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Let us learn about the comprison between cetace and sirenia.
Comparison # Cetacea:
1. Size:
Some attain a colossal size.
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2. Habitat:
Marine forms. Larger species prefer open sea, while smaller forms live in plenty in estuaries and river.
3. Habit:
Carnivorous and predaceous animals.
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4. Hairs:
Absent, except a few bristles around the muzzle which may also be absent in many whales.
5. Caudal fin:
Horizontally expanded which is usually divided into ‘flukes’.
6. Dorsal fin:
An adipose dorsal fin is present.
7. Eyes:
Small and devoid of third eyelid.
8. Nostrils:
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Nasal aperture present on the fore head as single or double blowhole is usually shifted to the dorsal side of the head. Valvular arrangement is highly developed.
9. Mammae:
Mostly absent, vestige may be present in foetus.
10. Hind limbs:
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Mostly absent, vestige may be present in foetus.
11. Forelimbs:
Short and transformed into flippers. Externally un-jointed and covered by skin. Lack movable joints. Number of digits—usually 5, may be 4 as in Balaena. Hyperphalangy (Gk. hyper, over, L. phalanx, bone of finger) and over, L. phalanx, bone of finger) and hyperdactyly are present. Usually have no trace of claws.
12. Teeth:
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Sometimes absent, if present, are conical, homodont, monophyodont and numerous. May be absent in Whale- bone whales.
13. Salivary glands:
Absent
14. Stomach:
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Divided into three chambers
15. Intestine:
Moderate in length.
16. Trachea:
Very short with a third bronchus.
17. Epiglottis and arytenoid cartilages:
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Prolonged into nasal passage.
18. Lungs:
Spacious—not lobed.
19. Diaphragm:
Oblique and less muscular.
20. Brain:
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Large with much convolutions.
21. Placenta:
Zonary.
22. Bones:
Spongy and contain oils.
23. Vertebral column:
Epiphyses are distinct. Cervical vertebrae are seven in number, more or less fused with one another and are disc-like in appearance.
24. Sternum:
Short
25. Ribs:
Lack heads
26. Skull:
Lack heads.
27. Scapula:
Bones are light. Nasals are short. Cranial cavity is large. Snout is formed of maxillae, premaxillae, vomer and mesethmoid cartilage. Tympanic bulla is ring-shaped and fused with the skull. Parietals are separate.
28. Examples:
Ganges dolphin, Plantanista gangetica; Sperm whale, Physeter; Porpoise, Phocaean; blue whale, Balaenopatera musculus.
Comparison # Sirenia:
1. Size:
Comparatively small.
2. Habitat
Live in sea near coast, also available
3. Habit:
Herbivorous and feed on aquatic.
4. Hairs:
Very sparse.
5. Caudal fin:
Horizontally flattened, either rounded or rhomboidal in outline.
6. Dorsal fin:
Absent.
7. Eyes:
Small and have third eyelid.
8. Nostrils:
Separate from one another and placed on the front of the head. Valves and pads are less developed.
9. Mammae:
Pectoral in position, close behind the forelimbs.
10. Hind limbs:
Usually absent.
11. Forelimb:
Short and transformed into flippers—enclosed by a common cutaneous covering. Movable joints are present. Number of digits—5. Hyperphalangy and hyperdactyly are absent.
12. Teeth:
Variable. Rhytina has no teeth, Halicore has rootless and enamelless molars, Manatus possesses numerous molars and no incisors
13. Salivary glands:
Well- developed
14. Stomach:
Not divided
15. Intestine:
Very long and with large caecum.
16. Trachea:
Short and having only two bronchi.
17. Epiglottis and arytenoid cartilages:
Not prolonged.
18. Lungs:
Elongated with associated air-sacs.
19. Diaphragm:
Oblique and highly muscular.
20. Brain:
Small with slight convolutions.
21. Placenta:
Diffuse.
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22. Bones:
Heavy and compact as an adaptation to take submerged weeds from the bottom.
23. Vertebral column:
Epiphyses are absent. Cervical vertebrae may be six (as in Manatus)compressed, but ankylosed, neural arches may be incomplete.
24. Sternum:
Usually broad
25. Ribs:
Double-headed.
26. Skull:
Bones are heavy. Nasals usually absent. Cranial cavity is narrow and elongated. Snout is formed of maxillae and pre-maxillae. Tympanic bulla is loosely attached with the skull. Parietals are united.
27. Scapula:
Elongated and narrow having all the normal mammalian features.
28. Examples:
Sex cows, Dugong; Manatee, Manatus.
