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No. IX.— THE CEPHALOCHOEDA : "AMPHIOXIDES." 



Bij H. O. S. Gibson, B.A., New College, Oxford. 



{Communicated by J. Stanley Gardiner, M.A., F.R.S., F.L.S.) 



(Plate 15 and 4 Text-figures.) 



Eead 3rd June, 1909. 



CONIENTS. 



Page 

 Distribution 214 



Anatomy — (1) The Larvje 217 



(2) The Metamorphosed Animals 233 



"What is Amphioxides ? 239 



How far is Ampliioxides primitive ? 244 



INTRODUCTION. 



The remarkable pelagic forms now generally known under the name of " Ainphioxides " 

 were first brought into prominence by the excellent monograph of Goldschmidt (1905) 

 based on the 29 animals collected by the ' Valdivia ' Expedition. His examination of 

 tliese led him to the belief tliat they represented a primitive family of pelagic Acraniates, 

 characterized by the possession of an unpaired median series of gill-slits and an 

 asymmetrical left-sided mouth and by the absence of atrium and oral cirrhi, from which 

 the Branchiostomidse were evolved by adaptation to a littoral existence. Subsequently 

 (1906) he has admitted the probability that " Am/ihioxides" is a " neotenically developed 

 Branchiostomid-larva," maintaining, however, his belief that its structure, as also that 

 of the well-known " Ainphioxus larva," represents a primitive phylogeuetic condition. 



Mr. Stanley Gardiner's large and carefully preserved collection — consisting of some 

 190 specimens — has given an admirable opportunity of testing these conclusions. It 

 does not, unfortunately, include any forms undergoing metamorphosis, to prove that 

 Amphioxides is a larval form : but it does contain two animals already metamorphosed, 

 which may with some probability be regarded as a link between Amphioxides pelarjiciis 

 and the remarkable form Asymmetroii lucayanuvi, well known from the description of 

 Andrews (1893). In addition, fresh points in the anatomy of Atnphioxides itself, which 

 are now brouglit to light, make its larval character almost certain. These new points 

 also weigh heavily against the theoretical conclusions as to the primitiveness of the 

 Amphioxides type of organisation. I am compelled to differ from Dr. Goldschmidt 

 as to these conclusions, as upon a number of smaller points : but I should like to pay a 

 tribute here to his admirably full and lucid treatment of the subject, which will have 

 greatly lightened the task of all subsequent investigators of these forms. 



SECOND SERIES.— ZOOLOGY, VOL. XXII. 29 



