59 



towards the tip, from the base of the gill, the leaflets at 

 first increase in size, the third pair being the largest, and 

 from here they gradually decrease towards the tip. The 

 aerating filaments stretch from the afferent to the efferent 

 vessels, and each leaflet is separated from its neighbours 

 by a slit. Hence water can pass in and out of the cavity 

 of the gill either by the hole at the base, or by the slits 

 between the gill leaflets. Looking through these slits on 

 the external side of the gill, the leaflet of the other side 

 may be seen (fig. GO, L. and LJ. This sketch shows a 

 part of the gill from the internal side. At the bottom is 

 the branchial gland, and from it three gill leaflets may be 

 seen running ventrally to the efferent axis of the gill. 

 The centre leaflet has been taken away except for a pari 

 at the base, in order to show 7 more clearly the two 

 alternating ones on the opposite side. Posteriorly, these 

 leaflets are attached to the outer side of the afferent vessel 

 which runs up the gill on the inner side of the spleen 

 (PL VIII, fig. 63), lessening in size as the gill narrows 

 to the tip. Thus the afferent vessel, covered over by the 

 general lining of the mantle cavity, forms the dorsal wall 

 of the cavity of the gill. Similarly on the outer edge of 

 the gill, the efferent vessel runs from its tip downwards, 

 but a thin sheet of connective tissue (fig. 60, C.T.), 

 running inwards into the gill cavity from the efferent 

 axis, along the median plane, separates the efferent vessel 

 from the cavity of the gill, and forms the ventral 

 boundary of the latter (figs. 60 and 63). 



The complex gill-leaflets are attached ventrally to 

 this sheet of tissue. First considering each leaflet as a 

 single sheet, it is seen that the aerating portion or 

 filament is on the external side of the attaching pillar, 

 and is consequently bathed in the water of the mantle 

 cavity. Hence blood brought into these filaments for 



