On the Ancestral Form of the Chordata.

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352 A. A. W. HTJBJBEOHT (this, according to Dohrn's interesting researches, is the case in one of the lowest of the vertebrate scale—Petromyzon), this invagination at the same time being directed towards the anterior termination of the notochord, and lying in its direct continuation (figs. 1 and 2), or (as is the case in the higher Vertebrates) not directly on the outer surface, but on that portion of the epiblast which has become the stomodEeum (fig. 6). In the latter case it arises as a median dorsal outgrowth from the mouth-cavity, directed towards that portion of the under surface of the brain where, between Prosencephalon and Metencephalon, the infundibulum travels downwards, this being at the same time the limit up to which the notochord extends forwards under the brain. The fact that an outgrowth from the brain thus grows downwards to meet this epiblastic invagination sufficiently indicates that in ancestral generations, where the hypophysis was a less rudimentary organ, some sort of connection existed between it and the cerebral thickening of the central nervous system. The constant presence in all Vertebrates of an organ so rudimentary as the hypophysis, and about the significance of which no plausible explanation has as yet been offered, has already been insisted upon above. Both facts are in favour of regarding it as a very ancient Structure, which was once of great importance, and had a different and at the same time a more definite physiological value. In tracing this ancestral significance, the relation to the brain and the somewhat less direct but, nevertheless, unmistakable relation to the notochord must not be lost sight of. We will now consider the ontogenetic and phylogenetic history of the Nemertine proboscis. In the lower Platyeltninths the researches of v. Graff, lately crowned by his brilliant monograph, have brought to light the different stages through which retractility of a portion of the tactile anterior extremity of the body, in which urticating elements are present, leads to the appearance of a definite proboscidian structure, which
ON THE ANCESTRAL FORM OF THE CHORDATA. 853 obtains a special musculature, and which finally (in Graff's Rhabdocoel family of the Proboscida) ha3 definitely become a proboscis, which is directly comparable to that of Nemertines, situated like it above the intestine, internally (externally when everted) clothed by the direct continuation of the outer layer of epiblast, and serving tactile and at the same time—through its nematocysts—aggressive purposes. The proboscis of Nemertines is thus directly related to this important structure of the lower flat-worms, as was already noticed in Gegenbaur's * Grundzuge ' (1870). We find urticating elements largely developed in the proboscidian coating of Palaeo- and Schizonemertiui, whereas in the Hoplonemertini the tactile significance may perhaps have come to predominate, if we judge from the extremely complicate arrangement and massive development of nervous tissue in the proboscis of these forms, which, moreover, is here provided with a central stylet-shaped armature. As to the ontogenetic development of the Neinertean proboscis, the great majority of authorities are in accordance that it develops as an iuvagination from the epiblast, which commences at the anterior extremity and gradually pushes backwards. Extensive details as to its successive developmental stages are, however, not yet to hand, only the principal fact above mentioned being generally accepted. It is highly important to notice that in this backward course the proboscis takes its way between the two anterior thickenings of the lateral nerve-cords, which in Carinella constitute the simplest Nemertean brain, and in other genera become more or less subdivided, the right and left halves being united by a thick commissure (fig. 3), ventral in relation to the proboscis, and by a thinner one dorsal to it. In all cases the proboscis passes through the ring of nerve tissue thus formed; in all cases the proboscidian sheath reaches forwards to the level of this nervous commissure, through which the proboscis passes (fig. 4). If we may look upon the spinal cord and brain of vertebrates as a dorsal coalescence of lateral trunks similar to those of the
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On the Ancestral Form of the Chordata.

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