442 P. W. GAMBLE AND J. H. ASHWORTH. 



fused basally. In A. ecaudata and A. Grubii (as Horst 

 [1889] first pointed out for the latter) the branches all spring 

 from a common stem. The mode of division of the branches 

 into leaflets is also somewhat different in the two cases. In 

 A. marina, A. cristata, and A. Claparedii the division is 

 nearly pinnate, though the leaflets are not strictly opposite, 

 but more nearly alternate. In A. ecaudata and A. Grubii 

 the branches bifurcate, and then the posterior of the two so 

 formed dichotomises near its tip. By a repetition of this 

 process the leaflets have a very distinctive appearance when 

 seen en masse, as contrasted with a pinnate type, all the 

 subdivisions taking place on the corresponding (posterior) 

 side of the secondary branches. The figures on PI. 22, fig. 

 7, will help to make this clear. 



The gills so formed are in some species retractile, though 

 never to the same degree as in Capitellids. This property 

 is best developed in the common lugworm, in which the whole 

 line of gills contracts from behind forwards, and so, as Milne 

 Edwards (1838) pointed out, assists in maintaining a circula- 

 tion of the blood. 



The gills of the Arenicolidas are usually considered to be 

 special structures rather than modifications of a dorsal cirrus, 

 on the grounds that in development each gill arises from a 

 slight evagination of the body-wall without the appearance 

 of any specially sensory structures such as in the Nereidi- 

 f ormia occur on the dorsal cirri before their transformation 

 into gills (Benham, 1893, p. 50). The gills of Arenicola, in 

 fact, develop from the first in connection with a capillary loop, 

 and function at once as respiratory structures. 



Cirri, in fact, have never been demonstrated in the 

 Arenicolidas. In A. cristata, however, certain large 

 papillEe occur on the tail (PL 22, fig. 1, and PI. 24, figs. 30 

 — 32), and these have been variously interpreted as cirri, 

 as accessory gills, or as merely somewhat large purely 

 epidermal processes. Stimpson (1854) and Liitken (1864) 

 have referred to them in describing American and West 

 Indian specimens. Levinsen (1883) suggested that they 



