Thus the elements of the Pelmatozoan ventral groove are now detected in so different a structure as the echinoid ambulacrum, while an aboral nervous system, the diminished representative of that in crinoids, has been traced in all Eleutherozoa except Holothurians.
- Diagrammatic sections across the ambulacra of A, C, Pelmatozoa, and B, D, Eleutherozoa, placed in the same position for comparison.
The Echinoderms may be divided into seven classes, whose probable relations are thus indicated: Cystidea Edrioasteroidea Pelmatozoa Holothurioidea Crinoidea Blastoidea Eleutherozoa Stelliformia Echinoidea Brief systematic accounts of these classes follow: Grade A.
The classes of the Eleutherozoa probably arose independently from different branches of the Pelmatozoan stem.
- Eleutherozoa normally elongate along the oro-anal axis, which axis and the dorsal hydropore lie in the sagittal plane of a secondary bilateral symmetry.