X AMPHIOXUS, CYCLOSTOMATA 395 



myotomes and clear supporting blocks of the median " fin," but there 

 is no modification of the connective tissue to form cartilage or bone. 

 Slender rods of chitin-like material support the pharyngeal wall between 

 the gill-slits, and others support the buccal cirri. 



The nervous system possesses the normal vertebrate character of a 

 thick-walled tube, with very small central cavity, lying dorsal to the 

 notochord. But it presents the quite peculiar feature that it is not 

 swollen at its anterior end to form a brain projecting forwards beyond 

 the tip of the notochord. In Amphioxus the central nervous system 

 comes to an end some distance behind the tip of the notochord and the 

 only suggestion of brain about it is that the central canal expands to 

 form a considerable cavity at its front end recalling the ventricular 

 cavity of the brain. This cavity frequently dips down ventrally in a 

 manner highly suggestive of the infundibulum and we are probably 

 justified in suspecting that the brainless character of Amphioxus is a 

 secondary acquirement, the once-present brain having atrophied, leaving 

 only the ventricular cavity as an indication of its previous existence. 

 Such reduction of the brain during the evolutionary history of Amphioxus 

 is readily understandable when we bear in mind (i) the passive method 

 of obtaining the food, and (2) the absence of the great organs of sense. 



The group Cyclostomata includes the lowly and degenerate fishes 

 known as Lampreys (Petromyzon — Fig. 173, A) and Hagfish or Borers 

 (Myxine — Fig. 173, B ; Bdellostoma). While the Cyclostomes are primarily 

 marine the Lampreys are anadromous and some species have become 

 permanent inhabitants of fresh water. These fishes further show the 

 remarkable peculiarity, unique amongst vertebrates, of being partially 

 parasitic in their habits, and it is this parasitism which has brought in 

 its train general degeneration and at the same time various speciaUzations 

 of structure. 



The general form of the body is eel-like, with a median fin varying 

 in the extent of its development in different genera and continuous 

 posteriorly round the tip of the protocercal tail. There are no paired 

 fins. Another important negative feature of the Cyclostomes is the 

 complete absence of anything of the nature of scales. The skin is soft 

 and glandular, and the Myxinoids possess along each side of the body 

 ventrally a row of very large epidermal glands (Fig. 173, B, g) -which 

 produce a sticky glutinous slime. This is produced at will in enormous 

 quantities ; it consists of very fine threads and no doubt is valuable as 

 a protection from the attacks of enemies. 



The alimentary canal commences with a stomodaeum or buccal cavity 



