446 MORGAN. [Vol. V. 



The relationship of Tornaria to the Trochophore (or even to 

 the Veliger) is much more difficult to safely determine than the 

 relationship to Auricularia. Balfour expressed himself — and 

 the view has been taken by others — as inclined to believe in a 

 relationship to the Trochophore. Metschnikoff has shown the 

 very great difficulties in the way of such a comparison. He 

 pointed out the absence of the anterior enteroccel in the Tro- 

 chophore, the differences in the position of the ciliated bands, 

 the presence of two muscle bands from the apical plate of the 

 Trochophore, and the ectodermal origin of the oesophagus of 

 the latter. To these we may now add that the apical plate of 

 these two forms is only superficially similar. The nerve-fibre 

 layer in Tornaria does not form nerve bundles and branching- 

 ganglion cells as in the Trochophore. The eyes are not at all 

 similar, but exceedingly different, most probably essentially so. 

 There is no structure in Tornaria similar to the head kidneys 

 of Trochophora. The ciliated bands have essentially different 

 histological structures. In the Trochophore they are formed 

 by large columnar cells, while in Tornaria the longitudinal band 

 is formed of exceedingly small and closely crowded cells. Thus 

 while we cannot positively deny that these larvae have had a 

 common form in the past, yet the evidence we have at present 

 goes against any such supposed relationship. 



So much for the more important larval forms. We may now 

 for a moment examine into the affinities between the adult 

 Balanoglossus and other groups. The Nemertinean is a form 

 to which many zoologists have turned to find similarities to 

 Balanoglossus. Both are long animals living in the sand, 

 having each a delicate and richly ciliated ectoderm ; the mouth 

 with a proboscis at one end, the terminal anus at the other. 

 Both have closed blood-vessels and serially arranged gonads. 

 I admit a sort of natural suggestiveness in these comparisons, 

 but the points of similitude formerly emphasized have been 

 largely shown to be false homologies, and I have been unable to 

 find any important resemblances which are common to the two 

 groups. The proboscis of the Nemertine is entirely different 

 from the so-called organ of Balanoglossus, and the nervous 

 system, gill slits, notochord, enteroccels of the latter have no 

 homologues to all appearances in the Nemertine, and there seem 

 to be no grounds for comparison between the larval forms. 



