EYES AND EARS OF VEBTEBMATES, 385 



anterior portion of the central nevTous system. The differ- 

 ence between the highly-developed eye of a cuttle-fish and 

 a bony fish, for example, consists in the fact that the rods 

 and cones (similar to those of the invertebrate eye) forming 

 3, layer (the bacillar layer) behind the retina, are in the ver- 

 tebrate eye turned away from, while in the invertebrates they 

 are directed toward the opening of the eye. 



The ear of Vertebrates is at first a primitive otocyst, or 

 ear-vesicle, which is gradually cut off and enclosed, forming 

 a cavity of the skull. As we rise towards the mammals, the 

 ear becomes more and more developed until the inner, 

 middle, and outer ear is formed ; the Eustachian tube being a 

 modification of the first branchial cleft, forming the spiracle 

 in the sharks {Selachii) . and Ganoids. 



In the lancelet a head is scarcely more set apart from the 

 rest of the body than in many invertebrates. In the fishes 

 and Amphibians the head is not separated by a neck from 

 the trunk ; in reptiles the neck begins to mark off a head 

 from the thorax, while in the birds and mammals the head 

 is clearly demarked, the degrees of cephalization and trans- 

 fer headward of those features subordinate to the intellec- 

 tual wants of the animal becoming more striking as we 

 ascend through the mammalian series to the apes, and finally 

 man. 



The development of Vertebrates can scarcely be epitomized 

 in a few lines. The mode of growth of Amphioxus is a 

 general expression for that of all Vertebrates, for all develop 

 from fertilized eggs, which undergo total or partial segmen- 

 tation of the yolk, become three-layered sacs and assume the 

 peculiar vertebrate characters, the development of the mam- 

 mals differing from that of the other classes only in compar- 

 atively unimportant features. 



Tlie Vertebrates or Ghordata are divided into three series 

 or sub-branches: the Ur.ochordata, the Acrania, and Grani- 

 ota. The Urochordata are represented by the class I'uni- 

 cata. The sub-branch Graniota is divided into six classes, 

 the Marsipobranchs, fishes, amphibians, reptilia, birds, and 

 mammals. 



