Evolution 



between the successive gill clefts. These branchial arches bear a 

 certain resemblance to the basket-work arrangement of cartilage 

 in the round-mouths, but for various reasons are not regarded as 

 having been derived from the latter. It is from the first pair of 

 these arches that the jaws are formed, organs which make their 

 first appearance in the lower fishes. The skeleton shows great 

 development in other directions. The notochord is present in 

 its primitive condition during the earlier stages of development, 

 but it becomes surrounded, and in many cases largely suppressed, 

 by the portions of the vertebrae. Each 

 vertebra consists of an upper and a lower 

 portion, the upper forming an arch round 

 the nerve cord and the lower bearing 

 lateral processes or ribs. In the higher 

 forms the two portions become united 

 round the notochord, and the resulting 

 vertebra may encroach inwards until it 

 becomes solid, the notochord then remain- 

 ing only as a series of small pieces of 

 cartilage between the successive units of 

 the vertebral column. There is also, of 

 course, a skeleton in connection with the 

 limbs, but this does not yet correspond in 

 detail to that of the other classes of verte- 

 brates. The brain is much more highly 

 developed than in the round-mouths ; in 

 many forms, particularly, there is a considerable development of the 

 cerebral hemispheres, a portion of the fore brain, and the seat of the 

 higher intelligence. The eyes and ears show the same main features 

 as in the higher groups. The ear has three semicircular canals, the 

 same number as in man, as against two in the lamprey and one in 

 the hag. Fishes are possessed of a peculiar ' sixth sense,' the 

 organs for which are situated in two lines running along the sides 

 of the body, the latter forming a familiar feature of a cod or 

 whiting. The nature of this sense is not definitely known, but it 

 appears to be of the nature of a very refined appreciation of wave 



90 



FIG. 69. Mouth of lamprey. 



