Chaetae of Arenicola 



45 



these remind one of the similar crests or " Sageblatter " of the 

 chaetae of some Aphroditidae and certain other Polychaeta. The 

 laminate portion of the chaeta bears on its surface numerous fine 

 processes, the pointed tips of which are directed at slightly different 

 angles ; those viewed in profile at the margin of the lamina form a 

 regular series of very fine teeth. In each interval between the 

 " Sageblatter " there is a denser transverse band, the presence of 

 which, at regular distances of about 10 to 12 /z, gives to the distal 

 portion of the shaft of the chaeta a transversely striated appearance, 

 which is seen clearly even under low magnification (about 50). This 

 well-marked striation and the great development of the crests on the 

 shaft are two characteristic features by which the chaetae of A. loveni 

 may be distinguished from those of any other species of Arenicola. 



The notopodial chaetae of other species apparently all possess crests 

 of a similar type to those of A. loveni, but of 

 more feeble development. The crests break 

 up into their individual teeth and the latter 

 become spread, by use, in different directions, 

 so that the shaft of the chaeta is covered, 

 to a greater or less extent in different species, 

 with numerous hair-like or spine-like pro- 

 cesses, the association of which with one 

 another to form comb-like crests, is, in many 

 cases, no longer obvious. 



The chaetae of young specimens (up to 

 about 48 mm. long) of A. cristata are 

 laminate along one side for a short distance, 

 but the lamina is markedly dentate and soon 

 breaks up into teeth. In older examples 

 the chaetae, even when newly formed, are 

 non-laminate. The tip of an unworn chaeta, 

 from a specimen 280 mm. long, is represented 

 in optical section in Fig. 12 A. The chaetae 

 of this species, when the tip has been worn Fig. IZ.A. cristata. A, Tip of an 



, ., , unworn notopodial chaeta, seen 



away by use, present the appearance shown in optical section, from a speci- 



T ,i- i o T i i i -I >en 280 mm. long ; B, End of a 



in Jblg. 12 B. In large examples, m which worn chaeta, from a specimen 

 .-, i , . i .*._* A ,1 403 mm. long. 



the chaetae attain a length of 9 mm., the 



spinous processes (teeth of the Sageblatter) are very numerous. 

 The chaetae of A. glacialis resemble those of A. cristata, but are 

 rather less spinous. 



