Chaetae of Arenicola 43 



is also a chaeta, the free end of which is spear-like (Fig. SB). 

 Spoon- and spear-headed chaetae are also present in the notopodia 

 of young larvae of A. cristata, in which only two or three segments 

 are indicated ; but the spoon-shaped chaetae are soon cast out, and 

 successive spear-headed chaetae, in which both shaft and head are 

 of gradually increased length, are formed during the rest of the 

 larval period. The tip of each of the later formed spears is drawn out 

 into a long, fine point. In post-larval specimens there are all 

 transitional forms, from spear-headed to laminate chaetae similar to 

 those of the adult, as shown in Fig. 9, which represents chaetae from 

 a post-larval A. marina, 4'3 mm. long. Each notopodium of this 

 worm contains one chaeta of the type shown in Fig. 9 A, accompanied 

 by from one to five of the longer capillary chaetae shown in Fig. 9 c. 

 Fig. 9 B represents a form transitional between those represented in 

 Figs. 9 A, c. Similar growth-phases are exhibited by the notopodial 

 chaetae of post-larval A. assimilis var. o finis. The spear-headed 

 chaetae probably do not persist long after the post-larval stage 

 has abandoned its free-swimming habit, for, in a young specimen 

 of A. marina, 17 mm. long, from the shore, spear-shaped chaetae 

 were not present ; the chaetae were all of the adult form, laminated 

 along one margin only. 



In post-larval stages of A. ecaudata 1 there are two kinds of 

 capillary chaetae (1) long tapering chaetae, similar to the majority 

 of those seen in late post-larval A. marina, and (2) shorter chaetae of 

 the kind shown in Fig. 10B, noteworthy for the presence of a slight 

 constriction about the junction of middle and distal thirds. A few 

 chaetae, like that shown in Fig. 10 A, are transitional forms suc- 

 ceeding spear-headed chaetae of earlier stages of development. 



These observations indicate that, throughout the genus, there is 

 a sequence of growth- forms of the notopodial chaetae, from spoon- 

 and spear-headed early phases to tapering chaetae, which alone are 

 found in the adult. 



THE XOTOPODIAL CHAETAE OF ADULT SPECIMENS. Each noto- 

 podial chaeta of an adult specimen is a yellow or golden, slender, 

 chitinous, tapering structure. It is inserted at its thicker, proximal 

 end, along with a number of similar chaetae, in a chaetal sac, which 

 is moved by protractor and retractor muscles (p. 35). The differences 

 between the chaetae of the various species consist in the presence or 



1 Also of A. branchialis, though these have not yet been as fully studied. 



