608 



COLLEGE ZOOLOGY 



Cyclostomes and fishes 



The lowest vertebrates, that is, the forms 

 most nearly related to the primitive chor- 

 dates such as the amphioxus are the cyclo- 

 stomes. These (Fig. 240) are eel-like verte- 

 brates without jaws and with cartilaginous 

 skeletons. Although the cyclostomes repre- 

 sent a primitive level of vertebrate develop- 

 ment, they are not themselves ancestral 

 vertebrates. 



The fossil record shows that the oldest and 

 most primitive of fossil vertebrates were 

 small, fishlike, jawless creatures called ostra- 

 coderms (Agnatha). 



Next above the ostracoderms come the 

 placoderms, which are extinct fishlike ani- 

 mals provided with jaws. The placoderms 

 appear to have given rise to the bony fishes 

 ( Osteichthyes ) and, as a side branch, to 

 the elasmobranchs. 



Amphibia 



The lungfishes resemble the amphibians 

 in so many developmental and structural 

 features that they were once thought to be 

 the actual amphibian ancestors. But it is 

 known that these features were also present 

 in their near relatives, the crossopter^'gians 

 (bony fishes), from which it now seems 

 probable that the amphibians are actually 

 derived. The lungfishes now are regarded as 

 a side branch from the same ancestral stock 

 as that from which the crossopterygians 

 originated. 



Reptilia 



The reptiles are descendants of ancient 

 amphibians. Because of their method of re- 

 production, they became the first completely 

 land vertebrates. The reptilian egg can be 

 laid on land, thus making water existence 

 unnecessary. The most primitive living rep- 

 tiles are the Rhynchocephalia; these are rep- 

 resented by the single living species Spheno- 

 don punctatus (Fig. 283) of New Zealand. 



Aves 



Birds sprang from reptilian ancestors; in 

 fact, they have appropriately been called 

 "glorified reptiles." This correctly implies 

 that the birds are closely related to the 

 reptiles, and one of the earliest known forms 

 [Archaeornis, Fig. 331) might actually be 

 called a flying reptile. The toothed birds 

 are considered the forerunners of the mod- 

 ern toothless birds. 



Mammalia 



Mammals are of special interest since this 

 class of vertebrates includes man. The earl- 

 iest living mammals, the monotremes (egg 

 layers) are descended from reptilian ances- 

 tors. Because of their reptilian characters, 

 they are considered veritable "missing links." 

 Although the monotremes are the most 

 primitive of the mammals, they are so spe- 

 cialized in many ways that they cannot 

 themselves be regarded as ancestral types. 

 Above the monotremes are placed the mar- 

 supials (pouched mammals), and finally 

 the Eutheria (placentals), which are the 

 highest of all animals. The primates, the 

 group that includes man, are discussed in a 

 later section. 



VARIATIONS WITHIN 

 A SPECIES 



An attempt was made earlier in this text 

 to define a species. There it was noted that 

 the individuals of a species may differ 

 markedly among themselves. Such differ- 

 ences are known as variations. Careful stud- 

 ies of variations have revealed that they are 

 of two principal types: (1) genetic or in- 

 herited variations and ( 2 ) environmental or 

 noninherited modifications. This distinc- 

 tion is of the greatest importance, since only 

 inherited variations can be passed on to 

 offspring and can have any influence in 

 changing the genetic constitution of the spe- 



