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The position of the pores near the centre of the ambulacrals in Bothriocidaris need not be regarded as primitive, since other early Palaeozoic genera, not to mention the young of living forms, show that the podia originally passed out between the plates, and were only gradually surrounded by their substance; thus the original structure of the echinoid ambulacra differed from that of the early asteroid in the position of the radial vessels and nerves, which here lie beneath the plates instead of outside them.

An Upper Silurian echinoid, however, Palaeodiscus, is believed by W.J.

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.

This would leave the Echinoid scheme remarkably simple, with the Melonitoida and Cidaroida as divergent branches from an ancestor like Bothriocidaris; but while the former branch soon decayed, the latter continues to flourish at the present day.

The rows of plates in an Echinoid which are not perforated for the emission of the "tube-feet."