584 



INVEKTEBKATA 



CHAP. 



AFFINITIES OF THE ENTEROPNEUSTA. 



The question of the vertebrate affinities of the Euteropneusta 

 stands or falls with the homology of the nerve cord, notochord, and 

 gill-slits with the similarly-named structures in the higher vertebrates. 

 Our own view, that the two sets of structures are really homo- 

 logous, and that Enteropneusta are a degenerate offshoot from the 

 base of the vertebrate stem, will be supported by evidence given 

 when the development of the Cephalochorda is discussed, since 

 these are the lowest forms admitted by all to be true Vertebrata. 



coe 



coe' 



FIG. 427. Longitudinal frontal sections through two embryos of Dolichoglossus pusillus 

 in order to illustrate the development of the body-cavities. (After Davis.) 



A, through younger embryo the proboscis coelom is arising as an anterior' evagination of the gut. 

 B, through older embryo the collar coelom and trunk coelom are arising by the transverse division of 

 posterior-directed tongues of the anterior coelom. coe 1 , the proboscis coelom ; coe 2 , the collar coelom ; 

 coe :{ , the trunk coelom. 



The development of Balanoglossus and Dolichoglossus is, however, 

 calculated to throw light on the previous history of the common 

 stock of Vertebrata and Enteropneusta ; it takes us, as Lankester 

 has well said, into prechordal times. It will be noted that the 

 description of the development of Dolichoglossus, given by Bateson, 

 is confirmed in all important points by the description of the 

 development of Balanoglossus as given by Heider and Morgan ; 

 we may therefore take it as being thoroughly well established. 

 Spengel's criticisms (1894) of Bateson's results, based on his own 

 observations on Tornaria larvae, made many years ago by methods 

 which are now superseded, are no longer valid, as he himself would 

 be the first to admit. 



