Exam 2

Question 1

List three common characteristics of animals that are not found in other multicellular eukaryotes

Answer 1

1. They are multicellular heterotrophs that ingest their food
2. Animals are mobile and lack a cell wall
3. Many animals have the diploid life cycle, and usually reproduce sexually. Many reproduce sexually and some asexually and some combine both life cycles.
4. Muscle and nerve tissues characterize animals
5. Animals are monophyletic-meaning both invertebrates (no backbone) and vertebrates (spinal cord, backbone) can trace their ancestry to the same ancestor.

Question 2

Summarize the colonial flagellate hypothesis as its relates to the origin of animals

Answer 2

1. It states that animals are descended from an ancestor that resembled a hollow, spherical colony of flagellated cells.
2. The choanoflagellate (collared flagellates) most likely resemble the last unicellular of living animals, and molecular data illustrates that they are the closest living relatives of animals.
3. A choanoflagellate is a single cell with a flagellum.
4. The transition from colonial flagellates to multicellular animals may have begun by aggregation of a few flagellated cells.
5. Individuals cells within the colony may have specialized.
6. Two tissue layers may have arisen by infolding of certain cells into a hollow sphere.
7. This hypothesis is attractive because of its implications regarding animal symmetry.

Question 3

Distinguish among the different body plans of animals

Answer 3

There are three types of symmetry (a pattern of similarity that is observed in objects) in the animal kingdom.

1. Asymmetry: no particular body shape (e.g., sponge)
2. Radial symmetry: two identical halves. describes body parts arranged around an axis, like spokes of a wheel (e.g., starfish). RS animals may be sessile (attached to a substrate or less motile) This symmetry allows the animal to reach out in all directions from one center.
3. Bilateral symmetry: definite right and left halves, or complementary halves. Only one longitudinal cut down the center produces mirror halves. Animals tend to be active and to move forward at an anterior end. The localization of the brain and sensory organs at the anterior end is called cephalization.
4. The first tissue layers that appear are called germ layers. These give rise to organs and organ systems. Diploblastic – Two tissue layers (ectoderm, endoderm), Triploblastic – Three tissue layers (ectoderm, mesoderm, endoderm)
5. Animals have 3 tissue layers that could be protostome or deuterostome
6. Cleavage: cell division without cell growth first development event after fertilization.
7. Blastula: hollow sphere of cells with an indentation that forms an opening called a blastopore.
8. A coelom is a body cavity lines by mesoderm; if the mesoderm lines the cavity completely, it is a true coelom.

Question 4

Differentiate between protostome and deuterostome development

Answer 4

1. Protostomes – First embryonic opening becomes the mouth, The blastopore is associated with the mouth, Spiral cleavage, which cells divide without an increase in size, The fate of cells is fixed-each contributes to development in only one particular way. Coelom forms by a splitting of the mesoderm, which has arisen from cells near the blastopore.
2. Deuterostomes – Second embryonic opening becomes the mouth, The blastopore is associated with the anus, Radial cleavage where the new daughter cells sit atop the previous cells, the fate of these cells is indeterminate, Coelom forms by the fusion of mesodermal pouches from the primitive gut

Question 5

Explain why sponges are considered to be the simplest animals

Answer 5

Sponges have no symmetry, lack true tissues, and have only the cellular level of organization

Question 6

Discuss how a sponge respires, feeds, and reproduces

Answer 6

1. Saclike bodies are perforated by many pores. They have a canal system that allows water to move through their bodies.Beating collar cells (choanocytes) produce currents through pores in the wall into a central cavity and out through the osculum.
2. It is a sessile filter feeder. They stay in one place and filter food from the water. Collar cells engulf and digest food particles in food vacuoles.
3. Reproduction. Asexually by budding, which can produce large colonies. Fragmentation occurs when sponges are chopped up; each piece can start a complete sponge. They reproduce sexually when eggs and sperm are released into a central cavity; the zygote. develops into a flagellated larvae.

Question 7

Compare the anatomical features of comb jellies to those of cnidarians, such as hydras

Answer 7

Both have tentacles. Come jellies have long tentacles covered with sticky filaments. Hydras have 4 to 6 tentacles contacting nematocysts that surround the mouth. Both contain jellylike material called mesoglea

Question 8

List the basic features of lophotrochozoans.

