GENERAL CONCEPTIONS. 



further classes, of which the largest and most varied is that of the bony fishes, 

 or Teleosts. Another class, known as the Elasmobranchs, 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 Lepidosiren 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. The two types are easily distin- 

 guished by the presence or absence, respectively, of the tail in the adult. 



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 embryology 

 being very fragmentary at the present time, save for a rather extended series 

 of observations on the development of lizards. As regards birds, it may be noted 

 that embryologists have worked chiefly upon the chick, which iias been for a 

 century the classic object of embryological study. There are comparatively few 

 observations on the development of 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. 



The second class is that of the Marsupials, familiar to us in America 

 through the common opossum. In Australia there are many genera and species 

 of marsupials. 



The third class comprises the majority of well-known mammals, and may 

 be termed the Placentalia, and, for embryological purposes, it is convenient to 

 consider the Placentalia as forming two principal subclasses, the animals with 

 claws and the animals with hoofs, the Unguiculates and the Ungulates. Of the 

 Unguiculates, we shall have occasion to refer to the Insect-Ivor a, of which the mole 

 may serve as a type; the Cheiroptera, or bats; the Rodents, including the rats. 

 guinea-pigs, rabbits, etc.; the Carnivora, cats, dogs, and allied animals; and, finally, 

 the Primates, which include the lemurs, monkeys, apes, and man. 



Of the Ungulates, we shall have occasion to refer chiefly to the pig and the 

 sheep. The following table presents in their proper order those animals which 

 we shall have occasion to consider. 



Annelida 

 Atriozoa 



Tunicata (Ascidia) 



Cephalochorda 

 Amphioxus 



