COMPARISON WITH FISHES; CJECILIANS 605 



zonule, the (secondary) absence of oil-droplets in most species, and the 

 green rods of the retina. The 'canals of Schlemm' of anurans are prob- 

 ably unique, and the ciliary muscles are surely not the same thing as the 

 teleostean 'tensor chorioideae'. 



But the amphibian eye is of course not wholly new. Though its iris 

 muscles, like those of elasmobranchs (and teleosts), represent independ- 

 ent inventions, their beginnings are perhaps seen in the contractility of 

 the unmodified iris epithelium of Protopterus. The mid- ventral 'ciliary 

 process' may have been inherited ultimately from the similar structure 

 in the chondrosteans, and may thus be a distant cousin of the campanula 

 Halleri. A strong point is the identical course of blood supply to the 

 vitreal vessels in anurans, Protopterus, and Polypterus (shared also with 

 Amia and with the catfishes among the teleosts). The urodeles probably 

 lack such vessels only through secondary loss. Most striking of all is the 

 resemblance of the visual-cell patterns of Protopterus and the amphib- 

 ians, emphasized diagrammatically in Plate I. When one considers how- 

 ever that the visual cells are phylogenetically the oldest and most funda- 

 mental elements in the whole eye, it should perhaps not be surprising 

 that in the present instance they seem especially reliable illuminants of 

 the dim pathway of phylogeny. 



(C) C^CILIANS 



The Caecilia or Gymnophiona are legless, worm-like amphibians which 

 are restricted to the tropics. The single, homogeneous family contains 

 55 species in nineteen genera. All, except the aquatic Typhlonectes, 

 spend most of their time underground. Their eyes are very small, but 

 have well-developed retinae and are useful for the registration of light- 

 intensities and directions. The most important sense-organ is the unique 

 retractile tentacle, which has both tactual and olfactory capacities. Sev- 

 eral adjuncts of the eye have been commandeered by this more useful 

 organ. The eyes of the Ceylonese Ichthyophis glutinosus and of Hypo- 

 geophis alternans, a resident of the Seychelles, have been the most com- 

 pletely investigated. 



The orbit is capacious, but is largely filled by the Harderian gland, 

 here serving to lubricate the sensory tentacle instead of the eye.* The 

 eyeball is two-thirds of a millimeter in diameter in a large Ichthyophis, 



*This is not the only instance in which the Harderian has ceased to serve the eye primarily, 

 and has taken on a new funaion (see pp. 424, 455, and 635). 



