PRINCIPAL MODIFICATIONS OF THE VERTEBRATE TYPE. 23 



guished by other peculiarities. The higher forms referred to all have the 

 organ known as the allantois, an appendage of the embryo, which is lacking 

 in the lower forms. The comparative anatomist finds many points of re- 

 semblance between the various classes of fishes, on the one hand, and the 

 amphibia, on the other, and indicates this relationship by the use of the term 

 Ichthyopsida, which means " fish-like." In our present classification the term 

 Ichthyopsida is synonymous with Anamniota. The comparative anatomist 

 further recognizes a close relationship between birds and reptiles, and puts 

 these together under the common designation of Sauropsida, or " reptile-like." 



As regards the fishes, many classifications are more or less in vogue at the 

 present time. For the purposes of this book, the following names for the 

 classes have been adopted as names generally understood and sufficiently 

 exact to meet our needs : The lowest fishes are the hag-fishes and lampreys, 

 constituting the group of Marsipobranchs. Next comes the group compris- 

 ing the sturgeon and its allies, for which we have retained the old term of 

 Ganoids. To these fishes the central position in the system must be assigned, 

 and it is probable that the higher fishes are more or less directly descended 

 from Ganoid-like forms. They fall into three further classes, of which the 

 largest and most varied is that of the bony fishes, or Teleosts. Another class, 

 known as the Elasmohranchs , comprises the sharks, skates, rays, and electric 

 fishes. The last class is known as the Dipnoi, or lung fishes, which comprise 

 only three living forms, the Ceratodus, living in Australia, the Protopterus 

 in Africa, and the Tepidosiren in South America. 



The amphibia are divided into two classes, the Urodela, of which the 

 newts and salamanders are familiar examples, and the Anura, of which the 

 frogs and the toads are the best known representatives. 



As to the reptiles, it is unnecessary to consider their classification, as we 

 shall not have much occasion to refer to them, our knowledge of their em- 

 bryology being very fragmentary at the present time, save for a rather ex- 

 tended series of observations on the development of lizards. As regards birds, 

 it may be noted that embryologists have worked chiefly upon the chick, which 

 has been for a century the classic object of embryological study. There are com- 

 paratively few observations on the development in other species of birds. 



Mammals are divided into three principal classes. Of these, the lowest is 

 that of the Monotremes, of which the only living representatives are found in 

 Australia and neighboring islands, a very few species concerning the develop- 

 ment of which very little is as yet known, but which are of importance, as they 

 resemble in certain respects the reptiles and assist us in drawing comparisons 

 between the reptilian and the mammalian types. Of this class, the Australian 

 duck-bill may be mentioned as typical. 