Answer 8

1. They are bilaterally symmetrical in at least one stage of development.
2. As embryos, they have three germ layers.
3. As adults, they have the organ level of organization. . 4. They have a protostome type of development.
4. Some have true coelom.
5. The two groups of lophotrochozoans are lophophorans (bryozoans, brachiopods, phoronids) and trochozoans (flatworms, rotifers, molluscs, annelids).

Question 9

Describe the basic anatomy and physiology of a planarian

Answer 9

Planarians live in freshwater habitats. They have a head that is bluntly arrow shaped. The head has auricles that function as sense organs, and it has two light-sensitive eye spots. They have three kinds of muscle layers including an outer circular layer, an inner longitudinal layer, and a diagonal layer. Planarians have excretory organs that function in osmotic regulation and water secretion. They can reproduce sexually and asexually. They are hermaphroditic and practice cross-fertilization. They also reproduce asexually by regeneration. They have undergone cephalization. Their heads have lateral extensions with chemosensory and tactile cells. They have a ladder-type nervous system, and paired ganglia function as a primitive brain.

Question 10

Summarize the steps in the life cycles of Schistosoma and Taenia.

Answer 10

The Schistosoma life cycle starts with the larvae penetrating the skin of a human, the primary host, and mature in the liver. Adult worms live and copulate in blood vessels of the human gut. Eggs migrate into the digestive tract and are passed in feces. Ciliated larvae hatch in water and enter a snail, the secondary host. In the snail, a mother sporocyst encloses many developing daughter sporocysts. Daughter sporocysts enclose many developing larvae. This larvae break out of daughter sporocysts, escape the snail, and enter water.
The Taenia life cycle starts with the primary host ingesting meat containing bladder worms. The bladder worm attaches to human intestines where it matures into a tapeworm. As the tapeworm grows, proglottids mature, and eventually fill with the eggs. Eggs leave the primary host in feces, which may contaminate water or vegetation. Livestock may ingest the eggs, becoming a secondary host as each larvae becomes a bladder worm encysted in muscle. Rare or uncooked meat from secondary hosts contains many bladder worms.

Question 11

Identify morphological features of molluscs

Answer 11

Molluscs have a three part body plan. This includes the visceral mass, the mantle, and the foot. The visceral mass contains internal organs, the mantle may secrete shell and/or contribute to the development of lungs or gills, and the foot is a muscle adapted for locomotion, attachment, or food capture. Molluscs have a nervous system that consists of several ganglia connected by nerve cords. Their true coelom is reduced and is largely limited to the region around the heart. Their hearts pump hemolymph through vessels into hemocoel.

Question 12

Identify morphological features of bivalves

Answer 12

Bivalves have shells that have two hinged parts closed by powerful muscles. They don’t have a head or radula. They have little cephalization and an open circulatory system. Their ciliated gills hang down within the mantle cavity. Beating of the cilia causes water to enter the mantle cavity. Bivalves are filter feeders, meaning that they capture tiny food particles suspended in water. In freshwater clams, the sexes are separate, fertilization is internal, and specialized larvae are released from the clam.

Question 13

Identify morphological features of rotifers

Answer 13

Rotifers are trochozoans that are abundant in freshwater. Marine and terrestrial forms exist. have a crown cilia known as a corona that resembles a rotating wheel This causes a rotating motion. It serves as both an organ of locomotion and aids direction of food to mouth. They can desiccate during harsh conditions and remain dormant for lengthy periods of time.

Question 14

Identify morphological features of annelids

Answer 14

Annelids have segmented partitions, or septa, that divide the coelom which acts as a hydrostatic skeleton. Annelids have a specialized digestive tract, a closed circulatory system, and a ventral solid nerve cord. Most of them are marine. They have setae (bristles) that help in movement. Most annelids are marine polychaetes. Their setae are in bundles on parapodia. Clam worms are predators. They have sex organs only during the breeding season.

Question 15

Identify the characteristics unique to ecdysozoans.

Answer 15

The ecdysozoans are a group of protostomes and include roundworms and arthropods, which periodically shed their outer covering. Ecdysis, where the name ecdysozoan comes from, means “stripping off”. They construct an outer covering called a cuticle. It protects and supports the animals, and it is shed by molting. The ecdysozoans have evolved separate sexes.

Question 16

Compare the anatomical features of roundworms and arthropods.

