654 THE EYE IN EVOLUTION 



Lutra ; this animal has a well-developed ciliary muscle and, in addition, 

 a powerful sphincter of the iris which appears to aid the deformation 

 of the lens after the manner of Sauropsidans so that its accommodative 

 range can cope with vision in air and also under water. In air the 

 animal is emmetropic and under water its visual acuity is sufficiently 

 good to allow it to capture its prey with considerable agility, primates 

 as a class possess the most effective range before senescence sets in 

 (up to 10 D in the ape ; up to 20 D in the human infant, decreasing 

 to 10 D at 21 years, thereafter rapidly diminishing). 



A resume of the occurrence and configuration of the ciliary musculature 

 may be useful at this stage. It is, of course, absent when the ciliary body as 

 such is absent or reduced to a flat ciliary zone (Cyclostomes, the coelacanth, 

 Dipnoans, Chondrosteans and Csecilians) ; it is also absent in Monotremes 

 and is vestigial in Rodents, Insectivora and Sirenia. The muscle is plain in 

 Fishes, Amphibians and Mammals ; striated in Reptiles and Birds. It is 

 represented by a small tensor choroide^ in Teleosts and Amphibians 

 (discontinuous in two strips above and below). This becomes a ciliary 

 MUSCLE in Reptiles, Birds and Mammals. Accessory musculature is re- 

 presented by a PROTRACTOR LENTis in Selachians (ectodermal) and Amphibians 

 except Csecilians (mesodermal ; dorsal and ventral in Anurans, ventral in 

 Urodeles) ; a retractor lentis is present in Teleosts (except eels) and Holo- 

 steans. A transversalis muscle is found in Chelonians, lizards, (?) Sphenodon 

 and (?) the pigeon. The segmentation of the ciliary muscle into Crampton's and 

 Briicke's muscle in most Reptiles and, in addition, into Miiller's muscle in 

 Birds has already been noted. In snakes the ciliary muscle has migrated to 

 the iris. 



Among all the activities of Vertebrates, the needs of the amphibious aninial 

 which reqviires to see both under water and in the air put the greatest strain 

 upon accommodation, a circumstance which applies both to fish which emerge 

 into the air and to land animals which go down into the water. The elimination 

 of the corneal refraction when it is immersed in water and its optical value in 

 air make the same eye strongly hypermetropic in the first medium and strongly 

 myopic in the second. So difficult is this optical transition that it is not 

 attempted by many forms. Thus certain fishes such as the climbing perch, 

 Anabas, which emerges on land crawling with the aid of the spines on the gill- 

 covers and on the anal fin, may be without effective accommodation or any 

 other detectable device for altering their relatively emmetropic state in water ; 

 in these vision in air must be so myopic as to serve merely for the detection of 

 light and shadow. Other fish such as the Indian mullet, Mugil corsula, have eyes 

 of the type designed for aerial vision with a lenticular-shaped lens ; this fish 

 swims feeding on the surface with the eyes out of water and its visual acuity 

 beneath the water must be relatively poor, a consideration which applies also 

 to such semi-aquatic animals as the ranid frogs, the crocodiles and the hippo- 

 potamus. Conversely, the penguins (unlike most other birds) are very myopic 

 in air ; while Sirenians, without accommodation and with a slight myopia in 

 air, appear to have so little visual acuity in either medium that vision can play 

 only a small part in their activities. 



WTiere the attempt is made to bridge over the optical transition demanded 

 by vision in two media, this may be accomplished in several ways. In the first 



