The excretory system begins to be formed, while the larva is still in the trochospere stage ([fig. 383], nph), and consists of a provisional excretory organ, which is placed in front of the future segmented part of the body, and occupies a position very similar to that of the provisional excretory organ found in some Molluscan larvæ (vide p. 681).
Hatschek, with some shew of reason, holds that the provisional excretory organs of Polygordius are homologous with those of the Mollusca.
In its earliest stage the provisional excretory organ of Polygordius consists of a pair of simple ciliated tubes, each with an anterior funnel-like opening situated in the midst of the mesoblast cells, and a posterior external opening. The latter is placed immediately in front of what afterwards becomes the segmented region of the embryo. While the larva is still unsegmented, a second internal opening is formed for each tube ([fig. 383], nph) and the two openings so formed may eventually become divided into five ([fig. 384] A), all communicating by a single pore with the exterior.
When the posterior region of the embryo becomes segmented, paired excretory organs are formed in each of the posterior segments, but the account of their development, as given by Hatschek, is so remarkable that I do not think it can be definitely accepted without further confirmation.
From the point of junction of the two main branches of the larval kidney there grows backwards ([fig. 384] B), to the hind end of the first segment, a very delicate tube, only indicated by its ciliated lumen, its walls not being differentiated. Near the front end of this tube a funnel, leading into the larval body cavity of the head, is formed, and subsequently the posterior end of the tube acquires an external opening, and the tube distinct walls. The communication with the provisional excretory organ is then lost, and thus the excretory tube of the first segment is established.
The excretory tubes in the second and succeeding segments are formed in the same way as in the first, i.e. by the continuation of the lumen of the hind end of the excretory tube from the preceding segment, and the subsequent separation of this part as a separate tube.
Fig. 384. Diagram illustrating the development of the excretory system of Polygordius. (After Hatschek.)
The tube may be continued with a sinuous course through several segments without a distinct wall. The external and internal openings of the permanent excretory tubes are thus secondarily acquired. The internal openings communicate with the permanent body-cavity. The development of the permanent excretory tubes is diagrammatically represented in [fig. 384] C and D.