RowQ
The Vault
RowQ
The Vault
CBSE Class 11 Biology · 10 questions · 24 marks
Animals differ far more from one another than plants do, so classifying them needs a set of shared structural yardsticks rather than a running list of species. This chapter builds those yardsticks first — levels of organisation, symmetry, germ layers, the presence and type of body cavity, and segmentation — then applies them phylum by phylum from sponges to chordates. Once the basis is clear, working out where an unfamiliar animal belongs becomes a matter of asking the right five questions.
Excretion in insects such as the grasshopper is carried out by:
Answer
Malpighian tubules are correct — these fine tubes open into the gut of arthropods and remove nitrogenous waste; flame cells belong to flatworms and nephridia to annelids.
An animal whose body cavity is not lined on all sides by mesoderm is described as:
Answer
Pseudocoelomate is correct — in roundworms such as Ascaris the mesoderm occurs only as scattered pouches between the gut and the body wall, so the cavity is a false coelom rather than a true one.
The water vascular system used for locomotion, capture of food and respiration is a unique feature of the phylum:
Answer
Echinodermata is correct — starfish and their relatives move using tube feet operated by water pressure in this system, which no other phylum possesses.
Which of the following is present in all chordates at some stage of life?
Answer
Notochord is correct — it is one of the four defining chordate features along with a dorsal hollow nerve cord, pharyngeal gill slits and a post-anal tail, and it is present at least in the embryo even where a vertebral column later replaces it.
Assertion (A): Adult starfish are radially symmetrical but their larvae are bilaterally symmetrical. Reason (R): Echinoderms are believed to have evolved from bilaterally symmetrical ancestors and acquired radial symmetry secondarily with a sedentary adult habit.
Answer
Both A and R are true and R is the correct explanation of A — the retention of bilateral symmetry in the free-swimming larva is direct evidence that radial symmetry in the adult is a later adaptation rather than an ancestral condition.
Distinguish between diploblastic and triploblastic organisation, giving one example of each.
Answer
In diploblastic animals the cells of the embryo are arranged in only two germ layers, an outer ectoderm and an inner endoderm, with a non-cellular jelly-like mesogloea in between; coelenterates such as Hydra are diploblastic. In triploblastic animals a third layer, the mesoderm, develops between the ectoderm and endoderm and gives rise to muscles, the skeleton, the circulatory system and the lining of the body cavity; all animals from Platyhelminthes upward, including the earthworm and humans, are triploblastic.
Describe the water canal system of sponges and explain its importance.
Answer
In a sponge, water enters the body through numerous minute pores called ostia in the body wall, passes into a central cavity called the spongocoel which is lined by flagellated collar cells or choanocytes, and finally leaves through a large opening at the top called the osculum. The beating of the choanocyte flagella maintains this one-way current. The system is important because a sponge has only a cellular level of organisation with no true tissues or organs: the flowing water brings in oxygen and food particles which the choanocytes trap and pass to other cells, and the same current carries away carbon dioxide and nitrogenous waste, so a single mechanism serves for feeding, respiration and excretion.
Explain the five main criteria used for the classification of animals, giving an example of each.
Answer
The first criterion is the level of organisation. In sponges the cells are loosely arranged and act more or less independently, which is the cellular level; in coelenterates such as Hydra similar cells form definite tissues, the tissue level; in flatworms tissues combine into organs, the organ level; and from roundworms upward organs work together in systems, the organ-system level, as in the earthworm and all higher animals. The second criterion is symmetry. Sponges are largely asymmetrical since no plane divides them into equal halves; Hydra and jellyfish are radially symmetrical because any plane through the central axis gives two identical halves; and animals such as the earthworm, insects and vertebrates are bilaterally symmetrical, being divisible into identical right and left halves by only one plane. The third criterion is the number of germ layers. Diploblastic animals such as coelenterates have only ectoderm and endoderm with mesogloea between them, while triploblastic animals from flatworms onward develop a true mesoderm as well. The fourth criterion is the nature of the body cavity. Acoelomates such as the tapeworm have no cavity between the gut and the body wall; pseudocoelomates such as Ascaris have a cavity that is not completely lined by mesoderm; and coelomates such as annelids, molluscs, arthropods, echinoderms and chordates have a true coelom fully lined by mesoderm, which allows organs to be suspended and to move independently of the body wall. The fifth criterion is segmentation. In the earthworm the body is divided both externally and internally into a serial repetition of similar segments, a condition called true metameric segmentation, whereas in animals such as the roundworm the body is unsegmented. Applying these five criteria in order allows an unfamiliar animal to be assigned to its phylum with confidence.
