444 F. W. GAMBLE AND J. H. ASHWORTH. 



in the absence of any good evidence of their sensory nature it 

 would be premature to call them cirri. 



Each of the remaining segments of the tail bears one or 

 two such tubercles, which are distinguished by their size and 

 hollow vascular nature from the scattered epidermal papillae. 

 The annuli upon which they are placed are thickened and 

 placed regularly at segmental intervals (PL 24, figs. 31, 32). 



Such tubercles, borne on either the last chaetigerous seg- 

 ments or on the tail, occur in all the American and West 

 Indian specimens we have examined. They are absent, 

 however, from the Neapolitan examples of A. cristata. 



4. Setse (PI. 23). 



The setse of Arenicola may be divided into two kinds : 

 (1) the capillary sette forming the notopodial pencil ; and (2) 

 the crotchets or chaetas present in the neuropodial ridge : 

 though, as we shall point out below (see p. 446), these two 

 kinds of setae in very young forms are not strictly confined to 

 the positions above named. 



Notopodial Setae. — The notopodial setae are similar 

 throughout the genus, though there are certain features which 

 appear to be characteristic of the various species. Each 

 notopodial seta is a slender capillary structure inserted at its 

 proximal end, along with many other similar setse, in a setal 

 sac, which is moved by special retractor and protractor 

 muscles. The seta has an almost uniform diameter for a con- 

 siderable portion of its length, but it tapers at its distal end 

 to a fine point. This tapering portion bears several rows of 

 very minute hair-like processes, beautifully and regularly 

 arranged like the barbs of a feather upon its shaft (figs. 13, 

 20, 23). 



The notopodial setse of adult specimens of A. marina, A. 

 Claparedii and A. cristata closely resemble each other. 

 They are moderately stout, and the hair-like processes upon 

 them are numerous and well marked. The notopodial sette 

 of A. cristata are very hairy, especially in old specimens, 

 and the barbs or processes on the set^ are more numerous 



