238 COMPARATIVE ANATOMY CHAP. 



the embryonic head nephridia or the head kidneys. (2) Those 

 which appear in the trunk segments ; these are the provisional trunk 

 kidneys. The permanent nephridia, on account of their frequently 

 strict segmental arrangement, are often called segmental organs, or 

 on account of their looped or winding course (in Oligochceta and 

 Hirudinea) looped canals. 



We will first describe the three sorts of nephridia separately, and then discuss 

 their morphological significance and their relations to each other. 



A. The embryonic head nephridia (head kidneys). These appear temporarily in 

 the larva or embryo, and are paired. Their inner end lies in the embryonic head 

 cavity. They have been observed in many Oligochceta and Polychceta. They are 

 ciliated canals, which are not in open commnnication with the head cavity. The 

 lumen of these canals is intracellular, i.e. the nephridia are rows of consecutive cells 

 perforated to form a canal. In this point the embryonic head nephridia agree with 

 the permanent nephridia of the Oligochceta and Hirudinea, and with the canals of 

 the water- vascular system of the Platodes. They are occasionally branched (e.g. in 

 the larvse of Echiurus and Polygordius), like the water-vascular system of the Platodes 

 and the nephridia of many Nemertina. Lateral branchings of the principal canals 

 also occur in the permanent nephridia of the Hirudinea and Oligochceta. Terminal 

 cells provided with bundles of cilia (flames) often occur at the inner ends of the 

 branched or simple nephridia ; the flames project into the lumen of the canal, in 

 which they undulate. These terminal cells resemble those of the water- vascular 

 system in the Platodes. 



B. The embryonic or provisional trunk nephridia. These have till now 

 been observed in comparatively few cases ; it is, however, probable that they are 

 widely distributed. They occur (like permanent nephridia) in strictly segmental 

 arrangement as paired canals in those generally anterior^ segments, in which in 

 adults permanent nephridia are wanting. 



Among the Oligochceta it has been proved that in Rhynchelmis, in the 5 

 anterior trunk segments in which nephridia are wanting in adult animalg, 

 5 pairs of provisional nephridia, which degenerate later, attain development 

 in the embryo. In the Capitellidce the nephridia are always wanting in 

 a large number of anterior segments (thorax and anterior part of the 

 abdomen), but this is only the case in adult animals. In the young animals, 

 however, we meet with provisional nephridia in most of these segments, which are 

 the better developed the younger the animal, and the further forward the segment 

 to which they belong. In other words, the nephridia arise first anteriorly, and their 

 degeneration proceeds in order from before backward (in the thoracic and in 

 some of the anterior abdominal segments) in proportion as the permanent 

 nephridia of the abdominal region attain development. In Nereis cultrifera 

 (Fig. 159) there are found in the larval stage 5 pairs of provisional or larval 

 trunk nephridia in the 5 anterior segments, in which in adult animals we meet 

 with no nephridia. As in most Oligochceta nephridia are wanting in some of the 

 anterior segments, it is probable that provisional nephridia occur in these segments 

 in the larval forms. The same may be conjectured of the Polychceta. In the 

 Oligochceta the nephridia are wanting in the genital segments, except in the 

 Lumbricidce. But here also provisional nephridia attain development in these 

 segments at early stages. In the Hirudinea provisional nephridia are developed in 

 early embryonic or larval stages (2 pairs in Nephelis, 3 pairs in Hirudo, 4 pairs 

 in Aulostoma) which disappear early. As the larvae are, at the time when pro- 

 visional nephridia are present, still unsegmented, it cannot be certainly decided what 



