Chaetae of Arenicola 



41 



those of the neuropodia in the greater length of its shaft, is present, 

 but for a short time only, in a few of the last-formed notopodia of 

 very young specimens. A notopodium never contains more than one 

 crotchet, and after this has been cast out it is not replaced by a chaeta 



Fig. 7. A. cristata. Outline of a larva, about -7 mm. long, to show the chaetae. 

 PR. Prostomium. 



of a similar kind, but henceforward capillary chaetae only are formed 

 in the notopodium. The evidence afforded by the young stages of 

 A. cristata described above shows that, in this species, the crotchets 

 are lost from the notopodium within a very short time of the 

 attainment of the full number of chaetiferous segments. Probably 

 this is also the case in other species, for in none of the notopodia of 

 post-larval stages of A. marina (of which examples only 3 6 mm. 

 long have been examined) or of A. assimilis has the 

 writer been able to find a crotchet. 



C 



NOTOPODIAL CAPILLARY CHAETAE OF YOUNG SPECI- 

 MENS. The capillary chaetae successively present in 

 any one notopodium exhibit a series of changes of 

 form, the principal phases of which, traced chiefly 

 in A. pusilla, cristata and marina, may be noticed 

 here. Within twenty-four hours after hatching, the 

 larva of A. pusilla or A. cristata acquires its first 

 chaetae a pair situated some distance behind the 

 middle of its length. There are no elevations of 

 the body-wall, that is, no actual notopodia, but 

 the two chaetae indicate the position of the first A,' First notopodiai 

 notopodia. The free end of the chaeta (Fig. 8 A) 25 e mm IM r 



B 



is almost spoon-shaped, the shaft being continued, 



but tapered, along the axis of the flat " bowl " of 



the " spoon." In older larvae, with three or four 



segments, each notopodium contains a chaeta similar to that just 



described, but in each of the anterior two or three segments there 