Answer 16

Roundworms are nonsegmented, generally colorless worms. They contain a pseudocoelom, a false body that cavity that is incompletely lined by mesoderm. It provides a space for internal organs and can serve as a hydrostatic skeleton.
Arthropods have rigid, jointed exoskeleton composed of chitin. They must molt as they grow. They are segmented, but some segments are fused into regions. They have a head, thorax, and abdomen. They have a well-developed nervous system including a brain and ventral nerve cord and simple or compound eyes. They have a variety of respiratory organs including gills, book lungs, and tracheae.
Roundworms and arthropods have some type of skeleton, and both of them are either unsegmented or have segments that are fused together. They are ecdysozoans that periodically shed their outer covering

Question 17

Describe the five characteristics responsible for the success of the arthropods

Answer 17

1. Arthropods have a rigid exoskeleton with freely movable jointed appendages. The exoskeleton is a strong but flexible outer covering composed mainly of chitin, a strong nitrogenous polysaccharide. The exoskeleton serves for protection, attachment for muscles, locomotion, and prevention of desiccation.
2. Segmentation can be observed because each segment has a pair of jointed appendages, although some segments of arthropods have fused into regions (head, thorax, abdomen). Jointed appendages are hollow tubes moved by muscle. The appendages are modified for food gathering, reproduction, and locomotion. Some appendages associated with sensory structures are used for tactile purposes.
3. Arthropods have a well-developed nervous system. Their brain is connected to a ventral solid nerve cord. The head has various sensory organs. They have compound eyes that have many complete visual units, and each collects light independently. The lens of each individual unit focuses the image on the light-sensitive membrane of a few photoreceptors. In simple eyes, a single lens brings the image to focus into many receptors, each receiving a portion of the image. Many arthropods have well-developed touch, smell, taste, balance, and hearing. They display complex behaviors and methods of communication.
4. Arthropods use a variety of respiratory organs. Marine forms use gills with vascularized, thin-walled tissue specialized for gas exchange. Terrestrial forms have book lungs (spiders) or air tubes called tracheae (insects) for transporting oxygen directly to cells.
5. Metamorphosis is a drastic change in form and physiology that occurs as a larvae becomes an adult. It contributes to the success of arthropods. A larvae eats food and lives in environments different from the adult. This causes competition between the immature and adults of a species to be reduced. This reduction in competition allows more members of the species to exist at one time.

Question 18

List the two major groups of animals in the Deuterostomia.

Answer 18

Deuterostomes include invertebrate echinoderms and chordates

Question 19

Identify the major morphological structures of the sea star, an echinoderm.

Answer 19

Sea stars have structures outside of their body wall. Spines from exoskeletal plates offer protection. Pincer-like structures around the bases of spines keep the surface free of small particles. Skin gills are used for respiration. On the oral surface, each arm has a groove lined by tube feet.

Question 20

Describe how sea stars move, feed, and reproduce.

Answer 20

Sea stars use their tube feet are used to open clams, bivalves, and oysters. Once open, the sea star inverts its cardiac stomach and pushes through the crack, secreting enzymes. Digestion begins externally. It continues in the pyloric stomach using enzymes from the digestive gland found in each arm. Locomotion depends on a water vascular system. Water enters through the sieve plate and passes through a series of canals. Contraction of an ampulla forces water into the tube foot. Expansion and contraction of tube feet allow movement. Echinoderms have no respiratory, excretory, or circulatory system. Sea stars reproduce sexually and asexually. They reproduce asexually by fragmentation. Each fragment can regenerate a whole animal as long as a portion of the central disk is present. Spawning is the sexual reproduction method in echinoderms.

Question 21

Identify the four basic characteristics of a chordate

Answer 21

1. Notochord –Dorsal-supporting rod located below nerve cord-It provides support and is replaced by the vertebral column in vertebrates.
2. Dorsal Tubular Nerve Cord–In vertebrates, the spinal cord-Protected by vertebrae–contains a fluid-filled canal-Anterior portion becomes brain in most chordates
3. Pharyngeal Pouches–These openings function in feeding, gas exchange, or both-Aquatic vertebrates become gills–Seen only as embryos in most vertebrates–Terrestrial vertebrates modified for various purposes-In humans, the first pair of pouches becomes the auditory tubes.
4. Postanal Tail–Extends beyond the anus–In humans, tail disappears in the embryo stage.

