486 ESSENTIALS OF ZOOLOGY 



Cyclostomes, the simplest vertebrates, are most closely related to 

 Amphioxus, Avhich has been suggested as the protochordate most 

 similar to vertebrates. These very primitive fish have an eellike 

 body without paired fins and without jaws. They have from seven 

 to fourteen branchial (gill) apertures in different species, and all 

 of them possess skeletons composed of cartilage. The cartilaginous 

 skull is not entirely closed dorsally but resembles a trough with 

 bars over the roof. The anterior end of the nerve cord has expanded 

 to become a brain. Next in order of complexity are the Elasmo- 

 branchii, which possess well-developed, paired appendages (fins) 

 and jaws. They also have a cartilaginous skeleton, but the skull 

 is much more complete dorsally. The number of gill arches is re- 

 duced to five, but the apertures are uncovered as in cyclostomes. 

 The number of aortic arches has been reduced from the sixty to 

 ninety pairs of Amphioxus or seven pairs of lamprey to five pairs. 

 The group of ganoid fishes, which was the dominant Devonian ani- 

 mal, is generally conceded to have Elasmobranch ancestry. Most 

 ganoids have more or less cartilage along with the bony structure 

 of the skeleton. Their gills are covered with an operculum, and 

 there are only four aortic arches. The bony ganoids are usually 

 thought to be the ancestors of true bony fish. 



There is an extinct form of Amphibia, Stegocephalia, which shows 

 relations to the ganoid fishes and for this reason the ganoids are 

 usually named as ancestors of Amphibia. Some authors hold that 

 the lungfishes, which represent an independent branch of the Elas- 

 mobranch group, are the ancestors of Amphibia because of their 

 ability to breathe air and live out of water.' However, the former 

 view of the phylogenetic relation between fish and amphibia is most 

 commonly held. Amphibia have well-developed bony skeleton with 

 paired appendages for locomotion on land. Lungs have appeared 

 as a means of aerial respiration, and the aortic arches have been 

 reduced to three. 



Reptiles are supposed to have descended from Stegocephalia also, 

 with most modern reptiles coming by way of Rhynchocephalia which 

 is represented by one living species, Spkenodon punctatum. The 

 snakes, lizards, crocodiles, and the extinct dinosaurs have probably 

 branched frODi this group, while the turtles are thought to have 

 descended through Therornorpha, another extinct branch of Stego- 

 cephalia. The dinosaurs are credited with the ancestry of birds by 