Give the distinguishing features of the classes Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves and Mammalia, with one example of each.
Answer
Chondrichthyes are marine fish whose endoskeleton is entirely cartilaginous. The mouth is ventral, the notochord persists through life, the gill slits are separate and not covered by an operculum, and the skin is rough with placoid scales. The air bladder is absent, so these fish must swim continuously to avoid sinking, and the tail is asymmetrical or heterocercal; fertilisation is internal and many are viviparous. An example is Scoliodon, the dogfish. Osteichthyes include both marine and freshwater fish with a bony endoskeleton. The mouth is mostly terminal, four pairs of gills are covered by an operculum, the skin bears cycloid or ctenoid scales, and an air bladder is present to regulate buoyancy. Fertilisation is usually external and most are oviparous. An example is Labeo, the rohu. Amphibia can live both on land and in water. The skin is moist and lacks scales, respiration occurs through gills, lungs and the skin, the heart has three chambers, and the body has two pairs of limbs. They are cold-blooded, fertilisation is external and development passes through a larval tadpole stage. An example is Rana tigrina, the common frog. Reptilia are creeping or crawling animals with a dry skin covered by epidermal scales, which is periodically shed. They breathe only by lungs even if aquatic, the heart is three-chambered though the ventricle is partly divided, and they are cold-blooded. Fertilisation is internal and the shelled eggs are laid on land. An example is Naja naja, the cobra. Aves are characterised by the presence of feathers and by forelimbs modified into wings. The bones are hollow and pneumatic to reduce weight, the beak is toothless, the heart is four-chambered, and the birds are warm-blooded with air sacs connected to the lungs supplementing respiration. An example is Columba, the pigeon. Mammalia are distinguished by mammary glands that produce milk to nourish the young, by hair on the skin, and by external ears or pinnae. They possess two pairs of limbs adapted for walking, running, climbing, swimming or flying, have a four-chambered heart, are warm-blooded, and most are viviparous with internal fertilisation. An example is Macropus, the kangaroo.
A marine biologist dissects four preserved specimens dredged from the same reef. Her notes read: Specimen W — soft unsegmented body, calcareous shell, muscular foot, feeds by scraping algae with a file-like structure. Specimen X — body divided into identical rings inside and out, closed circulatory system, coiled tubes opening to the exterior in each ring. Specimen Y — hard jointed outer covering, paired jointed legs, blood flowing freely in body spaces. Specimen Z — spiny surface, five arms, tiny tube feet in grooves along each arm. (a) Name the phylum of specimen W and the file-like feeding structure mentioned. (b) What is the condition described in specimen X called, and what are the coiled excretory tubes? (c) Specimen Y's blood flows freely in body spaces. Name this type of circulatory system and the phylum. (d) By what mechanism does specimen Z move, and to which phylum does it belong?
Answer
(a) Specimen W belongs to phylum Mollusca. The file-like feeding organ is the radula, a rasping ribbon set with rows of horny teeth used to scrape algae off the rock surface. (b) The division of the body into a serial repetition of identical rings both externally and internally is true metameric segmentation, and the phylum is Annelida. The coiled excretory tubes present in each segment are nephridia, which remove nitrogenous waste and also regulate water balance. (c) The blood of specimen Y flows freely through spaces in the body rather than being confined to vessels, so it has an open circulatory system, and the chitinous jointed exoskeleton with paired jointed appendages places it in phylum Arthropoda. (d) Specimen Z moves by means of tube feet operated by its water vascular system, in which water pressure extends and retracts the feet so the animal creeps over the sea bed; it belongs to phylum Echinodermata.
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