Question 22

Name the two groups of nonvertebrate chordates

Answer 22

Lancelets (Cephalochordates)
Sea Squirts (Urochordates)

Question 23

Describe two features of each of the two groups of non vertebrate chordates

Answer 23

Lancelets (Cephalochordates)
–Shallow marine environments, filter feed partly buried in sandy substrates, they feed on microscopic particles filtered from a constant stream of water that enters the mouths and exits through gill slits into an atrium that opes at the atriopore, segmented muscles, dorsal hollow nerve cord has periodic branches, the notochord extends from head to tail

Sea Squirts (Urochordates)–tunicates--Adults contain a thick outer covering (tunic); an excurrent siphon squirts out water when it is disturbed.–Marine organisms, filter feeders, bilateral symmetry (larvae)– Larvae undergo metamorphosis to develop into sessile adults  Water passes into a pharynx and out numerous gill slits, the only characteristics that remain in adults. 
Many biologists hypothesize that these organisms are directly related to the vertebrates. Larva may have become sexually mature without developing other adult characteristics, and then evolved into a fishlike vertebrate

Question 24

Describe four characteristics that are unique to vertebrates

Answer 24

1. Vertebral column: Replaces the embryonic notochord, It is part of a flexible, strong endoskeleton that is also evidence of segmentation. remnants of the notochord are seen in the invertebrate disks.
2. Skull: An anterior component of the main axis of vertebrate endoskeleton; Encloses and protects brain, it encases the brain. High degree of cephalization, Complex sense organs, The ears-equilibrium devices in water-function as sound-wave receivers in land vertebrates.
3. Endoskeleton: Living tissue that grows with the animal, Protection for internal organs. Attachment for muscle: permit rapid and efficient movement. Paired appendages characteristic: evolved from pectoral and pelvic fins of fish.
4. Internal organs Large coelom and complete digestive tract, Closed circulatory system with blood contained within blood vessels, Respiratory system consisting of gills or lungs: provide proficient gas exchange, Kidneys–Serve as excretory and water: excrete nitrogenous waste and regulate water, regulating organs that regulate water balance in the body, sexes are separate and reproduction is sexual.

Question 25

Explain how the terms tetrapod, gnathostome, and amniote relate to vertebrate evolution csv

Answer 25

1. Amphibians are the first vertebrates to live on land and to have four limbs (tetrapods). Adaptions for land existence. (amphibians still have to reproduce in water) Evolved from bony fishes. 2. Jaws for predation. The earliest vertebrates were fishes; most of which have jaws. Jawed fish and other vertebrates are gnathostomes, animals with jaws. Fish also had a bony skeleton, lungs, and fleshy fins, which were predictive for a land existence. 3. Amniotes develop within an aquatic environment but of their own making. In placental mammals, the fertilized egg develops inside the female, where it is surrounded by an amniotic membrane. They produce amniotic egg that surrounds the embryo in amniotic fluid. It allows reproduction on land. another feature for living on land includes watertight skin, and can be seen in reptiles and mammals

Question 26

Identify the geologic era and periods in which chordates and the first vertebrates appear

Answer 26

Chordates (including vertebrates) appeared at the start of the Cambrian period. (Paleozoic era) End of Ordovician, jawless and jawed fishes Jawed fishes and all other invertebrates (end Silurian) Amphibians evolved from bony fishes by the Devonian

Question 27

List several features of jawless fishes

Answer 27

1. They have a cartilaginous skeleton and persistent notochord 2. They have smooth non-scaly skin 3. They have cylindrical bodies and are up to a meter long 4. Many lampreys are filter feeders similar to their ancestors. 5. Parasitic lampreys have a round muscular mouth equipped with teeth; they attach themselves to fish and suck nutrients from the host's circulatory system. 6. Small, jawless, and finless ostracoderms are the earliest vertebrate fossils.

