84 KAKM NAKAYAN BAHL 



ducts of the pharyngeal nephridia are seen to have acquired 

 a lumen, while the nephridia th(anselves have not yet developed 

 their adult form and are still solid. These ducts are intra- 

 cellular, but are surrounded by the muscular tissue of the 

 strands passing from the pharynx to the body-wall for part 

 of their length. 



The usual order of antero-postericjr development is reversed 

 in the case of pharyngeal nephridia. In the two stages of 

 development mentioned above, the nephridia of the sixth 

 segment are advanced further in development than those of 

 the fifth, and the latter than those of the fourth segment. 



There is only one pair of nephridia anterior to the pharyngeal 

 ones, i. e. the one belonging to the third segment, the first two 

 segments of the embryo being anephrous. This most anterior 

 pair, although integumentary in character, follows the pharyn- 

 geals in their time and rate of development. 



The ducts in these early stages are very thin with intra- 

 cellular lumen, and are therefore to be looked upon as the 

 elongated terminal ducts of the primary pairs of pharyngeal 

 nephridia rather than as outgrowths from the walls of the 

 pharynx. Three successive thickenings on the lateral pharyn- 

 geal wall mark the places of entrance of the ducts into the 

 pharyngeal lumen. It is a remarkable fact that not only do 

 the terminal ducts acquire a lumen before the formation of the 

 canals in the nephridia themselves, but that they also open 

 into the cavity of the pharynx long before the nephridia are 

 able to function at all. 



The pharyngeal nephridia, like the integumentary ones, do 

 not develop a ' funnel ', but we have to note that at an early 

 stage when the terminal ducts have been formed and the 

 nephridia are developing their adult structure, the pharyngeal 

 pair come into connexion with the intersegmental septa 

 not in front of but behind them. The dorsal blood-vessel 

 and the lateral oesophageal act as the afferent and the efferent 

 vessels to these nephridia, and the branches of these vessels 

 near their points of origin and entrance into the main vessels 

 lie on the septal supports. 



