As in fishes also, the sensory canal system must have been highly developed on the skulls of many labyrinthodonts, and the impressions left by these canals have been utilized by morphologists for homologizing the various elements of the cranial roof with those of Crossopterygians.
Further, the morphologists of the 'fifties appear, with few exceptions, to have accepted a preliminary scheme with regard to the Arthropod head and Arthropod segmentation generally, which was misleading and caused them to adopt forced conclusions and interpretations.
It is held by some morphologists that the mollusc body is unsegmented, and therefore is to be compared to a single segment of a Chaetopod or Arthropod.
In the failure of previous g, ovary; k, kidney; m, mouth; n, supra morphologists to adapt oesophageal ganglion; nr, nerve ring in the details of developsection.
That doctrine took some few years to produce its effect, but it became evident at once to those who accepted Darwinism that the natural classification of animals, after which collectors and anatomists, morphologists, philosophers and embryologists had been so long striving, was nothing more nor less than a genealogical tree, with breaks and gaps of various extent in its record.