DEVELOPMENT OF NEPHRIDIA OF PHBRETIMA 69 



The ridge-shaped mass of nephridial cells grows in the middle, 

 and we soon get a sort of papilla-like protuberance ; this papilla 

 elongates further into a long loop, having its two limbs close 

 together. At this stage, while the two ends of the loop forming 

 the proximal part are attached to the body-wall, the loop 

 itself forming the distal part lies free in the coelomic cavity 

 (fig. 11 c). This loop now elongates further, and, side by side 

 with the elongation of the loop, we find its two limbs getting 

 more and more closely pressed together so as to form one 

 compact lobe. A bend appears, at this stage, towards the base 

 of this lobe, and we now get two more or less distinct divisions 

 of the embryonic nephridium, the one distal to the bend 

 (fig. 11 d) and lying free, and the other proximal to the bend 

 and connected with the body-wall. The distal portion is now 

 a compact structure and goes to form the short straight lobe 

 of the adult nephridium. Although it has visibly lost its 

 double character, we must note that it is really double morpho- 

 logically, having been formed by a close apposition of the two 

 limbs of the loop. 



The proximal portion of the developing nephridium, the part 

 connecting the bend with the body-wall, is now the seat of 

 further growth. In this portion the double nature of the nephri- 

 dial loop persists for a time (fig. 11 d), but, soon after, the two 

 limbs of the loop at its proximal end, which are really the 

 two opposite ends of the original ridge, come closer together 

 and elongate further. This further elongation of the part 

 proximal to the bend results in the formation of a twist, a little 

 way from the bend, resulting in a condition of the nephridium 

 represented in fig. He. Elongation and twisting go on further 

 until we get the long twisted loop fully formed, with the 

 number of twists characteristic of the adult nephridium. The 

 straight lobe and the twisted loop having been fully formed, 

 the proximal end connecting the nephridium with the body-wall 

 becomes narrow and slender and forms the terminal duct of 

 the nephridium. This duct has meanwhile grown through the 

 thickness of the body-wall, and opens to the exterior in front 

 of the row of setae occurring in the middle line of each segment. 



