96 VERTEBRATE ZOOLOGY 



After living in the larval state for from three to four years, during 

 a few weeks in the winter it undergoes a profound metamorphosis of 

 structure and habits and emerges as a small juvenile lamprey. 

 During the metamorphosis the buccal funnel is developed; the eyes 

 come to the surface and begin to function; the median fin becomes 

 specialized; the skull and branchial basket become more elaborate, 

 the gall bladder is lost except for a few vestiges. The most remarkable 

 transformation, however, concerns the pharyngeal region and the 

 food- concentrating mechanism. The dorsaLpart. which represents 

 gr^yj ^ o rnrnes pinched off from the old pharynx 



+.hg.i. hrpfllrg into the rnouth. The endo- 



becomes pinched off frr>m the ventral floor of the pharynx 

 and coiled up into a thyroid aland. The middle part of the pharynx 

 remains as the blind, lung-like branchial sac which operates independ- 

 ently of the mouth. 



All of these facts seem to point unmistakably to an affinity between 

 the cyclostomes and the cephalochordates. This helps to bridge the' 

 gap between the vertebrates and the lower chordates, It has been 

 suggested that Amphioxus is simply a case of psedogenesis, or ex- 

 treme racial senescence involving de*XTk>piiiunt arrrestecTin a larval 

 condition. According to tnis view, which is not acceptable to biolo- 

 gists^ln general, Amphioxus is a permanent larva of some species of 

 cyclostome. The fact that Ammocoetes spends as much as four years 

 in the larval stage lends color to this contention, for it indicates a 

 developmental retardation, that, if carried a step farther, might result 

 in a permanent paedogenetic condition akin to that which occurs in 

 the Amphibia (Axolotl larva of Amblystoma). There are, however, 

 arguments against this view which would be out of place here. We 

 have already put ourselves on record as supporting the theory that 

 the ancestor of the vertebrates was Amphioxus-like. Possibly, how- 

 ever, such an ancestor was more like Ammoccetes than like Amphi- 

 oxus. 



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