Crotchets of Arenicola 



51 



A. pusilla, Fig. 24). A crotchet from a specimen of A. assimilis 

 var. affinis, 208 mm. long, is drawn in Fig. 22 B. It is 1 1 mm. in 

 length, and its characters the elongate edentulous rostrum (a little 

 worn at the tip) making a wide angle with 

 the shaft are such as would be expected 

 in a crotchet from so large a specimen. 



A. ylacialis. The crotchets of this 

 species are similar in general form to those 

 of A. marina, and have no post-rostral 

 dilatation (Fig. 23). A neuropodium of 

 one of the fragmentary type specimens 

 yielded large crotchets of the type shown 

 in Fig. 23 c, in which the rostrum is longer 

 and is curved almost like the blade of a 

 scythe. This represents probably the final 

 growth- phase. 



A. pusilla. The crotchets of this species 

 exhibit a full rounded curvature of the 

 region behind the rostrum, so that the free 

 end of the crotchet resembles the head and 

 beak of a swan. This character is shown 

 best in small or medium sized specimens 

 (Fig. 24 A, B), and is especially marked 

 in the crotchets of the type specimen 

 (Fig. 24 B). It is also obvious, but to a 

 less degree, in the large crotchets 

 (Fig. 24 c), which represent the latest 

 growth-phase seen by the writer, of a 

 massive specimen 160 mm. in length. 



The head-like curvature is more strongly 

 marked in this species than in A. cristata. 

 As A. pusilla is one of the most difficult 

 species to identify, this character is of 

 considerable value, for in the two species 

 with which A. pusilla is most easily 

 confused, namely, A. marina and assimilis 

 var. affinis, the crotchets do not present any 

 dilatation of the post-rostral portion. 



A. cristata. The sequence of growth-phases (Figs. 25, 26) has 

 been found to be similar to that described for A. marina. In 

 crotchets (Fig. 26 D, E), which show the final phase of growth, from 



E 2 



Fig. 22. A. assimilis var. affinig. 

 A, Crotchet from a specimen 

 136 mm. long, from Otago Har- 

 bour, N.Z. ; B, Crotchet from a 

 specimen 208 mm. long, from 

 Kerguelen. 



