Structure and Development of the Nephridia. 387 



or fiftb somite in advauce of the growing zone, is shown in lougitu- 

 dinal section in Fig. 11, Piate 23. The anterior end of the organ 

 extends across the thin ectodermal region just hehiud the segmentai 

 liniit, and its li})» are continnous with the remaius of the primitive 

 seplum (not shown in Fig. 11). The organ consists of a straight 

 uubranched tubule whose walls are composed of a continuous vacuo- 

 lated protoplasm eontaiuiug excretory graniiles. An axial ciliated 

 lumen is present, which freely commuuicates with the body-eavity 

 anterior to the septum by means of a minute opening through which 

 tbe cilia project. Nuclei are present in the walls of the organ, 

 which as yet shows no division into well-defined eells. As yet the 

 lumen does not communicate with the exterior, although the posterior 

 extremity of the organ is closely applied to the ectoderm and has 

 apparently become continuous with the latter. Fig. 34, Piate 24 

 represents a tangential section through the ventro-lateral region 

 of a larva of 13 — 14 somites, showing the two most anterior nephridia. 

 The nephridia are the 2"'^ and 3'^'^ larvai nephridia (2'^'^ provisionai 

 and V^ definitive, of somites V and VI respectively), the first pair 

 of larvai nephridia having already disappeared. The nephridia 

 bave increased in size considerably as compared with the stage 

 represented in Fig. 11, Piate 23, but are otherwise essentially as 

 described above. The posterior fusion with the body wall is bere 

 shown; the fusion takes place slightly behind and internally to the 

 row of ventral hooked setae. The lumen is incomplete posteriorly. 

 Anteriorly the continuity of the nephrostomial border with the septum 

 is clearly shown. 



At this period the nephridium is, in brief, a simple tubule with 

 intracellular lumen, of the kiud frequently described in other larvai 

 Annelids (cf. Meyer, 1887 — 88, Lang, 1889). It is peculiar, however, 

 in being anteriorly in open communication with the body-cavity. In 

 this peculiarity it presents a distinct contrast to the pronephridia de- 

 scribed by Meyek and Lang, which are described as closed inter- 

 nally and as hearing a marked resemblance to the flame-cells of 

 Turbellaria — a resemblance which has formed the basis of much 

 phylogenetic speculation. The pronephridia of Arenicola, however, 

 possess a distinct coelomic aperture of the kind described, as I bave 

 observed with perfect clearness an indefinite number of times. The 

 anterior region of these organs in fa et, together with a portion of 

 the adjoiuing septum, constitutes the primitive uephrostome, from 

 which the adultnephrostome is directly derived, aswill shortly be shown. 



