The skull generally lacks a sagittal crest; and the condyle of the lower jaw is transversely elongated.
The latter is, in comparison with mammals, represented by its middle portion only, the vermis; in a sagittal section it shows an extremely well developed arbor vitae, produced by the transverse, repeated folding of the whole organ.
The first is a ventral flexure in the antero-posterior or sagittal plane; the result of this is to approximate the two ends of the alimentary canal.
- Sagittal Median Section of Bladder, Prostate and Rectum, showing one of the ejaculatory ducts.
There is generally no sagittal crest to the skull; and the condyle of the lower jaw is transversely elongated.
- In the skull there is a sagittal crest; the tympanic bulla is filled with cancellous tissue; the condyle of the lower jaw is rounded; and the premaxillae, or anterior bones of the upper jaw, have the full number of incisor teeth in the young state, the outermost of these being persistent through life as an isolated tooth.
- Eleutherozoa normally elongate along the oro-anal axis, which axis and the dorsal hydropore lie in the sagittal plane of a secondary bilateral symmetry.
Odontoid process of second vertebra semi-cylindrical; skull with a sagittal crest; and the condyle of the lower jaw rounded.