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Whale is placed in mammals and not in Pisces because it
A. Is aquatic in habit
B. Does not have fins
C. Respires through gills
D. Possess mammary glands

Answer
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Hint: Whales are a widespread and diverse group of fully aquatic placental marine mammals. The whales are open ocean creatures that feed, breed, bear children, suck and raise their young at sea. Whales vary in size between 2.6 metres and 135 kg of dwarf whale to 29.9 metres and 190 tonnes blue whale.

Complete answer:
The need for milk is an integral part of the development of any young mammal, but being aquatic causes breastfeeding significantly more difficult. Breastfeeding their young with milk is one of the things that distinguishes mammals, so whales certainly have mammary glands and produce milk. Animals that are completely confined to the sea, such as whales, have developed 'mammary slits' unique skin folds that enclose the feeding glands. The whale has the biggest mammary glands on Earth with each around 1.5m long and weighing the same as a baby elephant. Whale mothers will yield 200 litres of milk a day with a calorie count of 35-50 percent. This helps the whale calf to put on weight at an exponential rate of 100 kg per day. Within mammary slits, whales have inverted nipples. When the calf is prepared to consume, they dive underneath the mother and chug the mammary slits. The nipple is activated and the mother feeds milk to the calf. After the stimulus, the whale allows the nipples out of the mammary slits, and the young calf covers its mouth around it. This implies that the milk is channeled straight to the mouth. This assures that no fluid is lost in the water. The baby whales roll their tongue in a U-shape. The tongue is then pressed to the top and positioned around the nipple. The mother of the whale then keeps pouring milk straight into the mouth of the calf. The mammary glands have specific muscles that the very first contract, and afterward the milk is released straight into the mouth.
So, the correct solution is (D).

Note: The milk of the whale does have a very thick consistency. The milk is nearly solid. The density is due to the high body fat that can reach up to 50% of the fat in some species of whale. One of the primary reasons why whales can effectively breastfeed submerged is the consistency of the milk. Breastfeeding in whales persists till the calf is strong enough to live by itself and to hunt for its own food.