50 



Arenicolidae 



number, (2) gradual reduction and eventual loss of the teeth 

 behind the rostrum, (3) increase of the angle between the rostrum 

 and shaft, and (4) elongation of the rostral region, which is seen 

 only in crotchets from very large specimens. The crotchets of other 



species of Arenicola exhibit corre- 

 sponding changes. 



A. assimilis. 1 Crotchets from a 

 post-larval example of the var. 

 affinis, 7*6 mm. long, are shown in 

 Fig. 21. The left one is drawn with 

 the rostrum almost in optical sec- 

 tion ; the right one, which is shown 



Fig. 20. A. marina (250 mm. long). The 

 ventral end of a neuropodial chaetal sac, 

 showing the formation of crotchets. 



Fig. 21. A. assimilis var. affinis. Crotchets 

 from a post-larval specimen, 7-6 mm. long. 



with the surface of the rostrum focussed, demonstrates that the 

 series of teeth extends round the base of the rostrum, and that the 

 sub-rostral process is the lowest and smallest of the series. Fig. 22 A 

 represents a crotchet, from a specimen 136 mm. long. This is 

 similar in form to that of A. marina (Fig. 19), and does not present 

 any dilatation of the post-rostral region (contrast the crotchets of 



1 The neuropodia of A. assimilis and pusilla, being much shorter than those 

 of A. marina, contain fewer crotchets ; an average number, in specimens about 

 five inches long, is 30 to 40 in each neuropodium of the branchial region. 



