CLADISTIAN AND DIPNOAN EYES 589 



but cannot now be considered at all close to the roots of the amphibian 

 stock. The lungfishes strictly speaking (Dipnoi or Dipneusti), repre- 

 sented only by Protopterus, Lepidosiren, and Neoceratodus (living re- 

 spectively in African, South American, and Australian rivers) , are not 

 too close to the amphibians either. The latter arose from the Cross- 

 opterygii, which were offshoots from an extinct dipnoan line. But unless 

 and until the eye of the newly-discovered sole living crossopterygian 

 fish, Latimeria chalumnce, is sometime described, we have only the dip- 

 noans to indicate to us how the amphibian eye may have evolved from 

 its ultimate chondrostean ancestor (see Fig. 60, p. 135). The cladistians 

 may be expected to be of some help also, for their connection with the 

 chondrosteans is very close to the stem of the dipnoan-crossopterygian 

 line. 



Cladistians — Nothing is known concerning Calamoichthys, and the 

 eye of Polypterus has had no more complete studies than the sketchy one 

 of Leydig in 1854. The sclera exhibits the usual piscine hyaline-cartilage 

 cup. In the chorioid there is a silvery layer, but it is unclear whether this 

 is a guanin tapetum lucidum or an argentea (see p. 240). There are 

 vitreal vessels, with their main vascular supply coming in at the mid- 

 ventral point of the ora as in Amid and amphibians (suggesting that 

 the primitive chondrosteans may have had such vessels — see Fig. 60). 

 There is no trace of any mechanism of accommodation. The retina is 

 quite unknown; but the optic nerve has been described as having a num- 

 ber of branches, so that the optic papilla is multiple (see p. 367) . 



Dipnoans 



See also pages: 



135-6 Fig. 60, taxonomy, anatomy 200, 216-7 visual cells, oil-droplets 



150, 160, 220, 222-3 pupil 263-4, 216-7 accommodation 



187 lack of area centralis 525, 537 dermal color changes 



Only the eye of Protopterus has been given any complete descriptions 

 (by Hosch in 1904, Grynfeltt in 1911), and these have been faulty. 

 All three genera are said to have nothing like a falciform process, and 

 no accommodatory structures. There is of course no canal of Schlemm. 

 Lepidosiren is claimed to lack the oblique muscles; little is known about 

 its eyeball. The dearth of knowledge about Lepidosiren is of no great 

 importance, since this form is in the same family as Protopterus. But 

 Neoceratodus deserves a thorough investigation, for this large fish has 

 none of the appearances of degeneracy characteristic of the Lepido- 



