Some little time ago a preliminary note upon Phoronis was read before the Eoyal Society, and therein it was stated that a study of the anatomy of this group had led one to believe that its nearest phyletic allies were to be found in the Hemichorda, a group comprising a few species with primitive chordate characters.

In attempting to class Phoronis with these, an important difficulty presented itself. It has been usual to define the Chordata as having three essential characters in common, either temporarily in ontogeny or permanently throughout life, i.e., a dorsal nervous system, paired gill-slits, and a notochord.

With regard to these features, there can be no doubt that the nervous system of Phoronis is dorsal in position, and its arrangement is exactly comparable to that of Balanoglossus; but, on the other hand, there are no gill-slits in the adult Phoronis, and the notochord is as certainly absent.

In the case of the gill-slits, we find upon critical examination that they are extremely variable in extent of number and position in the different Chordata, and in one species, Rhabdopleura, they are, according to present knowledge, entirely absent. For these reasons it is well not to insist on the presence of gill-slits as an essential character. With respect to the notochord a different condition prevails-the hypoblastic origin and vacuolated character of this organ mark it out from all other skeletal structures, and it figures more or less prominently in all the Chordata.