S I REN I AN S 409 



their aqueous, with the small number of rugose ciliary processes for 

 which there is room on the small ciliary body (Fig. 140) . 



In sympathy with the 'grazing' habit the cornea is horizontally oval, 

 being 11.0 x 7.5mm. in Dugong; but this is as far as the eyeball goes 

 toward the ellipsoidality of full aquatic adaptation (compare Fig. 104, 

 p. 259). We should expect the eyeball to be flattened — and it is, a bit, 

 in Trichechus; but it is practically spherical in Dugong. We should also 

 expect to find a spherical lens close to a broad, flat cornea. Instead, 

 though the anterior segment is remarkably small, the cornea is arched 

 and the lens is far from spherical, being the flatter in Dugong. In this 

 genus, measurements of adult lenses have been given as 6.9 x 4.4mm., 

 7.0 x4.0mm., etc. Different investigators have variously computed the 



Fig. 140 — Sirenian eyes. 



a, a manatee, Trichechus manatus. x 3 !4 . After Piitter. b, dugong, Dugong dugon. xWi. 



Redrawn from Pettit and Rochon-Duvigneaud. 



quotient of the horizontal and axial diameters of the dugong lens as 

 1.75, 1.57, 1.25; of the manatee lens, 1.40-1.24. These are hardly ideal 

 relationships for under-water vision. 



Though the lens is very distant from the retina and the visual field 

 is very large — the eye of Dugong being strikingly human-like not only 

 in size but in the proportioning of its parts, and with a retinal extent 

 equalling 265° of the eyeball's circumference — the eye is not as good 

 an air-seeing one as it looks superficially. Dugong dugon is five diopters 

 myopic in air, and must be fearsomely hypermetropic in water since it 

 has no power of accommodation. Rochon-Duvigneaud could not make 

 out any ciliary muscle at all. 



Despite the 'diurnal' gross aspect of the sirenian eye (compare Fig. 

 140 with Fig. 71, p. 173), it has no devices for high visual acuity. The 



