6. The Nephridia of Lanice conchilega, Malmgren

Abstract

The excretory system in this species has never been adequately described; it presents an extremely interesting condition from a morphological point of view. There are four well-developed nephridia in somites 6–9, inclusive. Each of these commences by an internal aperture or nephrostome, and consists of a bent tube or loop, the inner side (the side nearer the median plane) of the loop being connected with the nephrostome, while the outer passes downwards and opens into a longitudinal tube common to all the four nephridia of a side. Four openings, corresponding to the four nephridia, place the longitudinal tube in communication with the exterior; these openings are close behind the upper ends of the 2nd to the 5th uncinigerous tori respectively; the 1st uncinigerous torus being in the 5th somite. The longitudinal tube is continued backwards on each side through somites 10–13, representing four more coalesced nephridia; but in this region there are neither internal nor external openings, nor any loops similar to those in the more interior region; the longitudinal tube is simple, almost cylindrical, showing slight indentations between the successive somites, which mark where the successive nephridia have coalesced. The outer side of the whole longitudinal tube is in contact with the ventral longitudinal muscles, while the upper and inner side is beneath the oblique muscles. The internal openings already mentioned are situated immediately behind the notopodial fascicles of setae of somites 5–8 inclusive. The longitudinal tube extends into the 5th somite, but I could not find there an external opening. Behind the 1st to the 4th somites are traces of incomplete septa, of which that behind the 4th is the most complete. Attached to the front of the latter septum is a nephrostome, but I could not trace any connection between this and the part of the tube in the 5th somite. There are two other well-marked nephrostomata attached to the septa behind somites 2 and 3, and these openings lead into tubes seen in somites 3 and 4. I could not find external openings in the two latter somites. There are thus eleven nephridia represented altogether,—three rudimentary, in somites 3, 4, 5; four perfect, in somites 6–9; and four imperfect, in somites 10–13; the eight posterior being all in communication, their distal parts having fused to form a longitudinal tube. This is the first case in which such a longitudinal coalescence of nephridia has been discovered, and its morphological similarity to the condition in Vertebrates is obvious.