this water flow also facilitates essential physiological processes such as respiration, by bringing in dissolved oxygen and removing carbon dioxide, and excretion, by carrying away metabolic waste products. The presence and structure of choanocytes are defining characteristics of sponges, highlighting their unique filter-feeding lifestyle.

An incomplete digestive system is characterized by having a single opening that serves as both the mouth for ingestion and the anus for egestion. This means that food and waste products pass through the same opening, leading to a less efficient digestive process as digestion and egestion cannot occur simultaneously. Organisms with an incomplete digestive system typically have a sac-like body plan. A classic example of animals exhibiting an incomplete digestive system includes members of the phylum Platyhelminthes, such as flatworms. In contrast, a complete digestive system possesses two separate and distinct openings: a mouth for the intake of food and an anus for the expulsion of undigested waste. This allows for a unidirectional flow of food, enabling specialized regions along the digestive tract to perform different functions like mechanical digestion, chemical digestion, absorption, and elimination sequentially. This separation of functions makes the complete digestive system far more efficient, allowing for continuous feeding and more thorough nutrient extraction. Most advanced animal phyla, including Chordates, Annelids, Arthropods, and Molluscs, possess a complete digestive system.

Asymmetrical animals are those organisms that lack any definite body plan or symmetry, meaning that no plane passing through the center of their body can divide them into two equal or identical halves. This absence of symmetry is often associated with a sessile or very slow-moving lifestyle, where there is no particular advantage to having a streamlined body or a distinct anterior end for directed movement. Their body organization is typically very simple, without well-defined tissues or organs arranged in a symmetrical pattern. The cells are often loosely aggregated, and their forms can be highly irregular and variable. Sponges, belonging to the phylum Porifera, are the most prominent example of asymmetrical animals. Their irregular shapes are adapted to their filter-feeding lifestyle, where water currents bring food particles to them from various directions, making a symmetrical body plan unnecessary for survival.

Bilateral symmetry offers significant evolutionary advantages to animals, primarily facilitating more efficient and directed movement. Animals with bilateral symmetry possess distinct anterior and posterior ends, as well as dorsal and ventral surfaces, along with identifiable left and right sides. This body plan allows for the development of cephalization, where sensory organs and nervous tissue, including a brain, are concentrated at the anterior end. This concentration of senses enables the animal to effectively perceive and process information from its environment in the direction of movement, which is crucial for actively seeking food, locating mates, and escaping from predators. The streamlined body shape associated with bilateral symmetry reduces drag, making locomotion more efficient. Furthermore, the development of specialized appendages on either side of the body allows for coordinated and powerful movements, contributing to the overall success and diversity of bilaterally symmetrical animals in various habitats.

Pseudocoelomates are animals that possess a body cavity, known as a pseudocoel, which is not fully lined by the mesodermal epithelium. In these organisms, the mesoderm lines the body wall externally, but it does not completely surround the digestive tract or other internal organs. Instead, the digestive tract is suspended within the pseudocoel, which is a fluid-filled space, but it is not covered by a mesodermal layer from all sides. This pseudocoelomic fluid plays several crucial roles: it acts as a hydrostatic skeleton, providing turgor and rigidity against which muscles can contract, thus aiding in movement. It also facilitates the free movement of visceral organs and assists in the circulation of nutrients and waste products throughout the body, as these animals typically lack a true circulatory system. Roundworms, belonging to the phylum Aschelminthes or Nematoda, are the most well-known examples of pseudocoelomates, along with rotifers and some other minor phyla.

Schizocoelomates and enterocoelomates represent two distinct developmental pathways for the formation of a true coelom, which is a body cavity completely lined by mesoderm. In schizocoelomates, the coelom develops from a splitting of the mesodermal tissue. During embryonic development, a solid mass of mesodermal cells forms near the blastopore. This mesodermal mass then splits internally, creating a cavity that expands to become the coelom. This mode of coelom formation is characteristic of protostomes, which include phyla such as Annelids, Arthropods, and Molluscs. In contrast, enterocoelomates form their coelom from mesodermal pouches that bud off from the archenteron, which is the primitive gut. During gastrulation, the archenteron develops outpocketings that pinch off to form the mesoderm, and the cavities within these pouches fuse to form the coelom. This developmental pattern is typical of deuterostomes, encompassing phyla like Echinoderms, Hemichordates, and Chordates. These differences in coelom formation reflect fundamental evolutionary divergences between major animal groups.

Animals exhibit different patterns of organisation:
The cellular level of organisation:
* Cells are loosely arranged without the formation of tissues.
* There is a division of labour among the cells, e.g., sponges.
Tissue level of organisation:
* Cells which perform a similar function are grouped into tissues.
* The tissues perform a common function, e.g., cnidarians.
Organ level of organisation:
Different kinds of tissues aggregate to form an organ to perform a specific function e.g., flatworms and other hyper phyla.
Organ system level of organisation:
* The tissues are organized to form organs and organ systems.
* All the organ systems function in a coordinated manner.

