ETE8 AND EARS OF VERTEBRATES. 405 



anterior portion of the central nervous 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 

 a 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. 



The Vertebrates are divided into two series or sub-branches: 

 the Acrania and Craniota ; the former type is represented 

 by but a single genus, order and class, the lancelet or Am- 

 phioxus. The sub-branch Craniotais divided into six class- 

 es, the Marsipobranchs, fishes, amphibians, reptilia, birds 

 and mammals. 



