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TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



the neurocranium and the visceral arches as there is in existing fishes. The floor of the 

 brain was also the roof of the orobranchial chamber and this continuous mesodermal tissue 

 extended downward between the gill-chambers, moulding itself around them and preserving, 

 so to speak, a natural cast both of the partitions between the gill pouches and of the tubes 

 for the nerves and blood vessels. 



Stensio has demonstrated that many of the peculiar features of the central nervous 

 system of the modern lamprey (Fig. 3 J) were present in the ostracoderms, and for this 

 and other reasons modern cyclostome structures may now be interpreted in the light of 

 evidence from ostracoderm anatomy and vice versa. But whereas in the ostracoderms the 

 orobranchial or visceral skeleton was merely the undifferentiated supporting tissue sur- 

 rounding the gill-chambers, which was continuous above with the endoskeletal floor of the 

 braincase, in adult lampreys the branchial skeleton (Fig. 3B) has become a somewhat 

 irregularly fenestrated basket surrounded by thin muscle bands. This branchial basket 

 differs from that of the shark in the lack of definite joints or articulations and in the some- 

 what irregular form of the cartilaginous tracts surrounding the branchial chambers. In 

 embryo sharks the contrast between the neurocranium and the visceral arches is very sharp 

 even from the first appearance of the gill-bars, but in the pre-gnathostome stage, as we 

 have seen, this contrast was far less conspicuous. 



branchial basket 



B 



naso-hupophyseaj , 



■^ . braincaui/u 



opening ■ ^ 



votochord 



mouth 



A 



Fig. 3. A. Diagram showing relations of brain to orobranchial chamber in larval lamprey. Based on figures of Parker 

 and of Goodrich. 



B. Cartilaginous syncranium of adult lamprey. After W. K. Parker. 



According to Stockard the so-called tongue of cyclostomes represents the mandibular or Meckelian bars of gnathostomes, 

 since it has similar relations to the visceral pockets and clefts. 