The cavity between the body wall and the gut wall is called coelom. If the animals do not have coelom, they are called acoelomates. e.g., flatworms. In some animals, the body cavity is not fully lined by the mesodermal epithelium. The mesoderm is formed as scattered pouches between the ectoderm and endoderm. Such a body cavity is called a pseudocoel. The animals which have pseudocoel e.g. roundworms.
If the coelom develops within the mesoderm and is lined by mesodermal epithelium it is called eucoelom. The animals which have true coelom are called eucoelomates. If the body cavity is formed by splitting of mesoderm, the animals are called schizocoelomates e.g., Annelids, arthropods and mollusks. If the body cavity is formed from the mesodermal ‘ pouches of archenteron, the animals are called enterocoelomate animals, e.g., echinoderms, hemichordates and chordates.
(5 Marks)
IV. Essay Questions

• The animals are marine, diploblastic and radially symmetrical.
• They have eight external rows of ciliated comb plates which help in locomotion.
• Bioluminescence is seen.
• They lack nematocysts but have lasso cells which help in food capture.
• Digestion is by both extracellular and intracellular.
• Sexual reproduction is seen.
• Fertilization is external and development is indirect.
• Cydippid larva is seen, e.g., Pleurobrachia.

Platyhelminthes and Aschelminthes are two distinct phyla within the animal kingdom, exhibiting several key differences. Platyhelminthes, commonly known as flatworms, have a dorso-ventrally flattened body, while Aschelminthes, or roundworms, are round in cross-section. Both are bilaterally symmetrical and triploblastic. However, Platyhelminthes are acoelomates, meaning they lack a true body cavity, whereas Aschelminthes possess a pseudocoelom, which is a body cavity not fully lined by mesoderm. Platyhelminthes exhibit pseudo-segmentation, while Aschelminthes have an unsegmented body. Excretion in Platyhelminthes occurs through flame cells, which are specialized protonephridia, while in Aschelminthes, excretion is carried out by rennet glands. Platyhelminthes are typically monoecious, meaning sexes are not separate, and some species show remarkable regeneration capacity. In contrast, Aschelminthes are dioecious, with separate sexes and often exhibiting sexual dimorphism, and they generally lack regeneration capacity. Larval stages are present in Platyhelminthes, but typically absent in Aschelminthes. Examples of Platyhelminthes include tapeworms and planarians, while Ascaris is a common example of Aschelminthes.

• Molluscs are terrestrial or aquatic with organ system level of organisation.
• They are triploblastic, bilaterally symmetrical, coelomate animals.
• Body is divided into head, foot and visceral hump.
• The digestive system is complete.
• Nephridia are the excretory organs.
• Open type of circulatory system is seen.
• Blood contains a copper-containing respiratory pigment called hemocyanin.
• They are oviparous.
• Development is indirect with a veliger larva, e.g., Pila and Octopus.

• The Hemichordates have both invertebrate and vertebrate characters.
• They are worm-like, tuberculous animals.
• They are bilaterally symmetrical, triploblastic coelomate animals with organ system level of organisation.
• The body is divided into proboscis, collar, and trunk.
• They are ciliary feeders.
• The circulatory system is simple and open.
• Excretion is by a single proboscis gland or glomerulus situated in the proboscis.
• Sexes are separate.
• Fertilization is external.
• Development is indirect with tomaria larva, e.g., Balanoglossus.

• They are marine found in shallow waters.
• They lead a burrowing mode of life.
• They are fish-like with notochord, nerve cord and pharyngeal gill slits throughout their life.
• Closed type of circulatory system is seen without heart.
• Excretion is by protonephridia.
• Sexes are separate.
• Fertilization is external.
• Development is indirect with a larva e.g. Amphioxus.

• The body is covered by hairs.
• They are found in a variety of habitats.
• The presence of the mammary gland is the most unique feature of mammals.
• They have two pairs of limbs.
• The skin consists of sweat glands and sebaceous glands.
• Exo skeleton includes horns spines, scales claws, etc.
• Teeth are thecodont heterodont and diphyodont.
• The heart is four-chambered and posses a left systematic arch.
• Mammals have a large brain when compared to other animals.
• Their kidneys are metanephric and are ureotelic.
• All are homeothermic.

All chordates possess three distinct features that are visible at some stage of their life cycle. The first feature is the presence of a notochord, which is a flexible rod-like structure located below the nerve cord and above the alimentary canal, providing structural support to the body. The second feature is the dorsal nerve cord, which lies above the notochord and below the dorsal body wall, serving as the main nervous system component. The third feature is the presence of pharyngeal gill slits, which are openings in the pharyngeal region that appear in all chordates at some point during their development. However, the fate of these structures changes dramatically in mature adult chordates. The notochord, which is prominent in larval stages, may be partially or completely replaced by the vertebral column or backbone in adult chordates, providing a more rigid skeletal support system. The dorsal nerve cord undergoes significant development and enlargement to form the brain and spinal cord in mature adults, becoming the central nervous system. In contrast, the pharyngeal gill slits are typically retained only in aquatic chordates like fish; in terrestrial chordates such as mammals, birds, and reptiles, these gill slits are either lost or modified into other structures during development, such as the Eustachian tube and middle ear components in mammals.

Chordates and non-chordates represent two major divisions within the animal kingdom, distinguished by several fundamental anatomical features. The most defining characteristic of chordates is the presence of a notochord, a flexible rod-like structure, at some stage in their life cycle, which is completely absent in non-chordates. Chordates possess a dorsal, hollow, and single nerve cord, whereas non-chordates typically have a double, ventral, and solid nerve cord. Another key feature of chordates is the presence of pharyngeal gill slits, which are perforations in the pharynx, at least during their embryonic development; these gill slits are absent in non-chordates. The heart in chordates is typically placed ventrally, while in non-chordates, if present, the heart is usually located dorsally or laterally, or may even be absent. Chordates also possess a post-anal tail, a posterior extension of the body beyond the anus, which is absent in non-chordates. Furthermore, the alimentary canal in chordates is positioned ventral to the nerve cord, whereas in non-chordates, the alimentary canal is generally placed dorsal to the nerve cord.