ON THE STRUCTURE OF ^ENIGMA iMNIGMATICA. 265 



lu front of the adductor muscle the axis of the left branchia 

 is suspended in the mantle cavity by a deep suspensory fold 

 attached to the lower surface of the visceral mass just below 

 the lower limb of the left kidney^ and has lost all connection 

 with the left mantle lobe. These relations are continued for- 

 ward as far as the posterior edge of the foot, where the direct 

 and reflected lamellfe of the outer demibranch become con- 

 tinuous with the external left labial fold and the direct 

 lamella of the inner demibranch passes into the internal left 

 labial fold as described above (p. 260). 



The fold formed by the united upper ends of the reflected 

 filaments of the right and left inner demibranchs acquires 

 no attachment till it reaches the anterior end of the great 

 extractor byssi muscle. Here it is pushed over to the left 

 and unites with the body wall close alongside of the attach- 

 ment of the direct lamella of the left inner demibranch, and 

 its blood-vessel passes into the blood sinus lying below the 

 recurrent limb of the left kidney, this sinus discharging its 

 blood into the left auricle (text-figure 2, b and c, v). 



As may be seen in fig. 1, the anterior end of the right 

 outer demibranch curves round dorsally behind the byssus 

 cavity. As explained above (p. 261) its reflected lamella 

 becomes continuous with the right external labial fold, and 

 its direct lamella with the right internal labial fold behind 

 the byssus (see text-figure 2, d and e). The anterior fila- 

 ments of both lamella of the right outer demibranch are 

 very short, but those of the direct lamella of the right inner 

 demibranch are very long and, as may be seen in fig. 1, d.h^, 

 extend below and to the left of the byssus cavity as far 

 forward as the foot. As is shown in text-figure 2, A to e, 

 and in fig. 10, these elongated anterior filaments of the right 

 inner demibranch are connected with the ventral surface of 

 the body, below the bjssus muscle, by a thin membrane, x, 

 which stretches across to the left side and eventually, as in 

 A, becomes attached to the left inner labial fold. Thus, as a 

 consequence of the displacement of organs several times 

 referred to, the inner demibranch of the right side is carried 



