WITH NOTES ON THE WEST INDIAN SPECIES. 



295 



MORPHOLOGY OF THE ENTEROPNEUSTA. 



In his monumental monograph of the Enteropneusta, Professor Spengel was led 

 to negative conclusions as to the outside affinities of the group. This result may be 

 partly accounted for by the fact that he was handicapped in being obliged to make 

 use of an unsuitable form, namely, Ptychodera minuta (the common species of the Bay 

 of Naples), as the basis of his work. I cannot help thinking that the theoretical 

 aspect of his labours might have assumed a different complexion if he could have 

 started with such a form as Ptychodera flava. 



As a treasury of facts it would not be easy to overestimate the value of this, 

 the eighteenth monograph issued by the management of the Zoological Station at 

 Naples ; and I hope I have made it clear in the foregoing pages, how much later 

 workers, like myself, are indebted to Professor Spengel for the great work which he 

 has accomplished. 



The result of my own observations, which have, intermittently, extended over the 

 best part of three years', has been not only to confirm my belief in the theory of 

 the Chordate affinities of the Enteropneusta, which was first definitely advocated by 

 Bateson, and has been accepted by most, if not all subsequent naturalists who have 

 dealt with the group, with the exception of Spengel, but, to carry to my own mind 

 the conviction that the Enteropneusta stand much nearer the direct line of Chordate 

 descent than has generally been supposed. 



Perhaps it may be admitted that I have brought forward a sufficient number 

 of new facts to justify a restatement of the case for the Enteropneusta. 



The views contained in Bateson's standard work on the direct development of 

 Balanoglossus (published during the years 1883 — 1886) were naturally and properly 

 based upon similarity of structure and origin. Spengel denied this similarity since it 

 fell short of identity. It now remains to found the theory upon change of function. 

 Such a theory not only dispenses with the necessity of the identity of structures, in 

 widely separated forms, which are supposed to be genetically related, but it requires that 

 they should be different. 



I think it right to assume that it would be quite out of place for me to attempt 

 the formidable task of discussing Dr Gaskell's Theory of the Origin of Vertebrates^. 

 I have quite enough on my hands in stating the case for the Enteropneusta. I may 

 be permitted to say that I, for one, regard Dr Gaskell's work as an important con- 

 tribution to the history and theory of the subject. Dr Gaskell has himself spoken of 

 his theory as an " earthquake hypothesis," and it may probably be regarded as the 

 culmination of that line of thought (namely, the reference of the Vertebrata to an 

 Articulate ancestry) which originated with Et. Geofifroy St. Hilaire and has numbered 



1 I made the acquaintance of Pt. flava in July 1896. 



2 W. H. Gaskell, " On the origin of Vertebrates, deduced from the study of Ammocoetes," Journ. Anat. 

 and Physiol., Vol. xxxii., p. 513 and Vol. xxxiii., p. 154. 



w. III. 41 



