X CYCLOSTOMATA 399 



and shorter, dwindling away until eventually nothing remains but their 

 opening into the urinogenital sinus. 



The skeleton of the Cyclostomes shows an interesting mixture of 

 primitive features and features which are clearly specialized. The noto- 

 chord never becomes replaced by a vertebral column but persists through- 

 out life. Cartilaginous neural arches make their appearance in the 

 Lampreys but they never become completed dorsally. In the Myxinoids 

 they are represented by a continuous plate of cartilage in the tail region^ 

 continued into fin rays which support the median fin. Similar fin rays 

 are present in the Lampreys but here they are separate from the neural 

 arches owing to the latter being incomplete dorsally. The cranium is 

 an open trough of cartilage. 



The cartilaginous branchial arches instead of being separate hoops 

 have become in the Lampreys joined together to form a continuous 

 basket-work. This is an adaptation to the peculiar method of breathing, 

 rendered necessary by the fact that the ordinary ingress for water — the 

 mouth — is liable to be blocked when the Lamprey is adhering to any 

 solid body. Water is expelled from the pharynx through the gill-openings 

 as in other fishes by muscular contraction of the pharyngeal region. In 

 this process the elastic basket-work is compressed ; but as soon as the 

 muscles -are relaxed it regains its volume, dilates the pharynx, and so 

 causes an inrush of water through the gill-openings. The Lampreys are 

 unique amongst vertebrates in this feature that the gill-openings serve 

 for inspiration as well as expiration. In the Myxinoids, in which this 

 mode of breathing does not occur, the branchial skeleton has become 

 reduced to inconspicuous vestiges. 



The blood system of the Cyclostome is arranged on the same general 

 plan as that of other fishes. The most conspicuous peculiarity is to be 

 found in the venous system — the right and left cardinal veins approaching 

 the mesial plane and undergoing fusion in the neighbourhood of their 

 opening into the duct of Cuvier. This fusion is followed by the complete 

 disappearance of the left duct of Cuvier, so that the whole of the blood 

 from the cardinal veins passes into the heart through the right duct. 



The brain is in the Lampreys comparatively primitive as regards its 

 general features. It is much elongated, it is only slightly enlarged as 

 compared with the spinal cord, and its various regions lie in a straight 

 line one behind the other. The membranous non-nervous character of 

 the brain roof which in the Dogfish is seen in the thalamencephalon and 

 medulla oblongata occurs here in the region of the mesencephalon as 

 well. The cerebellum is reduced to a small transverse ridge. The 

 thalamencephalon has two outgrowths from its roof, one in front of the 



