ANATOMY AND CLASSIFICATION OF THE ARENICOLID.E. 449 



(see PI. 24, fig. o7), but in old specimens there is a very large 

 number of crotchets in each neuropodium. Two of the 

 neuropodia of a large specimen of A. cristata (300 mm. 

 long) have been treated with caustic potash, and the entire 

 bands of cheetas isolated. Each band contains 187 fully 

 formed chgette, and about fifteen others in course of formation 

 at the ventral end of the series. 



A. marina. — Crotchets of post-larval specimens (4"3 mm. 

 long) are '04 mm. to '05 mm. long, and bear two, or very 

 occasionally three, sharply marked teeth behind the rostrum, 

 and also a well-marked process ending in a long fine point 

 under the rostrum (fig. 8). In older specimens 17 mm. long, 

 which have assumed the adult characters and mode of life, 

 the crotchets are '1 mm. to '12 mm. long, and bear two well- 

 marked teeth but no process (fig. 11). Specimens about 

 100 mm. long have chgetee which are "4 mm. to "5 mm. long, 

 each of which bears two or three very small teeth and also a 

 small process (fig. 10). The crotchets of very large specimens, 

 especially those of the " Laminarian " variety, may attain a 

 length of "8 mm. to '85 mm. From the time of their forma- 

 tion they are devoid of teeth, but bear a small rather blunt 

 process beneath the rostrum (fig. 9). It is interesting to 

 note that as the animal grows in size the rostrum of the 

 crotchet, which in the eai'ly stages is at right angles to the 

 shaft, in later formed chast^e makes a considerably greater 

 angle with the shaft, the angle increasing with the age of 

 the worm from which the chaetse are obtained, so that in a 

 chaeta from a large worm, e. g. a Laminarian specimen about 

 250 mm. long, the angle between the rostrum and shaft is 

 almost 130" (cf. figs. 8 — 11). Concurrently there is a 

 reduction in the size of the teeth of successive generations 

 of chaetaB; so that although in post-larval stages the teeth 

 are large and comparable to the rostrum in size, they are 

 entirely absent from the chsetee of very old specimens 

 (cf. figs. 8, 9). 



A. Claparedii. — The crotchets are nearly always strongly 

 curved, sometimes being bent almost into the form of a 



