ON THE ANCESTRAL FORM OP THE CHORDATA. 353 



obtains a special musculature, and which finally (in Graff's 

 Rhabdocoel family of the Proboscida) has definitely 

 become a proboscis, which is directly comparable to that of 

 Nemertines, situated like it above the intestine, internally (ex- 

 ternally 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 ' Grundziige ' (1870). We find urti- 

 cating elements largely developed in the proboscidian coating 

 of Palaeo- and Schizonemertini, whereas in the Hoplonemer- 

 tini the tactile significance may perhaps have come to predomi- 

 nate, 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 Nemertean pro- 

 boscis, the great majority of authorities are in accordance that 

 it develops as an invagination 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 thicken- 

 ings 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 pro- 

 boscis 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 



