138 



ZOOLOGY 



SECT. 



confined to a germinal disc situated at one end of the elongated 



egg. The blastoderm thus formed extends gradually over the 



surface of the yolk, which it only completely encloses at a late 



stage, when the gill-clefts are all formed. 



Bdellostoma differs from Petromyzon and 



resembles the majority of the Craniata 



in the mode of development of the central 



nervous system, which is formed, not 



from a solid ectodermal keel, but from an 



open medullary groove the lips of 



which bend inwards and unite to form a 



medullary canal. 



4. GENERAL REMARKS. 



The Lampreys and Hags are undoubtedly 

 the lowest of craniate Vertebrata, but are 

 in many respects so highly specialised that 

 it is a matter of great difficulty to deter- 

 mine their affinities with the remaining 

 classes. The structure of the vertebral 

 column and of the cranium are undoubtedly 

 primitive in the extreme ; but in the deve- 

 lopment of what may be called the accessory 

 portions of the skull, such as labial carti- 

 lages, they show a singularly high degree 

 of specialisation. The branchial basket is 

 quite sui generis, the theory that its ver- 

 tical bars are true branchial arches, dis- 

 placed outwards during development, being 

 quite unproved. The absence of functional 

 jaws is very remarkable, seeing that in the 

 remaining Craniata these structures always 

 bound the mouth at a period when the 

 skull is in the stage of development in 

 which it remains permanently in Cyclo- 

 stomes : it is quite possible that their func- 

 tionless condition may be due to degenera- 

 tion accompanying the evolution of a 

 suctorial mouth. The brain, in spite of its 

 small size, is in some respects of a more 

 advanced type than that of some of the 

 true Fishes. The circumstance that the 

 pituitary pouch perforates the skull-floor from above and be- 

 comes early associated with the olfactory sac is unique among 

 the Vertebrata. The kidney of Bdellostoma is of the most 

 primitive type, and the presence of a large pronephros is a 



FIG. 820. Palaeospondylus 

 gunni (magnified), c. cirri ; 

 p.a. parachordal and auditory 

 region ; t.p. trabecular re- 

 gion ; x, backward processes 

 of skull. (After Traquair.) 



