THE ENTEROPNEUSTA. 29 



it may be desirable to discuss briefly the new light which these 

 facts throw upon the affinities of the Enteropneusta, 



In 1881 MetschnikoflT published a detailed comparison of 

 Balanoglossus with the Echinoderms, comparing Tornaria with 

 Bipinnaria, showing that the resemblance is close, and con- 

 cluding with the suggestion that Balanoglossus should be in- 

 cluded among the Echinodermata in a separate division, 

 " Bilateralia." The branchial structures he compared to the 

 openings from the body cavities of Echinoderms. This view, 

 as thus expressed, receives no support from further observa- 

 tions, and would now appear to be untenable. 



As mentioned above, all the Enteropneusta possess a sup- 

 porting structure which is comparable with the notochord in 

 every way, except in extent and in the persistence of its con- 

 nection with the alimentary canal. Its resemblance to that of 

 Amphioxus is especially striking, for in Amphioxus the noto- 

 chord projects a long way in front of the mouth. It moreover 

 possesses gill-slits which are not only without parallel, except 

 among the Chordata, but also in structure, position, and de- 

 velopment, agree exactly with those of Amphioxus, in which 

 the slits acquire the same U-shaped form. 



The agreement in the position of the blood-vessels and 

 skeleton of the gill bars is also very close. The fact of their 

 gradual increase in number from before backwards throughout 

 life is another common feature. 



The position and mode of origin of the central nervous 

 system is also similar in both forms ; the invagination of the 

 dorsal cord in Balanoglossus being, however, only partial, while 

 that of Amphioxus is complete. 



The mesoblastic pouches suggest the same resemblance, 

 differing only from those of Amphioxus in number, being one 

 median and four lateral, while those of Amphioxus are one 

 median and twenty-eight lateral. As I have already pointed 

 out, the fate of this anterior pouch is in the two animals closely 

 similar. In both it is divided into two as the notochord grows 

 forward. In Amphioxus the division is complete, while in 

 Balanoglossus it is partial. In both, the backwardly-projecting 



