ANATOMY AND CLASSIFICATION OF THK AKENICOLID.E. 511 



corresponds iu position to the second nephridium of A. 

 marina. 



The first and last uephridia of A. marina are subject to 

 considerable variation ; the first pair especially are often 

 appreciably smaller than any of the sncceeding nephridia. 

 Out of about 160 specimens examined, we have found eight 

 instances, in each of which there is some considerable de- 

 parture from the normal condition. Six of these specimens 

 show reduction or loss of the first nephridium. In one case 

 the first nephridium of each side is very small, its secreting 

 portion being only as thick as an ordinary pin ; in a second 

 example the first nephridium of the left side is normal, but 

 that of the right side has no funnel ; in a third specimen the 

 first nephridium on each side has no funnel ; in the fourth 

 and fifth examples the first nephridium of one side has a very 

 small funnel, while the corresponding one of the other side is 

 represented by a funnel only ; and in the sixth specimen the 

 first nephridium of the left side is totally absent, the corre- 

 sponding right one being present but without funnel. 



In the other two instances of variation the last nephridia 

 are affected. In a large specimen (250 mm. in length) the 

 sixth nephridium of the right side is normal, but the corre- 

 sponding one of the left side consists of funnel only. Another 

 specimen, 100 mm. long^ has only five pairs of nephridia 

 opening on the fourth to the eighth chaetigerous annuli, the 

 sixth pair being totally absent. 



From the above records it is evident that the funnel and 

 the rest of the nephridium are to some extent formed in- 

 dependently, for in the examples of A. marina mentioned 

 above there are four cases in which the funnel is absent, and 

 three in which the nephridium is represented by a funnel only. 



The first nephridium in A. Grubii is sometimes represented 

 by a funnel only (see p. 513), and we have also dissected and 

 figured a specimen of this species in which a funnel is present 

 in the segment behind the last normal nephridium (PI. 26, 

 fig. 54). 



It therefore appears probable that the funnel develops at 



