

CHAPTER XIX 



INTRODUCTION TO SUB-PHYLUM IV, CRANIATA. 

 THE CYCLOSTOMATA 



ALL the remaining Vertebrata are distinguished by possessing a 

 skull and brain, and are grouped together as Craniata. The Craniata 

 are separated by a deep gap from the lower forms : but they them- 

 selves present a fairly continuous and graded series from the lowest 

 to the highest forms, and their comparative anatomy, especially when 

 we take into account the fossil representatives of the sub-phylum, 

 gives us a fairly good idea of the course which the evolution of 

 Vertebrata has pursued ; so much so indeed, that the group might 

 be compared to the fairly reliable and complete records of a country 

 during the historical period, whilst the Hemichorda, Cephalo- 

 chorda and Urochorda represent the few scattered and scarcely 

 decipherable documents of prehistoric epochs. 



The Craniata are defined, as we have seen, by the possession of 

 a skull and a brain, though these are only two of the 

 many characters which distinguish them from the 

 other Vertebrata. The skull is composed of either cartilage or 

 bone ; and even in cases where the adult skull is completely bony, in 

 the embryo the bone is partly, at any rate, represented by cartilage. 

 Cartilage and bone are really only two peculiar modifications of 

 connective tissue whose fundamental characters it may be useful 

 to recall. There is in every form of connective tissue a gelatinous 

 ground substance traversed by fibres, and applied to these fibres are 

 cells, which are connected with one another by delicate threads of 

 protoplasm and which secrete the greater part of the ground 

 substance and fibres contained therein. In cartilage, the ground 

 substance becomes cheesy in consistence, the fibres being masked, 

 and the cells are arranged by twos and threes in little pockets. 

 These cells are surrounded by concentric layers of dense 'chondrin' 



