180 THE AMERICAN NATURALIST [Vol. XL 



one half as long as the entire animal; it is attached to the poste- 

 rior wall of the nuchal cavity so that its distal end alone is visible 

 (Fig. 3). It is a somewhat plume-like organ ; 

 the shaft of the feather is represented by a 

 fhittened triangular lamina, very long and 

 very narrow, which bears on its flattened 

 dorsal and ventral faces the structures cor- 

 res[)onding to the barbs. These are two 

 series of flattened more or less semicircular 

 sacs each of which runs transversely across 

 the shaft so that its cavity communicates 

 with each of the two lateral vessels described 

 below. 



Blood is conveyed through the gill by 

 means of two vessels each of which occu- 

 pies one margin of the shaft. That on the 

 right edge conveys blood from the suprarenal 

 the gill, that on the left carries blood from 

 the gill to the auricle. In the living animal the gill is usually so 

 rotated that the efferent vessel alone is visible. 



Attachment of the sli(>ll is ctlVctf.l bv n ImikI conijx.scd of the 

 paUial and the columellar niiisclc^ i 1 >. 'V\\v \)\\\\vA muscle 

 is ring-like and its flbers extend from tlie slu-ll mto llic mantle. 

 The columellar muscle is liorseshoe-shai)e(l and lies just internal 

 to the pallial muscle from which it is separated l)y no sharp bound- 

 ary; its fibers run from the shell into, the foot and it is of course 

 interrupted anteriorly where the hea<l is interposed between these 

 two structures. 



From this muscle band depends the mantle, a thin, tentacle- 

 fringed, membranous fold which lines the margiiuil part of the shell 

 and in front of the columellar nuiscle runs u[) to the apex. In this 

 region it forms the roof of a deep cavity, the luichal cavity, which 

 lies above the head and neck and is bounded at the sides and be- 

 hin<l bv the columeUar nmsclc and the front part of the visceral 

 ma.. " KKruluM,. th,' ma,Ml< fmm- ihr out. r u all of a groove-like 

 space, thr inanth' m<H,xr, rn( Iom.! l.-turm ii and tlir foot. Mantle 

 groove and nuchal cavity are of course continuous; l>oth together 

 constitute the mantle cavity. 



