On the Ancestral Form of the Chordata.

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362 A. A. W. HUBREOHT conversion of a hollow proboscidian sheath into a solid notochord, the more so as functionally the proboscidian sheath in Nemertines may already be looked upon as an axis, around which the other organs symmetrically arrange themselves as they do around the notochord in Vertebrates. It must at the same time be borne in mind that the muscular coating in this posterior portion is found to be considerably reduced and replaced by a more or less homogeneous and comparatively thin sheath. Having thus far considered the arguments which are at present available for insisting upon the homology between proboscis and hypophysis, on the one hand, and between proboscidian sheath and notochord on the other, it now remains to inquire whether there are points in the anatomy of Nemertines, beyond those just now exposed, which either corroborate or weaken the evidence hitherto advanced in favour of the suggestion that the Nemertines, more than any other known group of Invertebrates, resemble the ancestors of the Protochordata. I need hardly insist upon the fact that I do uot advocate any direct relation between existing Nemertines and existing Vertebrates; my argument goes no further than the attempt to show that the general plan of structure of a Nemertine is more in accordance with that of a vertebrate animal than is, for example, that of the Archiannelida, and that the link connecting Coelenterate ancestors with Vertebrate descendants has most probably comprised forms in which two lateral nerve-cords were present, ultimately coalescing dors ally, and in which an epiblastic proboscis served for purposes which have either been given up or have been replaced by others when the animals gradually exchanged the Platyelminth for the Chordate type. Simultaneously with this passage from the Coelenterate type to the Chordate the highly important processes must have been gone through which lead to the formation of a body-cavity, separate from thearchenteron with which,as embryology teaches us, certain diverticula are originally in open communication, ultimately becoming constricted off and developing into the splanchnic and somatic layers, which have the body-cavity between them.
ON THE ANCESTRAL 1'OJBM OF THE OHORDATA. 363 The brilliant researches of Lang on Gunda segmentata, and of Hatschek on the ' Development of the Amphioxus,' must here in the first instance guide us; and anybody having carefully perused those important contributions, and having compared them with each other, must have been struck by the great probability of the view advocated by Lang, that the alimentary diverticula of these Platyelminths are the fore-runners of the arrangement of the ccelom in the higher enteroccelous worms, and that through this link a glimpse is gained at the road along which Annelids may have developed out of an ancestral Platyelminth stock. 1 On the other hand, the stages in the development of Amphioxus, where a double set of lateral divevticula of the archenteron is present (fig. 12), which ultimately become converted into the mesoblastic somites, appear to be of very great importance, in so far as they render it highly probable that in the ancestry of vertebrates certain forms with metamerically placed alimentary caeca must have obtained, of which the larval stage of Amphioxus is the reminiscence. In the remaining vertebrates the primitive alimentary diverticula giving rise to the ccelom are reduced to two. This appears to be an ulterior simplification. An attempt to explain this simplification, and to bring the process of the formation of the ccslom in Amphioxus under the same head with that in the other vertebrates, has at the present moment not yet been made. It is, however, sure to be made some day by the leading authorities on the subject. For the present it may suffice to point out that the ulterior development of the mesoblastic somites in the bulk of the vertebrates re-establishes the homology with the more primitive processes in Amphioxus. For us this larval stage of Amphioxus is all the more interesting, because it must lead up to Platyelminths, corresponding with Gunda in the presence of alimentary caeca, nietame- 1 It must Lere be noticed that Lang has only very lately ('Biologisches Centralblatt,' May, 1883) emitted serious doubts concerning bis own propositions. It remains to be seen whether future investigations will not tend to confirm bis original suggestive hypothesis rather than these doubts.
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On the Ancestral Form of the Chordata.

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