SEA-FISHERIES LABORATORY. 225 



a large number in the neuropodia of the branchial region. 

 One of the neuropodia of a specimen only about six inches 

 long contained 110 fully formed chsetse and eight or nine 

 others in course of formation at the ventral end of the series. 

 The crotchets show considerable differences according 

 to the age of the specimen from which they have been 

 taken. Those of post-larval specimens (5'1 mm. long) are 

 "03 to -04 mm. long, and bear two, or occasionally three, 

 sharply marked teeth behind the rostrum and also a well- 

 marked process, ending in a fine point, under the rostrum 

 (fig. 15). In older specimens 17 mm. long, which have 

 assumed the adult characters and mode of life the 

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

 teeth. Specimens about 100 mm. long have chsetse which 

 are - 4 to '5 mm. long, each of which bears two or three 

 very small teeth and also a very small subrostral process. 

 The crotchets of large specimens, especially those of the 

 Laminarian variety, may attain a length of '85 mm. 

 From the time of their formation they are devoid of teeth, 

 but bear a rather blunt subrostral process. It is inter- 

 esting to note that as the animal grows in size the rostrum 

 of the crotchet, which in the post-larval stages is at right- 

 angles to the shaft, in later formed chsetse makes a con- 

 siderably greater angle with the shaft, the angle 

 increasing with the age of the worm from which the 

 chsetEe are obtained, so that in a chseta from a large worm, 

 e.g., a Laminarian specimen about 250 mm. long, the angle 

 between the rostrum and shaft is almost 130°. Concur- 

 rently there is a reduction in the size of the teeth of suc- 

 cessive generations of chaetae, so that although in post- 

 larval stages the teeth are large and comparable to the 

 rostrum in size, they are entirely absent from the cheetse 

 of very old specimens. (For method of isolation and 

 preparation of seta?, see under practical work). 

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