ON THE ANCESTRAL FORM OF THE CHORDATA. 355 



Gegenbaur, * Das Kopfskelet tier Selachier,' pi. ix, figs. 1 and 

 2) be interpreted as a reminiscence of this connection ? 



A further character which is common to the two epiblastic in- 

 vaginations, known as hypophysis and as proboscis respectively, 

 is the shifting of their external opening. Amongst Nemertines 

 examples are found which form a parallel to the larger bulk of 

 Vertebrates (fig. 6) where the hypophysis does not arise (as in 

 Petromyzon) independently on the outer surface, but where it 

 is an invagination directed upwards from the roof of the mouth 

 cavity. Both in Malacobdella and in Akrostomum (a genus of 

 Hoplonemertini, instituted by Grube, in which I place, for ex- 

 ample, M'Intosh's Amphiporus bioculatus and Amph. lias- 

 tatus, and of which I have myself examined several specimens) 

 the opening for the proboscis is not independently 

 situated at the anterior extremity, but is found on 

 the dorsal wall of the intestinal tract, just inside 

 the mouth (figs. 7 — 10). I have the strongest reason to 

 believe, upon which I will not here further enter, that this 

 is a secondary modification, and that the separate opening 

 is the original state of things, phylogenetically related to the 

 separate proboscis of certain Rhabdocoels. 



The facts here advanced may justify us in looking 

 upon the Platyelminth {s. str. Nemertean) proboscis 

 as the homologue of the vertebrate hypophysis, as 

 was implied in the first part of our proposition. 



The proboscidian sheath in Nemertines is a cavity closed on 

 all sides and lined by an epithelium. It is situated in the 

 median dorsal line, above the intestine, just inside the muscu- 

 lar body wall, to which it is more or less firmly attached. Mus- 

 cular fibres serve to a large extent towards the thickening of 

 the tube we are considering. 



It terminates in the immediate vicinity of the anus, and 

 reaches forwards to just in front of the cerebral ganglia, which 

 in Schizo- and Palseonemertini are situated at a short distance in 

 front of the ventrally situated mouth. In the Hoplonemertini 



