39 



lilameuts make up tiie reflected or ascending lamellae, 

 corresponding to the outer limbs of tlie W in the 

 diagram. 



The direct and reflected lamellae of one demibrancli 

 are quite continuous, and the reflection is simply a device 

 for increasing the area of the gills ^Yithout occupjdng an 

 awkwardly large space. The reflected lamella (fig. 20, 

 Br. a.) does not reach the axis of the ctenidium, 

 neither is it connected by cilia or other means with the 

 mantle or with the ctenidium of the opposite side. Nor 

 is there any connection between the two demibranchs of 

 one side except at the axis from which they arise. 



The Ctenidial Axis (fig. 20 and 45, Br. ax.) is a plate- 

 like ridge of connective tissue of considerable depth, 

 depending from the body wall. On the right side where 

 the adductor is inserted nearer to the hinge line than on 

 the left, the axis arises not far from the labial palps and 

 is attached to the adductor muscle. It continues ventrally, 

 gradually increasing in depth, until it passes the point 

 where the afferent branchial vessel from the renal organ 

 enters it, and just Avhere the pallial nerves from the 

 visceral ganglion enter the mantle. Here its course is 

 diverted and the ctenidial axis leaves the adductor and 

 is suspended from the mantle lobe. It terminates at a 

 point almost level Avith the end of the rectum, but its 

 posterior extremity does not diminish in depth, the 

 filament-bearing margin continues almost parallel to the 

 attached edge, and the posterior extremity has a slightly 

 bifurcated appearance as shown in fig. 1. On the left 

 side the ctenidial axis is attached to the adductor muscle 

 along its whole length, and does not extend quite so far 

 back as that of the right side. 



The axis is made up almost entirely of connective 

 tissue, with a good supply of muscle fibres (fig. 45). It is 



