Ichtliyopsida 459 



amphibians. No accessory auditory structures (tympanic cavity and 

 membrane) are developed. 



The lungs are well developed except that the left lung is usually 

 very much smaller than the right. This is apparently an adaptation 

 to the extreme narrowness of the body, because in reptilian snakes 

 there is a similar reduction of the left lung. 



The skeleton is fairly well ossified; the vertebrae, of which there 

 may be 250 or more, are amphicoelous. The skin is typically amphibian 

 in being thin and highly mucous, but in many genera it contains nu- 

 merous very small bony scales buried in its dermis. It is marked by 

 external transverse grooves encircling the body — increasing the ani- 

 mal's resemblance to an earthworm. 



Most of the Apoda are burrowing animals. One genus is aquatic. In 

 the burrowing species the eyes are very small, poorly developed, and 

 more or less deeply covered by skin or even lying beneath bones of the 

 skull. Presumably in compensation for poor sight or blindness, most of 

 these animals have a pair of sensory "facial tentacles." Each "ten- 

 tacle" is a minute papilla projecting from the bottom of a pit situated 

 between the eye and the nostril of one side. The papilla may, on occa- 

 sion, be distended (apparently by increase of blood-pressure within it) 

 and protruded from the mouth of the pit. A minute muscle retracts 

 the tentacle into the pit. The rich supply of nerves to the organ indi- 

 cates its sensory function. Its sensory nature is not definitely known, 

 but it is probably an organ of touch. Its like is not known in other 

 vertebrates. 



The Apoda, otherwise known as "caecilians" or "blindworms," 

 are widely distributed in the tropic regions of both Eastern and West- 

 ern Hemispheres, although not found in Madagascar. The best-known 

 genus is Ichthyophis, meaning "fish-snake," from southern Asia. 



Early Amphibians 



The Class Amphibia is very ancient. Fossil skeletal material identi- 

 fied as amphibian has been found in strata of the late Devonian Period 

 in Greenland (see Table 3, p. 411). The relative abundance of amphib- 

 ian fossil material in rocks of the Carboniferous Period indicates that 

 the group was in its prime at that time. 



The early amphibians were long-bodied animals with short legs 

 and a long tail. They differed from the modern in various ways. They 

 resembled fishes in that the girdle of the pectoral appendages was 

 attached to the rear of the skull, while there was no skeletal connect ion 

 between the pelvic girdle and the vertebral column. In modern amphib- 

 ians the pectoral girdle has no skeletal connection with the skull, and 

 the pelvic girdle is firmly joined to a single sacral vertebra. The early 



