DEVELOPMENT OF NEPHRIDIA OF PHERETIMA 85 



Secondary pharyngeal nephridia arise in a way different 

 from that of the secondary septal and integumentary ones. 

 They do not appear independently of the primary pair but 

 develop as buds on the nephridial ends of the pharyngeal 

 ducts. In fig. 19 are seen three buds in the fifth and two in 

 the sixth segment ; while in the fourth the primary nephridium 

 itself is not fully formed yet. As these buds develop into fully- 

 formed nephridia, their terminal ducts, longer than those of 

 the other types of nephridia, remain continuous with the 

 primary pharyngeal duct, or rather open into it. Thus we get 

 a large number of pharyngeal nephridia forming big tufts 

 and having their terminal ducts opening into these primary 

 ducts. The primary ducts themselves, although originally 

 very narrow and intra-cellular, enlarge and acquire a muscular 

 investment which makes their walls thick and tough as they 

 are in the adult condition. 



9. Comparison with the Development of ' Meganephric ' 

 AND the so-called ' Plectonephric ' Types of 

 Nephridia. 



I have referred in brief to the known facts of development 

 of these two types of nephridia in the historical part of this 

 paper. So far as the ' meganephric ' type of nephridia are 

 concerned, we can compare them only with the primary integu- 

 mentary nephridia of Pheretima, a pair in each segment. 

 The obvious differences between the meganephridia of Lum- 

 bricus and the primary pair of integumentary nephridia 

 in an embryo of Pheretima are the larger size of the former 

 and the presence of a ' funnel ' in them. In his recent memoir 

 on the development of nephridia in Criodrilus, as already 

 mentioned on p. 53 (15), Staff derives the nephridium 

 from the ' funnel-cell ' and the retroperitoneal group of cells 

 behind each septum, both being ultimately derived from the 

 nephridial string of cells between the ectoderm and the meso- 

 derm. In Pheretima, as we have seen, the primary 

 integumentary nephridia develop from the ' retroperitoneal ' 

 cells alone and there is no ' funnel-cell ' taking part in their 



