Animal Kingdom: common doubts

Question 1

What is the difference between cellular, tissue, organ and organ-system levels of organisation?

Accepted Answer

In the cellular level, as in sponges, cells are arranged as loose aggregates with a division of labour but no true tissues. The tissue level, in coelenterates and ctenophores, has cells of the same function grouped into tissues. The organ level appears in flatworms, and the organ-system level begins with aschelminthes and continues in all higher phyla.

Question 2

What is the difference between an incomplete and a complete digestive tract?

Accepted Answer

An incomplete digestive system has a single opening that serves as both mouth and anus, as in flatworms. A complete system has a separate mouth and anus, so food moves in one direction and different regions can specialise. Aschelminthes and all higher animals have a complete tract, which allows more efficient digestion.

Question 3

What is the difference between open and closed circulatory systems?

Accepted Answer

In an open system the blood is pumped out of the heart into cavities or spaces that bathe the tissues directly. In a closed system the blood stays within vessels. Arthropods and hemichordates have open circulation, while annelids and chordates have closed circulation, which gives better control of blood flow.

Question 4

What is the difference between radial and bilateral symmetry?

Accepted Answer

In radial symmetry any plane passing through the central axis divides the body into two identical halves, as in coelenterates and ctenophores. In bilateral symmetry only one plane, dividing the body into right and left halves, gives identical halves, as in annelids and arthropods. Sponges are mostly asymmetrical.

Question 5

Are echinoderms radially or bilaterally symmetrical?

Accepted Answer

Adult echinoderms are radially symmetrical, but their larvae are bilaterally symmetrical. So the answer depends on the stage you mean. This is because the larva has a body plan that is later modified. If an exam question says adult echinoderms, choose radial; if it mentions the larva, choose bilateral.

Question 6

What is the difference between diploblastic and triploblastic animals?

Accepted Answer

Diploblastic animals have two embryonic germ layers, an outer ectoderm and an inner endoderm, with an undifferentiated mesoglea between them, as in coelenterates. Triploblastic animals have a third layer, the mesoderm, between the other two, and include all animals from flatworms up to chordates.

Question 7

What is the difference between acoelomate, pseudocoelomate and coelomate animals?

Accepted Answer

Acoelomates, such as flatworms, have no body cavity. Pseudocoelomates, such as roundworms, have a cavity not fully lined by mesoderm, because mesoderm is present only as scattered pouches. Coelomates, such as annelids and chordates, have a true coelom lined by mesoderm. Do not call roundworms acoelomate.

Question 8

What is metamerism and in which phylum does it first appear?

Accepted Answer

Metamerism is the division of the body into a series of similar segments, each repeating some organs, and it first appears in Phylum Annelida. In earthworms the body is externally and internally segmented. This is also called metameric segmentation and is a feature carried on into arthropods.

Question 9

How does water move through a sponge?

Accepted Answer

Water enters through the many small pores called ostia, passes into the central cavity called the spongocoel, and leaves through the large opening called the osculum. Flagellated choanocytes line the spongocoel and canals and drive this current. The flow brings food and oxygen and carries away wastes, and digestion in sponges is intracellular.

Question 10

What are cnidoblasts and what do they do?

Accepted Answer

Cnidoblasts, or cnidocytes, are stinging cells on the tentacles and body of cnidarians such as Hydra. Each contains a capsule called a nematocyst that is used for anchorage, for defence and for capturing prey. They are the characteristic cell type of the coelenterates and are not found in ctenophores, which instead have comb plates.

Question 11

What is the difference between polyp and medusa?

Accepted Answer

A polyp is a sessile, cylindrical form, like Hydra, whereas a medusa is a free-swimming, umbrella-shaped form, like Aurelia. Some cnidarians exist in only one form, while others alternate between the two. Obelia shows this alternation, with polyps producing medusae asexually and the medusae forming polyps again through sexual reproduction.

Question 12

What are the characteristic features of Ctenophora?

Accepted Answer

Ctenophores, or comb jellies, are marine animals with eight external rows of ciliated comb plates that help in locomotion. They show bioluminescence, which means they produce light. They reproduce only sexually, fertilisation is external, and development is indirect. They are diploblastic and radially symmetrical, with tissue-level organisation.

Question 13

What are flame cells and which animals have them?

