EYES AND EARS OF. VERTEBRATES. 385 
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 (Sedachii) 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 or Chordata are divided into three series 
or sub-branches: the Urochordata, the Acrania, and Crani- 
ota. The Urochordata are represented by the class Tuni- | 
cata. The sub-branch Crantota is divided into six classes, 
the Marsipobranchs, fishes, amphibians, reptilia, birds, and 
mammals. 
