28 



Mr. W. Eateson. On the Development of [Dec. 18, 



glandular body which is attached to the end of the notochord. As 

 far as could be observed through the body wall of the living animal, 

 the pulsations in this region are those of the tube lying immediately 

 upon the notochord. Moreover, this structure gives off all the blood- 

 vessels of the body, both according to Spengel and from my own 

 observations, while none of them are supposed to arise from the 

 glandular portion, which has besides no distinct muscular wall. 



The " glandular body " lying at the sides of these structures is in 

 appearance closely comparable to the so-called " heart " of Asterias, 

 being perforated by a network of blood-vessels (Spengel). 



The posterior body-cavities are prolonged into the cavity of the 

 temporary tail, remaining separated by the vertical septum. 



It may be desirable to discuss briefly the new light which these facts 

 throw upon the affinities of the group Enteropneusta. 



In 1881 Metsctmikoff: published a detailed comparison of Balano- 

 glossus with the Echinoderms, comparing Tornaria with Bipinnaria, 

 showing that the resemblance is close, and concluding with the 

 suggestion that Balanoglossus should be included among the Echino- 

 dermata in a separate division, " Bilateralia." The branchial struc- 

 tures he compared to the openings from the body cavities of 

 Echinoderms. This view, as thus expressed, receives no support 

 from further observations, and would now appear to be untenable. 



As mentioned above, all the Enteropneusta possess a supporting 

 structure which is comparable with the notochord in every way, 

 except in extent and in the persistence of its connexion with the 

 alimentary canal. Its resemblance to that of Amphioxus is especially 

 striking, for in Amphioxus the notochord 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 development, 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, 