Question 28

Describe four characteristics shared by all jawed fishes

Answer 28

1. Ectothermy-they depend on the environment to regulate their body temperature. 2. Gills are used for gas exchange. Jawed fish have a single-looped, closed circulatory pathway with a heart that pumps the blood first to the gills (for oxygen exchange) and then to the rest of the body 3. Cartilaginous or bony endoskeleton- the endoskeleton of jawed fishes includes the vertebral column, a skull with jaws, and paired pectoral and pelvic fins. Jaws evolved from the first pair of gill arches of agnathans; the second pair of arches became support structures for the jaws. 4. Scales cover and protect the skin

Question 29

Discuss the evolutionary significance of lobe-finned fishes

Answer 29

Possess fleshy fins supported by bones. Lobe-finned fishes gave rise to modern lobe-finned fishes and amphibians

Question 30

List the seven characteristics that define the amphibians

Answer 30

1. They are tetrapods; they have four limbs. The skeleton is well-developed for locomotion 2. Amphibians have smooth and non-scaly skin. The moist skin plays an active role in water balance, respiration, and temperature regulation 3. If lungs are present in amphibians they are usually small and supplemented by gas exchange across porous skin 4. Double-loop circulatory system; oxygen-rich blood mixes with some oxygen-poor blood. A three-chambered heart with a single ventricle pumps and two atria pumps blood to both the lungs and the body. 5. Amphibians have sense organs that are adapted to life on land. The brain is larger than that of fishes; their cerebral cortex is more developed. A specialized tongue is used for catching prey. The eyelids keep their eyes moist. Amphibian ears are adapted for detecting sound waves; in turn, the larynx produces sounds 6. Amphibians are ectothermic, depending on the environment to regulate body temperature. If the winter temperature drops too low, temperature exotherms become inactive and enter torpor. 7. Amphibians return to the water to reproduce. They shed eggs into the water for external fertilization. Amphibian eggs are protected by a coat of jelly but not by a shell. The young hatch into aquatic larvae with gills (tadpoles). The aquatic larvae usually undergo metamorphosis to develop into a terrestrial adult. Some amphibians evolved mechanisms that allow them to bypass the aquatic larval stage and reproduce on land.

Question 31

Describe the features of the three groups of living amphibians

Answer 31

1. Salamanders and newts: Salamanders practice internal fertilization the males produce a spermatophore that females pick up with the cloaca (common receptacle for the urinary, genital, and digestive canals). They have a long body and tail, and two pairs of legs. Their s-shaped (sinusoidal) locomotion is similar to fish movements. They are carnivorous, feeding on insects, snails, etc. 2. Frogs and toads: Tailless as adults. hind limbs are specialized for jumping. Glands in skin secrete poison ; some tropical species often have brilliant warning coloration. They have the head and trunk fused frogs live near or in fresh water while toads live in damp places away from water 3. Caecilians: Legless, sightless, worm shaped. most burrow in soil and feed on worms, etc.

Question 32

Summarize the two hypotheses that explain the evolution of amphibians from lobe-finned fishes

Answer 32

1. Lobe-finned fishes that could move from pond-to-pond had an advantage over those that couldn't. 2. The supply of food on land and the absence of predators promoted adaption to land.

Question 33

List the seven features that define the reptiles

Answer 33

1. Reptiles have paired limbs adapted for climbing, running, paddling, or flying. 2. Reptiles have a thick, scaly skin that is impermeable to water. reptiles' protective skin prevents water loss but it also requires several molts a year. 3. Efficient breathing: their lungs are more developed than in amphibians; air rhythmically moves in and out of the lungs due to an expandable rib cage. 4. Efficient circulation- heart prevents mixing of blood. oxygen-rich blood is more fully separated from oxygen-poor blood. 5. Efficient excretion: the kidneys are well-developed and excrete uric acid so less water is required. 6. Ectothermic; require a fraction of the food per body weight of birds and mammals. They are behaviorally adapted to warm their body temperature by sunbathing 7. Well-adapted reproduction. The sexes are separate and fertilization is internal. the amniotic egg contains extra embryonic membranes. These membranes are not a part of the embryo and are disposed of after development. They protect the embryo, remove nitrogenous wastes, and provide oxygen, food, and water. The amnion is one extra-embryonic membrane; it fills with fluid to provide a "pond" for the embryo to develop.

Question 34

Explain why the reptiles are represented by more than one evolutionary lineage

Answer 34

1. The amniotes consist of three lineages a.) The turtles, in which the skull is anapsid, having no openings behind the orbit-eye socket. b.) All other reptiles, in which the skull has two openings behind the orbit (diapsid) c.) The mammals, in which the skull has one opening behind the orbit (synapsid) d.) the modern view of reptile evolution places turtles within the archosaurs with birds and crocodiles, meaning that anapsid are highly specialized diapsids. e.) The classical view is being presented that all other reptiles except turtles are diapsids. 2. The reptiles have no common ancestor; they are a paraphyletic group and not a monophyletic group. 3. Thecodonts are diapsids that gave rise to the ichthyosaurs which returned to the aquatic environment and the pterosaurs, which were terrestrial. They gave rise to crocodiles and dinosaurs. Dinosaurs: some had a bipedal stance and gave rise to birds, went extinct at the end of the Cretaceous period. Snakes evolved from lizards and lost legs as an adaptation to burrowing. Tuataras are the only member of an ancient group of reptiles that included the common ancestor of modern lizards and snakes. Birds share a common ancestor with crocodilians.

