Extant sirenians grow to between in length and can weigh up to . Steller's sea cow was the largest known sirenian to have lived, and could reach lengths of , and could weigh in at . A dugong's brain weighs a maximum of , about 0.1% of the animal's body weight. The body of sirenians is sparsely covered in short hair (vibrissae), except for on the muzzle, which may allow for tactile interpretation of their environment. Manatees are the only creatures to exhibit corneal avascularity, and lack blood vessels in the cornea, which prevents optical clarity and vision. This may be the result of irritations from or protection against their hypotonic freshwater environment.