LINE BY LINE EXPLANATION OF NCERT, CLASS 7 - SCIENCE, CHAPTER 1: NUTRITION IN PLANTS

 

aquatic mammals

Aquatic mammals
· Mammals are primarily terrestrial.
· However some mammals are well adapted for aquqtic mode of lide.
· They are not gill breathers but take air through lungs.
· Probably they reverted to water because of extreme competition on land for food and shelter.
· Depending upon the degree of aquatic adaptations the aquatic mammals are of two types: amphibious mammals, completely aquatic mammals.
1. Amphibious mammals:
o They live on land but go into water for food and shelter.
o They show only partial aquatic adaptations such as
a. Small external years.
b. Webbed feet.
c. Flattened tails.
d. Subcutaneous fat.
Ex. Walrus, hippopotamus, beavers.
2. Completely aquatic mammals:
o The members of two orders, cetacea (whales, dolphins, porpoises) and sirenia (manatees and dugong) are essentially aquatic forms.
· Various types of aquatic adaptations in aquatic mammals: the specifications of truly aquatic mammals (cetacea and sirenia) fall into three major categories.
1. Modifications of original structures.
2. Loss of structures.
3. Development of new structures.
a. Modifications of original structures:
1. Body shape: the body shape is one of the most important adaptations to aquatic life. The fish like streamlined body offers very little resistance and allows the animal to swim rapidly through water.
2. Large size body weight: large size reduce skin friction and heat loss. The large size creates no problem for support as water offers buoyancy.
3. Flippers : fore limbs are transformed into skin covered unjointed paddles or flippers having no separate indications for fingers. Flippers can only be moved as a whole at shoulder joint only. The broad and flattened flippers serves as balancers and provide stability during swimming.
4. Hyperdactyly and hyperphalangy: extra digits and extra phalanges, upto 14 or more in some cases, serves to enlarge the surface area of flippers for great utility during swimming in water.
5. High and valvular nostrils: nostrils are placed far back on the top of head so that animal can breathe under water without raising the head much out of water. nostrils are provided with valves which can be closed while diving in warter.
6. Mammary ducts: during lactation, ducts of mammary glands dialate to form large reservoirs of milk. The milk is pumped into the mouth of young by the action of a special compressor muscles. The arrangement facilitates suckling of the young under water.
7. Oblique diaphragm: oblique diaphragm makes thoracin cavity large. This large spaces provides more apace for lungs expansions.
8. Large lungs: large lungs and unlobulated lungs provide a big volume of air to be filled in. the dorsal lung also serves as hydrostatic organ in maintaining a horizontal posture during swimming.
9. Intra narial epiglottis: elongatedtubular and intranarial epiglottis when embraced by th eosft palate provides a continuous and separate air passage, thus allowing breathing and feeding simultaneously.
10. Endoskeleton: cranium becomes small but wider to accommodate the short and wide brain. Cervical vertebrae are fused into solid bony mass because of reduced neck. Sacrum is reduced. Ribs become arched dorsally to increase thoracic cavity. Bones are light spongy and in cetacea they are filled with oil.
11. Teeth: in toothed whales, teeth are monophyodont, homodont and numerous. As many as 250 teeth may be present. The teeth are used for seizing prey, prevent its escape and swallowing it without mastication. Usually the mortality of jaw is reduced as they have no function in mastication.
b. Loss of structures: due to loss of hairs, skin surface is smooth and glistening. Only a few sensory bristles are on snout and lips in some cases. Pinnae are absent. Due to thickening and immobility skin losses its muscles and nerves. Hind limbs are only represented by button-like knobs in the foetus but disappear in adults. Pelvis is also rudimentary. Scrotal sacs are also sbsent and remains inside the abdomen.
c. Development of new structures:
1. Tail flukes: tail flukes develop lateral expansion of skin, called tail fibres. These are not supported by fin rays. They help to propel through water by up and down movements. They enable rapid return to the surface after prolonged submersion.
2. Dorsal fin: most cetacea develop an unpaired adipose dorsal fin without internal skeletal support. It serves as a rudder or keel during swimming.
3. Blubber: it is thick subcutaneous layer of fat acting as heat insulators