Gills of Arenicola 59 



angles to that in which they are drawn. This gill-axis and the three 

 hundred gill-filaments it bears, affording a very large aerating surface, 

 would be a most efficient respiratory structure. 



All the known examples of A. loveni possess typically pinnate 

 gills, practically identical in their form and details with those of 

 A. cristata. 



Both types of gill are found in A. marina. The large Laminarian 

 form has pinnate gills similar to those of A. cristata ; the smaller 

 littoral form has fruticose gills. 



The second gill of a littoral example of A. marina, 120 mm. long, is 

 shown in PI. XIII, Fig. 43. The gill consists of nine axes, " webbed " 

 at their base, and a tenth is just making its appearance. The longest 

 dorsal axis is about 2 mm. in length, and its lateral branches are 



Fig. 31. A. marina (from Wood's Holl), dorsal Fig. 32. A. pusilla (from Unalaska). Dorsal 



axis of fifth gill. axis of fifth gill. 



typical of those found in this form of gill. The ventral axis is a 

 little unusual in that one of its branches is disproportionately 

 large. 



A gill-axis from a specimen of A. marina (200 mm. long) from 

 Wood's Holl, exhibiting a rather extreme form of the bushy type, is 

 shown in Fig. 31. The lateral branches are few and are subdivided 

 into a comparatively small number of very long filaments. The 

 presence of eleven or twelve such axes, of which the gill is composed, 

 in an area about 9x6 mm., produces the effect of a dense bush. 



The gills of nearly all the specimens of A. pusilla examined con- 

 form to the pinnate type (cf. PI. XIII, Fig. 41). Those of massive 

 examples (160 mm. long) of this species from Unalaska are, however, 

 of different form (Fig. 32) : the axes are proportionately very short 

 (2 5 mm.) and bear on each side only three, or at most four, branches, 



