They are now by many systematists united with the Acanthocephala and the Nematomorpha to form the group Nemathelminthes.
The study of the distribution of species dates back to the time of the early systematists, the study of vegetation to the time of the early botanical travellers.
The interesting little Rhodope veranyii, which has no odontophore, has been associated by systematists both with these simplified Opisthobranchs and with Rhabdocoel Planarians.
Yet as systematists their authors were no worse than Klein, whose Historiae Avium Prodromus, appearing at Lubeck in 1750, and Stemmata Avium at Leipzig in 1759, met with considerable favour in some quarters.
Be that as it may, he declares that characters drawn from the sternum or the pelvis - hitherto deemed to be, next to the bones of the head, the most important portions of the bird's framework - are scarcely worth more, from a classificatory point of view, than characters drawn from the bill or the legs; while pterylological considerations, together with many others to which some systematists had attached more or less importance, can only assist, and apparently must never be taken to control, the force of evidence furnished by this bone of all bones - the anterior palatal.