A further fact must be considered in deciding this question, which is the discovery of ramifying coelomic tubes, approaching close to, but not entering, the epidermis in the Polychaete Arenicola.
This form of nephridia leads to the shorter but essentially similar organs in the Polychaete Sternaspis, and to those of the Echiuroidea (q.v.) and of the Gephyrea.
Cleavage leads to the formation of an epibolic gastrula and ciliated embryo which hatches as a free-swimming larva remarkably like that of a Polychaete worm (D).
Of these 6, the two first to be considered are very plainly separable and represent the extremes of Polychaete organization.