Question 35

Define traits of birds that are related to flight

Answer 35

1. Feathers keep birds warm, and help birds fly and steer. Feathers are modified scales. Birds molt and replace their feathers annually. 2. Modified skeleton. The collarbone is fused (the wishbone) and the sternum has a keel. Other bones are fused to make the skeleton more rigid than the reptilian skeleton. The breast muscles are attached to the keel. 4. Modified respiration. Bird respiratory sacs are extensive, even extending into some larger bones. Using a one-way flow of air, air sacs minimize gas exchange and oxygenation of blood. Efficient supply of oxygen to muscles is vital for the level of muscle activity needed for flight. 5. Endothermic; they have the ability to maintain a constant, relatively high body temperature. 6. Well-developed sense organs and nervous system; birds have very acute vision, muscle reflexes are excellent, migration allows use of widespread food sources, an enlarged portion of the rain is responsible for instinctive behaviors. Bird vocalizations are distinctive and convey an abundance of information.

Question 36

Describe five features of mammals

Answer 36

1. Hair: provides insulation against heat loss, color can provide camouflage to blend in to its surroundings, can serve sensory functions. 2. Mammary glands: enable females to feed young without deserting them to obtain food. Nursing creates a bond between mother and offspring to ensure parental care while the young are helpless. 3. Skeleton: skull accommodates a larger brain relative to body size compared to reptiles', teeth are differentiated into molars and premolars, the vertebral column provides more movement. 4. Internal organs: gas exchange is efficiently accomplished by lungs, mammals possess a four-chambered and a double-loop circulatory system, kidneys are adapted to conserving water. The nervous system and sensory organs are highly developed. 5. Internal development: in most mammals, the young are born alive after a period of development in the uterus.

Question 37

Discuss the timeline of the evolution of mammals

Answer 37

Mammals share an amniote ancestor with reptiles. Immediate ancestors had a synapsid skull.Mammals evolved during the Mesozoic Era from mammal-like synapsids. Common ancestor of all mammal groups appeared in late Triassic/early Jurassic period. True mammals appeared during the Triassic period, about the same time as the first dinosaurs. The first mammals were small, about the size of mice. Some of the earliest mammalian groups were monotremes and marsupials. Placental mammals underwent an adaptive radiation later to occupy habitats vacated by the dinosaurs

Question 38

Identify several features that define each of the three living lineages of mammals.

Answer 38

1. Monotremes: mammals that have a single urogenital opening (cloaca) and lay hard-shelled amniote eggs. Represented by the duckbill platypus and two species of the spiny anteaters. A female duckbill platypus lays her eggs in a burrow in the ground where she incubates them. After hatching, the young lick milk seeping from modified sweat glands on the abdomen. The spiny anteater has a pouch formed by the swollen mammary glands and muscle; the egg moves from cloaca to pouch and hatches; the young remain for 53 days and live in the burrow where the mother feeds them. 2. Marsupials: begin development inside the mother's body but are born in a very immature state, the newborns crawl up into a pouch on their mother's abdomen. Inside a pouch they attach to the nipples of the mother''s mammary glands and continue to develop. Today, most marsupials are found in Australia where they underwent adaptive radiation for several million years without competition from placental mammals, only introduced recently. 3. Placental Mammals; Developing placental mammals are dependent on a placenta, an organ of exchange between maternal and fetal blood. The placenta supplies nutrients to and removes wastes from the blood of developing offspring. A placenta also allows a mother to move about while the offspring develop. The placenta enables young to be born in a relatively advanced state of development. Placental Mammals are active mammals; they posses acute senses and relatively large brain. The brains of placental animals have cerebral hemispheres proportionally larger than other animals. The young go through a long period of dependency on their parents after birth. Most are terrestrial, but some are aquatic, bats can fly. Humans are mammals