340 MOORE. [Vol. XIII. 



the passages are very strongly arched between them, but they 

 may be at opposite poles {pn"). They correspond to the 

 places of union between the nodules and the simple connecting 

 tubules, one afferent, one efferent, with the lumina of which 

 the converged passages are continuous. The plexuses increase 

 greatly in complexity with the growth of the animals, and 

 the nodules correspondingly gain in size and opacity. In the 

 newly hatched individual the nodules are very slight enlarge- 

 ments of a tortuous canal, and the lumen very slightly branched; 

 but as the animal grows the canal branches more and more, until 

 the enlarging nodule is completely honeycombed by a network 

 of passages (Fig. i8). Dorner (14) has already stated that the 

 tubules are more distinctly seen in the brown mass of young 

 individuals, owing to the fact that it becomes more massive 

 with age, but the change is due to an alteration in the character 

 of the tubules themselves, and not primarily to any increase in 

 a cellular envelope, as he supposed. 



Within the limits of a single nephridium the several plexuses 

 show a certain variation in complexity, and in general it may 

 be said that they become somewhat more simple as the external 

 end of the series is approached, and decidedly so in the acces- 

 sory plexus lobe. This is shown in Fig. i. In the last three 

 or four nodules composing this lobe {ap, ap') the interior 

 becomes more extensively excavated, but the passages can 

 scarcely be said to constitute plexuses, consisting as they do of 

 rounded recesses opening by wide apertures into an irregular 

 central chamber, which communicates at two points with the 

 connecting tubules, and in the case of the last one with the 

 system of tubule loops (Fig. i, //). 



Nuclei are absent from the plexus nodules, the one figured 

 as present in that position in my former paper being probably 

 an abnormal condition, or one so projected optically from a 

 different level. The nodules are very finely and densely gran- 

 ular, but in this species, owing to the almost total absence of 

 pigment, are not very opaque. This is especially true of the 

 accessory lobe, which is quite translucent and only slightly 

 granular. It is in every respect a transitional region between 

 plexus and simple tubule. 