Accepted Answer

Flame cells are the specialised excretory and osmoregulatory cells of flatworms such as Taenia and Fasciola. They help in osmoregulation and excretion. Other phyla have different organs for this role: nephridia in annelids, Malpighian tubules in arthropods, and a proboscis gland in hemichordates, while echinoderms have no excretory system.

Question 14

Are all annelids hermaphrodite?

Accepted Answer

No. Earthworms and leeches are monoecious, with both sexes in one individual, but Nereis is dioecious, with separate sexes. Annelids are triploblastic, coelomate and metamerically segmented, and Nereis moves with the help of parapodia. So a blanket statement that all annelids are hermaphrodite is wrong.

Question 15

Why is Arthropoda the largest phylum?

Accepted Answer

Arthropoda has the greatest number of species, making up over two-thirds of all named animal species, with insects being the largest group. They have jointed appendages, a chitinous exoskeleton, and an open circulatory system. Limulus, the king crab, is a living fossil among them.

Question 16

What is the radula and which phylum has it?

Accepted Answer

The radula is a file-like rasping organ of the mouth used in feeding. It is found in Phylum Mollusca, the second largest phylum. Molluscs have a soft body covered by a calcareous shell, usually with gills in the mantle cavity that serve for respiration and excretion, along with a muscular foot for movement.

Question 17

What is the water vascular system and which animals have it?

Accepted Answer

The water vascular system is a network of canals filled with fluid, used for locomotion, capture and transport of food, and respiration, found in echinoderms such as starfish. The body also has tube feet. Echinoderms are marine, have an endoskeleton of calcareous ossicles, and lack a separate excretory system.

Question 18

Is Hemichordata a chordate group?

Accepted Answer

No. Hemichordates are now treated as a separate non-chordate phylum, even though earlier they were included in Chordata. They have a stomochord in the collar region, which resembles the notochord of chordates but is not the same structure. Balanoglossus is the example, with a proboscis, collar and trunk, and open circulation.

Question 19

What are the fundamental features that define chordates?

Accepted Answer

Chordates have a notochord, a dorsal hollow nerve cord, paired pharyngeal gill slits, a post-anal tail, and a ventral heart. Notochord is the key feature. Non-chordates may lack these features, and when they have a nerve cord it is solid and ventral instead. Protochordates are the group formed by Urochordata and Cephalochordata.

Question 20

What is the difference between Urochordata and Cephalochordata?

Accepted Answer

In urochordates, such as Ascidia, the notochord is present only in the larval tail. In cephalochordates, such as Branchiostoma (amphioxus), the notochord extends from head to tail and is present throughout life. The two groups together are called protochordates, and both are marine.

Question 21

What is the difference between cartilaginous and bony fishes?

Accepted Answer

Cartilaginous fishes (Chondrichthyes) have a cartilage skeleton, placoid scales, no air bladder and no operculum, so they must keep swimming to avoid sinking. Bony fishes (Osteichthyes) have a bony skeleton, four pairs of gills covered by an operculum, and an air bladder that regulates buoyancy.

Question 22

How many chambers are in the heart of fishes, amphibians, reptiles, birds and mammals?

Accepted Answer

Fishes have a two-chambered heart, amphibians have three chambers, and most reptiles have three, though crocodiles have four chambers. Birds and mammals have a four-chambered heart. So the common line that all reptiles have three chambers is not completely right, and the crocodile exception is often asked.

Question 23

Which features of birds help them fly?

Accepted Answer

Several bird features suit flight. The forelimbs are modified into wings and the body is covered with feathers. The endoskeleton is fully ossified, but the long bones are pneumatic, hollow with air cavities, which keeps the skeleton light. Air sacs connected to the lungs supplement breathing, and birds are warm-blooded with a fully four-chambered heart. Flightless birds such as the ostrich share the same body plan.

Question 24

Is the platypus a mammal even though it lays eggs?

Accepted Answer

Yes. What makes an animal a mammal is chiefly the mammary glands, which produce milk for the young, and the platypus, Ornithorhynchus, has them. It also shares the other mammalian features: hair on the skin, a four-chambered heart and a constant body temperature. Most mammals are viviparous and give birth to live young, so the egg-laying platypus is the standard exception to that rule.

Animal Kingdom: common doubts, answered

Levels of organisation