Adaptation to the aquatic environment:
The shape of the body should always be streamlined in its most diverse variants: flattened from the sides (crucian carp), flattened in the dorsal-abdominal direction (leech), round in cross section (eel), teardrop-shaped (swimming beetle), torpedo (squid).
The body must minimize water friction. This is achieved by the peculiarities of its integuments: mucus cover (fish); very smooth (“polished”) hard surface (sea turtle), soft layer on the surface of a hard body (whale).
Extremities: have a swimming membrane (frog), converted to fins (dolphin), converted to flippers (seal).
Special outgrowths and adaptations for movement in water may include: a swimming membrane around the body or a special “umbrella” type (cuttlefish, jellyfish), a water-jet (“jet”) engine (squid, dragonfly larva), a tail with a fin (fish).
You also have to breathe in the water, and such breathing is organized according to certain rules. Respiratory organs are different: gills (fish), respiratory tube (ranatra), air intakes (water beetles, bugs), air storage under water in the form of a bubble (silver spider), the formation of a bubble that replaces the lung (swimming beetles).
Organisms that live in the soil live by their own rules and also have special adaptations that relate to the shape of the body, its integuments, limbs and other features. Adaptations to the soil:
The covers of the body of an underground inhabitant should allow him to move freely in dense soil both forward and backward (it is not always possible to turn around in a narrow course). Here are some rules for the integument: slimy secretions that allow it to slide in the soil (worm), if there is wool, it is usually short (mole), the wool is smoothed back and forth (mole), the wool is resistant to abrasion (mole).
The shape of the body and limbs should also be specific. Long limbs will not allow you to move in a narrow hole, in addition, limbs are needed for digging the ground. The body should not cling to the vaults of the burrow, or should bend easily at a right or even sharp angle. Hence the following rules: short limbs, digging limbs (mole) or digging teeth (mole vole), thin and long body (geophile), streamlined body without protruding parts (mole).
The specificity of the sensory organs of a digger is also its adaptation to the peculiarities of the habitat. They can be arranged according to the following rules: auricles (mole) are reduced or absent, eyes (mole) are reduced or absent, tactile sensitivity is increased (vibrissae throughout the body).
The soil is dense and heavy, and may also lack air. These features also lead to physiological and anatomical adaptations: resistance to a lack of oxygen (asphyxia), a system of lacunae (cavities in which blood saturated with oxygen is stored); strong muscles and bones that resist compression (mole).
The inhabitants of the aerial habitat are diverse in their adaptations, because they are specialized for too different habitats. So those running on solid ground are not at all like climbing ones, and both are very different from flying ones. Therefore, it is convenient to divide all organisms here into subgroups with similar adaptations to the same habitat.